8-Channel, 1 MSPS, 8-/10-/12-Bit ADCs
with Sequencer in 20-Lead TSSOP
AD7908/AD7918/AD7928
Rev. C
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responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 www.analog.com
Fax: 781.461.3113 ©2006–2008 Analog Devices, Inc. All rights reserved.
FEATURES
Fast throughput rate: 1 MSPS
Specified for AVDD of 2.7 V to 5.25 V
Low power
6.0 mW max at 1 MSPS with 3 V supply
13.5 mW max at 1 MSPS with 5 V supply
Eight (single-ended) inputs with sequencer
Wide input bandwidth
AD7928, 70 dB min SINAD at 50 kHz input frequency
Flexible power/serial clock speed management
No pipeline delays
High speed serial interface SPI®/QSPI™/
MICROWIRE™/DSP compatible
Shutdown mode: 0.5 μA max
20-lead TSSOP package
GENERAL DESCRIPTION
The AD7908/AD7918/AD7928 are, respectively, 8-bit, 10-bit, and
12-bit, high speed, low power, 8-channel, successive approximation
ADCs. The parts operate from a single 2.7 V to 5.25 V power
supply and feature throughput rates up to 1 MSPS. The parts
contain a low noise, wide bandwidth track-and-hold amplifier that
can handle input frequencies in excess of 8 MHz.
The conversion process and data acquisition are controlled using
CS and the serial clock signal, allowing the device to easily interface
with microprocessors or DSPs. The input signal is sampled on the
falling edge of CS and conversion is also initiated at this point.
There are no pipeline delays associated with the part.
The AD7908/AD7918/AD7928 use advanced design techniques to
achieve very low power dissipation at maximum throughput rates.
At maximum throughput rates, the AD7908/AD7918/AD7928
consume 2 mA maximum with 3 V supplies; with 5 V supplies, the
current consumption is 2.7 mA maximum.
Through the configuration of the control register, the analog input
range for the part can be selected as 0 V to REFIN or 0 V to 2 ×
REFIN, with either straight binary or twos complement output
coding. The AD7908/AD7918/AD7928 each feature eight single-
ended analog inputs with a channel sequencer to allow a
preprogrammed selection of channels to be converted sequentially.
The conversion time for the AD7908/AD7918/AD7928 is
determined by the SCLK frequency, which is also used as the
master clock to control the conversion.
FUNCTIONAL BLOCK DIAGRAM
•
•
•
•
•
•
•
•
•
•
•
•
•
T/H
I/P
MUX
SEQUENCER CONTROL LOGIC
GND
SCLK
DOUT
DIN
CS
AD7908/AD7918/AD7928
REF
IN
V
IN
0
V
IN
7
A
V
DD
03089-001
8-/10-/12-BIT
SUCCESSIVE
APPROXIMATION
ADC
V
DRIVE
Figure 1.
PRODUCT HIGHLIGHTS
1. High Throughput with Low Power Consumption. The AD7908/
AD7918/AD7928 offer up to 1 MSPS throughput rates. At the
maximum throughput rate with 3 V supplies, the AD7908/
AD7918/AD7928 dissipate just 6 mW of power maximum.
2. Eight Single-Ended Inputs with a Channel Sequencer.
A sequence of channels can be selected, through which
the ADC cycles and converts on.
3. Single-Supply Operation with VDRIVE Function. The AD7908/
AD7918/AD7928 operate from a single 2.7 V to 5.25 V supply.
The VDRIVE function allows the serial interface to connect directly
to either 3 V or 5 V processor systems independent of AVDD.
4. Flexible Power/Serial Clock Speed Management. The conversion
rate is determined by the serial clock, allowing the conversion
time to be reduced through the serial clock speed increase. The
parts also feature various shutdown modes to maximize power
efficiency at lower throughput rates. Current consumption is
0.5 μA max when in full shutdown.
5. No Pipeline Delay. The parts feature a standard successive
approximation ADC with accurate control of the sampling
instant via a CS input and once off conversion control.
AD7908/AD7918/AD7928
Rev. C | Page 2 of 28
TABLE OF CONTENTS
Features .............................................................................................. 1
General Description ......................................................................... 1
Functional Block Diagram .............................................................. 1
Product Highlights ........................................................................... 1
Revision History ............................................................................... 2
Specifications ..................................................................................... 3
AD7908 Specifications ................................................................. 3
AD7918 Specifications ................................................................. 5
AD7928 Specifications ................................................................. 7
Timing Specifications .................................................................. 9
Absolute Maximum Ratings .......................................................... 10
ESD Caution ................................................................................ 10
Pin Configuration and Function Descriptions ........................... 11
Terminology .................................................................................... 12
Typical Performance Characteristics ........................................... 13
Performance Curves ................................................................... 13
Control Register .............................................................................. 15
Sequencer Operation ................................................................. 16
SHADOW Register .................................................................... 17
Circuit Information .................................................................... 18
Converter Operation .................................................................. 18
ADC Transfer Function ............................................................. 19
Handling Bipolar Input Signals ................................................ 19
Typical Connection Diagram ................................................... 19
Modes of Operation ................................................................... 21
Power vs. Throughput Rate ....................................................... 23
Serial Interface ............................................................................ 23
Microprocessor Interfacing ....................................................... 24
Application Hints ....................................................................... 27
Outline Dimensions ....................................................................... 28
Ordering Guide .......................................................................... 28
REVISION HISTORY
11/08—Rev. B to Rev. C
Changes to ESD Parameter, Table 5 ............................................. 10
6/06—Rev. A to Rev. B
Updated Format .................................................................. Universal
Changes to Reference Section ....................................................... 21
9/03—Rev. 0 to Rev. A
Changes to Figure 3 ........................................................................ 15
Changes to Reference section ....................................................... 18
AD7908/AD7918/AD7928
Rev. C | Page 3 of 28
SPECIFICATIONS
AD7908 SPECIFICATIONS
AVDD = VDRIVE = 2.7 V to 5.25 V, REFIN = 2.5 V, fSCLK = 20 MHz, TA = TMIN to TMAX, unless otherwise noted.
Table 1.
Parameter B Version1 Unit Test Conditions/Comments
DYNAMIC PERFORMANCE fIN = 50 kHz sine wave, fSCLK = 20 MHz
Signal-to-(Noise + Distortion) (SINAD)2 49 dB min
Signal-to-Noise Ratio (SNR)2 49 dB min
Total Harmonic Distortion (THD)2 −66 dB max
Peak Harmonic or Spurious Noise (SFDR)2 −64 dB max
Intermodulation Distortion (IMD)2 fa = 40.1 kHz, fb = 41.5 kHz
Second-Order Terms −90 dB typ
Third-Order Terms −90 dB typ
Aperture Delay 10 ns typ
Aperture Jitter 50 ps typ
Channel-to-Channel Isolation2 −85 dB typ fIN = 400 kHz
Full Power Bandwidth 8.2 MHz typ @ 3 dB
1.6 MHz typ @ 0.1 dB
DC ACCURACY2
Resolution 8 Bits
Integral Nonlinearity ±0.2 LSB max
Differential Nonlinearity ±0.2 LSB max Guaranteed no missed codes to 8 bits
0 V to REFIN Input Range Straight binary output coding
Offset Error ±0.5 LSB max
Offset Error Match ±0.05 LSB max
Gain Error ±0.2 LSB max
Gain Error Match ±0.05 LSB max
0 V to 2 × REFIN Input Range −REFIN to +REFIN biased about REFIN with
twos complement output coding
Positive Gain Error ±0.2 LSB max
Positive Gain Error Match ±0.05 LSB max
Zero Code Error ±0.5 LSB max
Zero Code Error Match ±0.1 LSB max
Negative Gain Error ±0.2 LSB max
Negative Gain Error Match ±0.05 LSB max
ANALOG INPUT
Input Voltage Ranges 0 to REFIN V RANGE bit set to 1
0 to 2 × REFIN V RANGE bit set to 0, AVDD/VDRIVE = 4.75 V to
5.25 V
DC Leakage Current ±1 μA max
Input Capacitance 20 pF typ
REFERENCE INPUT
REFIN Input Voltage 2.5 V ±1% specified performance
DC Leakage Current ±1 μA max
REFIN Input Impedance 36 kΩ typ fSAMPLE = 1 MSPS
LOGIC INPUTS
Input High Voltage, VINH 0.7 × VDRIVE V min
Input Low Voltage, VINL 0.3 × VDRIVE V max
Input Current, IIN ±1 μA max Typically 10 nA, VIN = 0 V or VDRIVE
Input Capacitance, CIN3 10 pF max
AD7908/AD7918/AD7928
Rev. C | Page 4 of 28
Parameter B Version1 Unit Test Conditions/Comments
LOGIC OUTPUTS
Output High Voltage, VOH VDRIVE − 0.2 V min ISOURCE = 200 μA, AVDD = 2.7 V to 5.25 V
Output Low Voltage, VOL 0.4 V max ISINK = 200 μA
Floating-State Leakage Current ±1 μA max
Floating-State Output Capacitance3 10 pF max
Output Coding Straight (natural) binary Coding bit set to 1
Twos complement Coding bit set to 0
CONVERSION RATE
Conversion Time 800 ns max 16 SCLK cycles with SCLK at 20 MHz
Track-and-Hold Acquisition Time 300 ns max Sine wave input
300 ns max Full-scale step input
Throughput Rate 1 MSPS max See Serial Interface section
POWER REQUIREMENTS
AVDD 2.7/5.25 V min/max
VDRIVE 2.7/5.25 V min/max
IDD4 Digital inputs = 0 V or VDRIVE
Normal Mode (Static) 600 μA typ AVDD = 2.7 V to 5.25 V, SCLK On or Off
Normal Mode (Operational) 2.7 mA max AVDD = 4.75 V to 5.25 V, fSCLK = 20 MHz
2 mA max AVDD = 2.7 V to 3.6 V, fSCLK = 20 MHz
Using Auto Shutdown Mode 960 μA typ fSAMPLE = 250 kSPS
0.5 μA max (Static)
Full Shutdown Mode 0.5 μA max SCLK on or off (20 nA typ)
Power Dissipation4
Normal Mode (Operational) 13.5 mW max AVDD = 5 V, fSCLK = 20 MHz
6 mW max AVDD = 3 V, fSCLK = 20 MHz
Auto Shutdown Mode (Static) 2.5 μW max AVDD = 5 V
1.5 μW max AVDD = 3 V
Full Shutdown Mode 2.5 μW max AVDD = 5 V
1.5 μW max AVDD = 3 V
1 Temperature ranges as follows: B version: −40°C to +85°C.
2 See Terminology section.
3 Sample tested @ 25°C to ensure compliance.
4 See Power vs. Throughput Rate section.
AD7908/AD7918/AD7928
Rev. C | Page 5 of 28
AD7918 SPECIFICATIONS
AVDD = VDRIVE = 2.7 V to 5.25 V, REFIN = 2.5 V, fSCLK = 20 MHz, TA = TMIN to TMAX, unless otherwise noted.
Table 2.
Parameter B Version1 Unit Test Conditions/Comments
DYNAMIC PERFORMANCE fIN = 50 kHz sine wave, fSCLK = 20 MHz
Signal-to-(Noise + Distortion) (SINAD)2 61 dB min
Signal-to-Noise Ratio (SNR)2 61 dB min
Total Harmonic Distortion (THD)2 −72 dB max
Peak Harmonic or Spurious Noise (SFDR)2 −74 dB max
Intermodulation Distortion (IMD)2 fa = 40.1 kHz, fb = 41.5 kHz
Second-Order Terms −90 dB typ
Third-Order Terms −90 dB typ
Aperture Delay 10 ns typ
Aperture Jitter 50 ps typ
Channel-to-Channel Isolation2 −85 dB typ fIN = 400 kHz
Full Power Bandwidth 8.2 MHz typ @ 3 dB
1.6 MHz typ @ 0.1 dB
DC ACCURACY2
Resolution 10 Bits
Integral Nonlinearity ±0.5 LSB max
Differential Nonlinearity ±0.5 LSB max Guaranteed no missed codes to 10 bits
0 V to REFIN Input Range Straight binary output coding
Offset Error ±2 LSB max
Offset Error Match ±0.2 LSB max
Gain Error ±0.5 LSB max
Gain Error Match ±0.2 LSB max
0 V to 2 × REFIN Input Range −REFIN to +REFIN biased about REFIN with twos
complement output coding
Positive Gain Error ±0.5 LSB max
Positive Gain Error Match ±0.2 LSB max
Zero Code Error ±2 LSB max
Zero Code Error Match ±0.2 LSB max
Negative Gain Error ±0.5 LSB max
Negative Gain Error Match ±0.2 LSB max
ANALOG INPUT
Input Voltage Ranges 0 to REFIN V RANGE bit set to 1
0 to 2 × REFIN V RANGE bit set to 0, AVDD/VDRIVE = 4.75 V to 5.25 V
DC Leakage Current ±1 μA max
Input Capacitance 20 pF typ
REFERENCE INPUT
REFIN Input Voltage 2.5 V ±1% specified performance
DC Leakage Current ±1 μA max
REFIN Input Impedance 36 kΩ typ fSAMPLE = 1 MSPS
LOGIC INPUTS
Input High Voltage, VINH 0.7 × VDRIVE V min
Input Low Voltage, VINL 0.3 × VDRIVE V max
Input Current, IIN ±1 μA max Typically 10 nA, VIN = 0 V or VDRIVE
Input Capacitance, CIN3 10 pF max
AD7908/AD7918/AD7928
Rev. C | Page 6 of 28
Parameter B Version1 Unit Test Conditions/Comments
LOGIC OUTPUTS
Output High Voltage, VOH VDRIVE − 0.2 V min ISOURCE = 200 μA, AVDD = 2.7 V to 5.25 V
Output Low Voltage, VOL 0.4 V max ISINK = 200 μA
Floating-State Leakage Current ±1 μA max
Floating-State Output Capacitance3 10 pF max
Output Coding Straight (natural) binary Coding bit set to 1
Twos complement Coding bit set to 0
CONVERSION RATE
Conversion Time 800 ns max 16 SCLK cycles with SCLK at 20 MHz
Track-and-Hold Acquisition Time 300 ns max Sine wave input
300 ns max Full-scale step input
Throughput Rate 1 MSPS max See Serial Interface section
POWER REQUIREMENTS
AVDD 2.7/5.25 V min/max
VDRIVE 2.7/5.25 V min/max
IDD4 Digital inputs = 0 V or VDRIVE
Normal Mode (Static) 600 μA typ AVDD = 2.7 V to 5.25 V, SCLK on or off
Normal Mode (Operational) 2.7 mA max AVDD = 4.75 V to 5.25 V, fSCLK = 20 MHz
2 mA max AVDD = 2.7 V to 3.6 V, fSCLK = 20 MHz
Using Auto Shutdown Mode 960 μA typ fSAMPLE = 250 kSPS
0.5 μA max (Static)
Full Shutdown Mode 0.5 μA max SCLK on or off (20 nA typ)
Power Dissipation4
Normal Mode (Operational) 13.5 mW max AVDD = 5 V, fSCLK = 20 MHz
6 mW max AVDD = 3 V, fSCLK = 20 MHz
Auto Shutdown Mode (Static) 2.5 μW max AVDD = 5 V
1.5 μW max AVDD = 3 V
Full Shutdown Mode 2.5 μW max AVDD = 5 V
1.5 μW max AVDD = 3 V
1 Temperature ranges as follows: B version: –40°C to +85°C.
2 See Terminology section.
3 Sample tested @ 25°C to ensure compliance.
4 See Power vs. Throughput Rate section.
AD7908/AD7918/AD7928
Rev. C | Page 7 of 28
AD7928 SPECIFICATIONS
AVDD = VDRIVE = 2.7 V to 5.25 V, REFIN = 2.5 V, fSCLK = 20 MHz, TA = TMIN to TMAX, unless otherwise noted.
Table 3.
Parameter B Version1 Unit Test Conditions/Comments
DYNAMIC PERFORMANCE fIN = 50 kHz sine wave, fSCLK = 20 MHz
Signal-to-(Noise + Distortion) (SINAD)2 70 dB min @ 5 V
69 dB min @ 3 V typically 70 dB
Signal-to-Noise Ratio (SNR)2 70 dB min
Total Harmonic Distortion (THD)2 −77 dB max @ 5 V typically −84 dB
−73 dB max @ 3 V typically −77 dB
Peak Harmonic or Spurious Noise −78 dB max @ 5 V typically −86 dB
(SFDR)2 −76 dB max @ 3 V typically −80 dB
Intermodulation Distortion (IMD)2 fa = 40.1 kHz, fb = 41.5 kHz
Second-Order Terms −90 dB typ
Third-Order Terms −90 dB typ
Aperture Delay 10 ns typ
Aperture Jitter 50 ps typ
Channel-to-Channel Isolation2 −85 dB typ fIN = 400 kHz
Full Power Bandwidth 8.2 MHz typ @ 3 dB
1.6 MHz typ @ 0.1 dB
DC ACCURACY2
Resolution 12 Bits
Integral Nonlinearity ±1 LSB max
Differential Nonlinearity −0.9/+1.5 LSB max Guaranteed no missed codes to 12 bits
0 V to REFIN Input Range Straight binary output coding
Offset Error ±8 LSB max Typically ±0.5 LSB
Offset Error Match ±0.5 LSB max
Gain Error ±1.5 LSB max
Gain Error Match ±0.5 LSB max
0 V to 2 × REFIN Input Range −REFIN to +REFIN biased about REFIN with twos
complement output coding
Positive Gain Error ±1.5 LSB max
Positive Gain Error Match ±0.5 LSB max
Zero Code Error ±8 LSB max Typically ±0.8 LSB
Zero Code Error Match ±0.5 LSB max
Negative Gain Error ±1 LSB max
Negative Gain Error Match ±0.5 LSB max
ANALOG INPUT
Input Voltage Ranges 0 to REFIN V RANGE bit set to 1
0 to 2 × REFIN V RANGE bit set to 0, AVDD/VDRIVE = 4.75 V to 5.25 V
DC Leakage Current ±1 μA max
Input Capacitance 20 pF typ
REFERENCE INPUT
REFIN Input Voltage 2.5 V ±1% specified performance
DC Leakage Current ±1 μA max
REFIN Input Impedance 36 kΩ typ fSAMPLE = 1 MSPS
LOGIC INPUTS
Input High Voltage, VINH 0.7 × VDRIVE V min
Input Low Voltage, VINL 0.3 × VDRIVE V max
Input Current, IIN ±1 μA max Typically 10 nA, VIN = 0 V or VDRIVE
Input Capacitance, CIN3 10 pF max
AD7908/AD7918/AD7928
Rev. C | Page 8 of 28
Parameter B Version1 Unit Test Conditions/Comments
LOGIC OUTPUTS
Output High Voltage, VOH VDRIVE − 0.2 V min ISOURCE = 200 μA, AVDD = 2.7 V to 5.25 V
Output Low Voltage, VOL 0.4 V max ISINK = 200 μA
Floating-State Leakage Current ±1 μA max
Floating-State Output Capacitance3 10 pF max
Output Coding Straight (natural) binary Coding bit set to 1
Twos complement Coding bit set to 0
CONVERSION RATE
Conversion Time 800 ns max 16 SCLK cycles with SCLK at 20 MHz
Track-and-Hold Acquisition Time 300 ns max Sine wave input
300 ns max Full-scale step input
Throughput Rate 1 MSPS max See Serial Interface section
POWER REQUIREMENTS
AVDD 2.7/5.25 V min/max
VDRIVE 2.7/5.25 V min/max
IDD4 Digital inputs = 0 V or VDRIVE
Normal Mode (Static) 600 μA typ AVDD = 2.7 V to 5.25 V, SCLK on or off
Normal Mode (Operational) 2.7 mA max AVDD = 4.75 V to 5.25 V, fSCLK = 20 MHz
2 mA max AVDD = 2.7 V to 3.6 V, fSCLK = 20 MHz
Using Auto Shutdown Mode 960 μA typ fSAMPLE = 250 kSPS
0.5 μA max (Static)
Full Shutdown Mode 0.5 μA max SCLK on or off (20 nA typ)
Power Dissipation4
Normal Mode (Operational) 13.5 mW max AVDD = 5 V, fSCLK = 20 MHz
6 mW max AVDD = 3 V, fSCLK = 20 MHz
Auto Shutdown Mode (Static) 2.5 μW max AVDD = 5 V
1.5 μW max AVDD = 3 V
Full Shutdown Mode 2.5 μW max AVDD = 5 V
1.5 μW max AVDD = 3 V
1 Temperature ranges as follows: B Version: −40°C to +85°C.
2 See Terminology section.
3 Sample tested @ 25°C to ensure compliance.
4 See Power vs. Throughput Rate section.
AD7908/AD7918/AD7928
Rev. C | Page 9 of 28
TIMING SPECIFICATIONS
AVDD = 2.7 V to 5.25 V, VDRIVE ≤ AVDD, REFIN = 2.5 V, TA = TMIN to TMAX, unless otherwise noted.1
Table 4.
Limit at TMIN, TMAX AD7908/AD7918/AD7928
Parameter AVDD = 3 V AVDD = 5 V Unit Description
fSCLK2 10 10 kHz min
20 20 MHz max
tCONVERT 16 × tSCLK 16 × tSCLK
tQUIET 50 50 ns min Minimum quiet time required between CS rising edge and start of
next conversion
t2 10 10 ns min CS to SCLK setup time
t33 35 30 ns max Delay from CS until DOUT three-state disabled
t43 40 40 ns max Data access time after SCLK falling edge
t5 0.4 × tSCLK 0.4 × tSCLK ns min SCLK low pulse width
t6 0.4 × tSCLK 0.4 × tSCLK ns min SCLK high pulse width
t7 10 10 ns min SCLK to DOUT valid hold time
t84 15/45 15/35 ns min/max SCLK falling edge to DOUT high impedance
t9 10 10 ns min DIN setup time prior to SCLK falling edge
t10 5 5 ns min DIN hold time after SCLK falling edge
t11 20 20 ns min 16th SCLK falling edge to CS high
t12 1 1 μs max Power-up time from full power-down/auto shutdown mode
1 Sample tested at 25°C to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of AVDD) and timed from a voltage level of 1.6 V. See Figure 2.
The 3 V operating range spans from 2.7 V to 3.6 V. The 5 V operating range spans from 4.75 V to 5.25 V.
2 Mark/space ratio for the SCLK input is 40/60 to 60/40.
3 Measured with the load circuit of Figure 2 and defined as the time required for the output to cross 0.4 V or 0.7 × VDRIVE.
4 t8 is derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 2. The measured number is then extrapolated
back to remove the effects of charging or discharging the 50 pF capacitor. This means that the time, t8, quoted in the timing characteristics is the true bus relinquish
time of the part and is independent of the bus loading.
TO
OUTPUT
PIN
50pF
I
OH
I
OL
C
L
200µA
200µA
1.6V
0
3089-002
Figure 2. Load Circuit for Digital Output Timing Specifications
AD7908/AD7918/AD7928
Rev. C | Page 10 of 28
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 5.
Parameter Rating
AVDD to AGND −0.3 V to +7 V
VDRIVE to AGND −0.3 V to AVDD + 0.3 V
Analog Input Voltage to AGND −0.3 V to AVDD + 0.3 V
Digital Input Voltage to AGND −0.3 V to +7 V
Digital Output Voltage to AGND −0.3 V to AVDD + 0.3 V
REFIN to AGND −0.3 V to AVDD + 0.3 V
Input Current to Any Pin Except
Supplies1
±10 mA
Operating Temperature Range
Commercial (B Version) −40°C to +85°C
Storage Temperature Range −65°C to +150°C
Junction Temperature 150°C
TSSOP Package, Power Dissipation 450 mW
θJA Thermal Impedance 143°C/W (TSSOP)
θJC Thermal Impedance 45°C/W (TSSOP)
Lead Temperature, Soldering
Vapor Phase (60 sec) 215°C
Infrared (15 sec) 220°C
ESD 1.5 kV
1 Transient currents of up to 100 mA do not cause SCR latch-up.
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
AD7908/AD7918/AD7928
Rev. C | Page 11 of 28
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
1
AD7908/
AD7918/
AD7928
TOP VIEW
(Not to Scale)
SCLK
20
2
DIN
19
3
CS DOUT
18
4
AGND
AGND
AGND
AGND
17
5
AV
DD
AV
DD
V
DRIVE
REF
IN
16
615
714
813
9
V
IN
7
V
IN
6
V
IN
0
V
IN
1
V
IN
2
V
IN
3
V
IN
4
V
IN
5
12
10 11
03089-003
Figure 3. Pin Configuration
Table 6. Pin Function Descriptions
Pin No. Mnemonic Description
1 SCLK Serial Clock. Logic input. SCLK provides the serial clock for accessing data from the part. This clock input is also
used as the clock source for the conversion process of the AD7908/AD7918/AD7928.
2 DIN Data In. Logic input. Data to be written to the control register of the AD7908/AD7918/AD7928 is provided on
this input and is clocked into the register on the falling edge of SCLK (see the Control Register section).
3 CS Chip Select. Active low logic input. This input provides the dual function of initiating conversions on the
AD7908/AD7918/AD7928, and also frames the serial data transfer.
4, 8, 17, 20 AGND Analog Ground. This is the ground reference point for all analog circuitry on the AD7908/AD7918/AD7928. All
analog input signals and any external reference signal should be referred to this AGND voltage. All AGND pins
should be connected together.
5, 6 AVDD Analog Power Supply Input. The AVDD range for the AD7908/AD7918/AD7928 is from 2.7 V to 5.25 V. For the 0 V
to 2 × REFIN range, AVDD should be from 4.75 V to 5.25 V.
7 REFIN Reference Input for the AD7908/AD7918/AD7928. An external reference must be applied to this input. The
voltage range for the external reference is 2.5 V ± 1% for specified performance.
16 to 9 VIN0 to VIN7 Analog Input 0 through Analog Input 7. These are eight single-ended analog input channels that are
multiplexed into the on-chip track-and-hold. The analog input channel to be converted is selected by using
Address Bit ADD2 through Address Bit ADD0 of the control register. The address bits, in conjunction with the
SEQ and SHADOW bits, allow the sequencer to be programmed. The input range for all input channels can
extend from 0 V to REFIN or 0 V to 2 × REFIN as selected via the RANGE bit in the control register. Any unused
input channels must be connected to AGND to avoid noise pickup.
18 DOUT Data Out. Logic output. The conversion result from the AD7908/AD7918/AD7928 is provided on this output as
a serial data stream. The bits are clocked out on the falling edge of the SCLK input. The data stream from the
AD7908 consists of one leading zero, three address bits indicating which channel the conversion result
corresponds to, followed by the eight bits of conversion data, followed by four trailing zeros, provided MSB
first; the data stream from the AD7918 consists of one leading zero, three address bits indicating which
channel the conversion result corresponds to, followed by the 10 bits of conversion data, followed by two
trailing zeros, also provided MSB first; the data stream from the AD7928 consists of one leading zero, three
address bits indicating which channel the conversion result corresponds to, followed by the 12 bits of
conversion data, provided MSB first. The output coding can be selected as straight binary or twos complement
via the CODING bit in the control register.
19 VDRIVE Logic Power Supply Input. The voltage supplied at this pin determines at what voltage the serial interface of
the AD7908/AD7918/AD7928 operates.
AD7908/AD7918/AD7928
Rev. C | Page 12 of 28
TERMINOLOGY
Integral Nonlinearity Negative Gain Error Match
This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. The
endpoints of the transfer function are zero scale, a position 1 LSB
below the first code transition, and full scale, a position 1 LSB
above the last code transition.
This is the difference in negative gain error between any two
channels.
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of the level of
crosstalk between channels. It is measured by applying a full-
scale 400 kHz sine wave signal to all seven nonselected input
channels and determining how much that signal is attenuated
in the selected channel with a 50 kHz signal. The figure is given
worst case across all eight channels for the AD7908/AD7918/
AD7928.
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Offset Error
This is the deviation of the first code transition (00 . . . 000) to
(00 . . . 001) from the ideal, that is, AGND + 1 LSB. Power Supply Rejection (PSR)
Variations in power supply affect the full-scale transition, but
not the converter’s linearity. Power supply rejection is the
maximum change in full-scale transition point due to a change
in power-supply voltage from the nominal value (see the
Performance Curves section).
Offset Error Match
This is the difference in offset error between any two channels.
Gain Error
This is the deviation of the last code transition (111 . . . 110) to
(111 . . . 111) from the ideal (that is, REFIN – 1 LSB) after the
offset error has been adjusted out. Track-and-Hold Acquisition Time
The track-and-hold amplifier returns to track mode at the end
of conversion. Track-and-hold acquisition time is the time
required for the output of the track-and-hold amplifier to reach
its final value, within ±1 LSB, after the end of conversion.
Gain Error Match
This is the difference in gain error between any two channels.
Zero Code Error Signal-to-(Noise + Distortion) Ratio
This applies when using the twos complement output coding
option, in particular to the 2 × REFIN input range with −REFIN
to +REFIN biased about the REFIN point. It is the deviation of the
midscale transition (all 0s to all 1s) from the ideal VIN voltage,
that is, REFIN − 1 LSB.
This is the measured ratio of signal-to-(noise + distortion) at
the output of the ADC. The signal is the rms amplitude of the
fundamental. Noise is the sum of all nonfundamental signals up
to half the sampling frequency (fS/2), excluding dc. The ratio is
dependent on the number of quantization levels in the
digitization process; the more levels, the smaller the
quantization noise. The theoretical signal-to-(noise +
distortion) ratio for an ideal N-bit converter with a sine wave
input is given by
Zero Code Error Match
This is the difference in zero code error between any two
channels.
Positive Gain Error Signal-to-(Noise + Distortion) = (6.02N + 1.76)dB
This applies when using the twos complement output coding
option, in particular to the 2 × REFIN input range with −REFIN
to +REFIN biased about the REFIN point. It is the deviation of the
last code transition (011. . .110) to (011 . . . 111) from the ideal
(that is, +REFIN − 1 LSB) after the zero code error has been
adjusted out.
Thus for a 12-bit converter, this is 74 dB; for a 10-bit converter,
this is 62 dB; and for an 8-bit converter, this is 50 dB.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7908/AD7918/
AD7928, it is defined as:
Positive Gain Error Match
This is the difference in positive gain error between any two
channels.
()
1
2
6
2
5
2
4
2
3
2
2
log20 V
VVVVV
dBTHD
++++
=
Negative Gain Error
This applies when using the twos complement output coding
option, in particular to the 2 × REFIN input range with −REFIN
to +REFIN biased about the REFIN point. It is the deviation of the
first code transition (100 . . . 000) to (100 . . . 001) from the ideal
(that is, −REFIN + 1 LSB) after the zero code error has been
adjusted out.
where V1 is the rms amplitude of the fundamental and V2, V3,
V4, V5, and V6 are the rms amplitudes of the second through the
sixth harmonics.
AD7908/AD7918/AD7928
Rev. C | Page 13 of 28
TYPICAL PERFORMANCE CHARACTERISTICS
PERFORMANCE CURVES
Figure 4 shows a typical FFT plot for the AD7928 at 1 MSPS
sample rate and 50 kHz input frequency. Figure 5 shows the
signal-to-(noise + distortion) ratio performance vs. input
frequency for various supply voltages while sampling at 1 MSPS
with an SCLK of 20 MHz.
Figure 6 shows the power supply rejection ratio vs. supply ripple
frequency for the AD7928 when no decoupling is used. The
power supply rejection ratio is defined as the ratio of the power
in the ADC output at full-scale frequency f, to the power of a
200 mV p-p sine wave applied to the ADC AVDD supply of
frequency fS
PSRR(dB) = 10 log(Pf/Pfs)
Pf is equal to the power at frequency f in ADC output; PfS is
equal to the power at frequency fS coupled onto the ADC AVDD
supply. Here a 200 mV p-p sine wave is coupled onto the AVDD
supply.
0 50 100 150 200 250 300 350 400 450 500
SNR (dB)
FREQUENCY (kHz)
–10
–30
–50
–70
–90
–110
4096 POINT FFT
AVDD = 5V
fSAMPLE = 1MSPS
fIN = 50kHz
SINAD = 71.147dB
THD = –87.229dB
SFDR = –90.744dB
03089-004
Figure 4. AD7928 Dynamic Performance at 1 MSPS
75
70
65
60
5510 100 1000
SINAD (dB)
INPUT FREQUENCY (kHz)
f
SAMPLE
= 1MSPS
T
A
= 25°C
RANGE = 0V TO REF
IN
AV
DD
= V
DRIVE
= 2.70V
AV
DD
= V
DRIVE
= 3.60V
AV
DD
= V
DRIVE
= 4.75V
AV
DD
= V
DRIVE
= 5.25V
03089-005
Figure 5. AD7928 SINAD vs. Analog Input Frequency for
Various Supply Voltages at 1 MSPS
Figure 7 shows a graph of total harmonic distortion vs. analog
input frequency for various supply voltages, and Figure 8 shows a
graph of total harmonic distortion vs. analog input frequency
for various source impedances. See the Analog Input section.
Figure 9 and Figure 10 show typical INL and DNL plots for the
AD7928.
PSRR (dB)
SUPPLY RIPPLE FREQUENCY (kHz)
0 1000500 900800700600400300200100
–40
–60
–80
–90
–20
–50
–70
0
–10
–30
AV
DD
= 5V
200mV p-p SINEWAVE ON AV
DD
REF
IN
= 2.5V, 1µF CAPACITOR
T
A
= 25°C
03089-006
Figure 6. AD7928 PSRR vs. Supply Ripple Frequency
–
50
–65
–75
–85
–90
–60
–70
–80
–55
AV
DD
= V
DRIVE
= 4.75V
AV
DD
= V
DRIVE
= 3.60V
AV
DD
= V
DRIVE
= 2.70V
fSAMPLE
= 1MSPS
T
A
= 25°C
RANGE = 0V TO REF
IN
AV
DD
= V
DRIVE
= 5.25V
10 100 1000
THD (dB)
INPUT FREQUENCY (kHz)
03089-007
Figure 7. AD7928 THD vs. Analog Input Frequency for
Various Supply Voltages at 1 MSPS
AD7908/AD7918/AD7928
Rev. C | Page 14 of 28
f
SAMPLE = 1MSPS
TA= 25°C
RANGE = 0V TO REFIN
AVDD = 5.25V
RIN = 1000Ω
RIN = 50Ω
RIN = 100Ω
RIN = 10Ω
–
50
–60
–55
–70
–65
–80
–75
–85
–9010 100 1000
THD (dB)
INPUT FREQUENCY (kHz)
03089-008
Figure 8. AD7928 THD vs. Analog Input Frequency for
Various Source Impedances
1.0
0.6
0.8
0.2
0.4
0
–0.2
–0.4
–0.6
–0.8
–1.0 0 1024 2048 30721536512 2560 3584 4096
INL ERROR (LSB)
CODE
AV
DD
= V
DRIVE
= 5V
TEMPERATURE = 25°C
03089-009
Figure 9. AD7928 Typical INL
0
1.0
0.6
0.8
0.2
0.4
–0.2
–0.4
–0.6
–0.8
–1.0 0 1024 2048 30721536512 2560 3584 4096
DNL ERROR (LSB)
CODE
AV
DD
= V
DRIVE
= 5V
TEMPERATURE = 25°C
03089-010
Figure 10. AD7928 Typical DNL
AD7908/AD7918/AD7928
Rev. C | Page 15 of 28
CONTROL REGISTER
The control register on the AD7908/AD7918/AD7928 is a 12-bit, write-only register. Data is loaded from the DIN pin of the
AD7908/AD7918/AD7928 on the falling edge of SCLK. The data is transferred on the DIN line at the same time that the conversion
result is read from the part. The data transferred on the DIN line corresponds to the AD7908/AD7918/AD7928 configuration for the next
conversion. This requires 16 serial clocks for every data transfer. Only the information provided on the first 12 falling clock edges (after
CS falling edge) is loaded to the control register. MSB denotes the first bit in the data stream. The bit functions are outlined in . Tabl e 7
MSB LSB
WRITE SEQ DON’TCARE ADD2 ADD1 ADD0 PM1 PM0 SHADOW DON’TCARE RANGE CODING
Table 7. Control Register Bit Functions
Bit Mnemonic Comment
11 WRITE The value written to this bit of the control register determines whether or not the following 11 bits are loaded to the
control register. If this bit is a 1, the following 11 bits are written to the control register; if it is a 0, the remaining 11 bits
are not loaded to the control register, and it remains unchanged.
10 SEQ The SEQ bit in the control register is used in conjunction with the SHADOW bit to control the use of the sequencer
function and access the SHADOW register (see the SHADOW register bit map).
9 DON’TCARE
8 to 6 ADD2 to
ADD0
These three address bits are loaded at the end of the present conversion sequence and select which analog input
channel is to be converted in the next serial transfer, or they can select the final channel in a consecutive sequence as
described in Table 10. The selected input channel is decoded as shown in Table 8. The address bits corresponding to
the conversion result are also output on DOUT prior to the 12 bits of data, see the Serial Interface section. The next
channel to be converted on is selected by the mux on the 14th SCLK falling edge.
5, 4 PM1, PM0 Power Management Bits. These two bits decode the mode of operation of the AD7908/AD7918/AD7928 as shown in
Table 9.
3 SHADOW The SHADOW bit in the control register is used in conjunction with the SEQ bit to control the use of the sequencer
function and access the SHADOW register (see Table 10).
2 DON’TCARE
1 RANGE This bit selects the analog input range to be used on the AD7908/AD7918/AD7928. If it is set to 0, the analog input
range extends from 0 V to 2 × REFIN. If it is set to 1, the analog input range extends from 0 V to REFIN (for the next
conversion). For 0 V to 2 × REFIN, AVDD = 4.75 V to 5.25 V.
0 CODING This bit selects the type of output coding the AD7908/AD7918/AD7928 uses for the conversion result. If this bit is set to
0, the output coding for the part is twos complement. If this bit is set to 1, the output coding from the part is straight
binary (for the next conversion).
Table 8. Channel Selection
ADD2 ADD1 ADD0 Analog Input Channel
0 0 0 VIN0
0 0 1 VIN1
0 1 0 VIN2
0 1 1 VIN3
1 0 0 VIN4
1 0 1 VIN5
1 1 0 VIN6
1 1 1 VIN7
AD7908/AD7918/AD7928
Rev. C | Page 16 of 28
Table 9. Power Mode Selection
PM1 PM0 Mode
1 1 Normal Operation. In this mode, the AD7908/AD7918/AD7928 remain in full power mode regardless of the status of any of
the logic inputs. This mode allows the fastest possible throughput rate from the AD7908/AD7918/AD7928.
1 0 Full Shutdown. In this mode, the AD7908/ AD7918/AD7928 is in full shutdown mode with all circuitry powering down. The
AD7908/AD7918/AD7928 retains the information in the control register while in full shutdown. The part remains in full
shutdown until these bits are changed.
0 1 Auto Shutdown. In this mode, the AD7908/AD7918/AD7928 automatically enters full shutdown mode at the end of each
conversion when the control register is updated. Wake-up time from full shutdown is 1 μs and the user should ensure that 1 μs
has elapsed before attempting to perform a valid conversion on the part in this mode.
0 0 Invalid Selection. This configuration is not allowed.
SEQUENCER OPERATION
The configuration of the SEQ and SHADOW bits in the control register allows the user to select a particular mode of operation of the
sequencer function. Tabl e 10 outlines the four modes of operation of the sequencer.
Table 10. Sequence Selection
SEQ SHADOW Sequence Type
0 0 This configuration means that the sequence function is not used. The analog input channel selected for each individual
conversion is determined by the contents of the ADD0 through ADD2 channel address bits in each prior write
operation. This mode of operation reflects the traditional operation of a multichannel ADC, without the sequencer
function being used, where each write to the AD7908/AD7918/AD7928 selects the next channel for conversion (see
Figure 11).
0 1 This configuration selects the SHADOW register for programming. The following write operation loads the contents of
the SHADOW register. This programs the sequence of channels to be converted on continuously with each successive
valid CS falling edge (see the section, SHADOW register bit map, and ). The channels
selected need not be consecutive.
SHADOW Register Figure 12
1 0 If the SEQ and SHADOW bits are set in this way, then the sequence functions are not interrupted upon completion of
the write operation. This allows other bits in the control register to be altered between conversions while in a sequence,
without terminating the cycle.
1 1 This configuration is used in conjunction with the ADD2 to ADD0 channel address bits to program continuous
conversions on a consecutive sequence of channels from Channel 0 to a selected final channel as determined by the
channel address bits in the control register (see Figure 13).
AD7908/AD7918/AD7928
Rev. C | Page 17 of 28
SHADOW REGISTER
MSB LSB
VIN0 VIN1 VIN2 VIN3 VIN4 VIN5 VIN6 VIN7 VIN0 VIN1 VIN2 VIN3 VIN4 VIN5 VIN6 VIN7
Sequence One Sequence Two
The SHADOW register on the AD7908/AD7918/AD7928 is a
16-bit, write-only register. Data is loaded from the DIN pin of
the AD7908/AD7918/AD7928 on the falling edge of SCLK. The
data is transferred on the DIN line at the same time that a
conversion result is read from the part. This requires 16 serial
clock falling edges for the data transfer. The information is
clocked into the SHADOW register, provided that the SEQ and
SHADOW bits were set to 0,1, respectively, in the previous
write to the control register. MSB denotes the first bit in the
data stream. Each bit represents an analog input from Channel 0
to Channel 7. Through programming the SHADOW register,
two sequences of channels can be selected, through which the
AD7908/AD7918/AD7928 cycle with each consecutive
conversion after the write to the SHADOW register.
Sequence One is performed first and then Sequence Two. If the
user does not wish to perform a second sequence option, then
all 0s must be written to the last 8 LSBs of the SHADOW
register. To select a sequence of channels, the associated channel
bit must be set for each analog input. The AD7908/AD7918/
AD7928 continuously cycle through the selected channels in
ascending order beginning with the lowest channel, until a
write operation occurs (that is, the WRITE bit is set to 1) with
the SEQ and SHADOW bits configured in any way except 1, 0,
(see Tabl e 10). The bit functions are outlined in the SHADOW
register bit map.
Figure 11 reflects the traditional operation of a multichannel
ADC, where each serial transfer selects the next channel for
conversion. In this mode of operation the sequencer function is
not used.
Figure 12 shows how to program the AD7908/AD7918/AD7928
to continuously convert on a particular sequence of channels.
To exit this mode of operation and revert back to the traditional
mode of operation of a multichannel ADC (as outlined in
Figure 11), ensure that the WRITE bit = 1 and the SEQ =
SHADOW = 0 on the next serial transfer. Figure 13 shows how
a sequence of consecutive channels can be converted on without
having to program the SHADOW register or write to the part
on each serial transfer. Again, to exit this mode of operation and
revert back to the traditional mode of operation of a multichannel
ADC (as outlined in Figure 11), ensure the WRITE bit = 1 and
the SEQ = SHADOW = 0 on the next serial transfer.
POWER-ON
DUMMY CONVERSION
DIN = ALL 1s
WRITE BIT = 1,
SEQ = SHADOW = 0
CS
DOUT: CONVERSION RESULT FROM
PREVIOUSLY SELECTED
CHANNEL A2 TO A0.
DIN: WRITE TO CONTROL REGISTER,
WRITE BIT = 1, SELECT CODING, RANGE,
AND POWER MODE.
SELECT A2 TO A0 FOR CONVERSION.
SEQ = SHADOW = 0
DIN: WRITE TO CONTROL REGISTER,
WRITE BIT = 1, SELECT CODING, RANGE,
AND POWER MODE.
SELECT A2 TO A0 FOR CONVERSION.
SEQ = SHADOW = 0
CS
0
3089-011
Figure 11. SEQ Bit = 0, SHADOW Bit = 0 Flowchart
POWER-ON
DUMMY CONVERSION
DIN = ALL 1s
CS
DOUT: CONVERSION RESULT FROM
PREVIOUSLY SELECTED CHANNEL A2
TO A0.
DIN: WRITE TO SHADOW REGISTER,
SELECTING WHICH CHANNELS TO
CONVERT ON; CHANNELS SELECTED
NEED NOT BE CONSECUTIVE
CHANNELS
DIN: WRITE TO CONTROL REGISTER,
WRITE BIT = 1, SELECT CODING, RANGE,
AND POWER MODE.
SELECT CHANNEL A2 TO A0
FOR CONVERSION.
SEQ = 0 SHADOW = 1
CS
CS
WRITE BIT = 1
SEQ = 1, SHADOW = 0
WRITE BIT = 1
SEQ = 1, SHADOW = 0
CONTINUOUSLY
CONVERTS ON
THE SELECTED
SEQUENCE OF
CHANNELS
WRITE BIT = 0
WRITE BIT = 0
WRITE BIT = 0
CONTINUOUSLY
CONVERTS ON THE
SELECTED SEQUENCE
OF CHANNELS BUT WILL
ALLOW RANGE, CODING,
AND SO ON, TO CHANGE
IN THE CONTROL
REGISTER WITHOUT
INTERRUPTING THE
SEQUENCE, PROVIDED
SEQ = 1 SHADOW = 0
03089-012
Figure 12. SEQ Bit = 0, SHADOW Bit = 1 Flowchart
AD7908/AD7918/AD7928
Rev. C | Page 18 of 28
POWER-ON
DUMMY CONVERSION
DIN = ALL 1s
CS
DOUT: CONVERSION RESULT FROM
CHANNEL 0.
CONTINUOUSLY CONVERTS ON A
CONSECUTIVE SEQUENCE OF
CHANNELS FROM CHANNEL 0 UP TO,
AND INCLUDING, THE PREVIOUSLY
SELECTED A2 TO A0 IN THE CONTROL
REGISTER.
CONTINUOUSLY CONVERTS ON THE
SELECTED SEQUENCE OF CHANNELS
BUT WILL ALLOW RANGE, CODING, AND
SO ON, TO CHANGE IN THE CONTROL
REGISTER WITHOUT INTERRUPTING
THE SEQUENCE, PROVIDED SEQ = 1
SHADOW = 0
DIN: WRITE TO CONTROL REGISTER,
WRITE BIT = 1, SELECT CODING, RANGE,
AND POWER MODE.
SELECT CHANNEL A2 TO A0
FOR CONVERSION.
SEQ = 1 SHADOW = 1
CS
WRITE BIT = 1
SEQ = 1, SHADOW = 0
WRITE BIT = 0
CS
03089-013
Figure 13. SEQ Bit = 1, SHADOW Bit = 1 Flowchart
CIRCUIT INFORMATION
The AD7908/AD7918/AD7928 are high speed, 8-channel, 8-bit,
10-bit, and 12-bit, single-supply ADCs, respectively. The parts
can be operated from a 2.7 V to 5.25 V supply. When operated
from either a 5 V or 3 V supply, the AD7908/AD7918/AD7928
are capable of throughput rates of 1 MSPS when provided with
a 20 MHz clock.
The AD7908/AD7918/AD7928 provide the user with an on-
chip, track-and-hold ADC, and a serial interface housed in a
20-lead TSSOP package. The AD7908/AD7918/ AD7928 each
have eight single-ended input channels with a channel
sequencer, allowing the user to select a channel sequence that
the ADC can cycle through with each consecutive CS falling
edge. The serial clock input accesses data from the part, controls
the transfer of data written to the ADC, and provides the clock
source for the successive approximation ADC. The analog input
range for the AD7908/AD7918/ AD7928 is 0 V to REFIN or 0 V
to 2 × REFIN, depending on the status of Bit 1 in the control
register. For the 0 to 2 × REFIN range, the part must be operated
from a 4.75 V to 5.25 V supply.
The AD7908/AD7918/AD7928 provide flexible power
management options to allow the user to achieve the best power
performance for a given throughput rate. These options are
selected by programming the PM1 and PM0 power
management bits in the control register.
CONVERTER OPERATION
The AD7908/AD7918/AD7928 are 8-, 10-, and 12-bit
successive approximation analog-to-digital converters based
around a capacitive DAC, respectively. The AD7908/AD7918/
AD7928 can convert analog input signals in the range 0 V to
REFIN or 0 V to 2 × REFIN. Figure 14 and Figure 15 show
simplified schematics of the ADC. The ADC is comprised of
control logic, SAR, and a capacitive DAC, which are used to add
and subtract fixed amounts of charge from the sampling
capacitor to bring the comparator back into a balanced
condition. Figure 14 shows the ADC during its acquisition
phase. SW2 is closed and SW1 is in Position A. The comparator
is held in a balanced condition and the sampling capacitor
acquires the signal on the selected VIN channel.
V
IN
7
V
IN
0
AGND
A
B
SW1
SW2
COMPARATOR
CONTROL
LOGIC
4kΩ
CAPACITIVE DAC
03089-014
Figure 14. ADC Acquisition Phase
When the ADC starts a conversion (see Figure 15), SW2 opens
and SW1 moves to Position B, causing the comparator to
become unbalanced. The control logic and the capacitive DAC
are used to add and subtract fixed amounts of charge from the
sampling capacitor to bring the comparator back into a
balanced condition. When the comparator is rebalanced, the
conversion is complete. The control logic generates the ADC
output code. Figure 17 and Figure 18 show the ADC transfer
functions.
V
IN
7
V
IN
0
AGND
A
B
SW1
SW2
COMPARATOR
CONTROL
LOGIC
4kΩ
CAPACITIVE DACCAPACITIVE DAC
03089-015
Figure 15. ADC Conversion Phase
Analog Input
Figure 16 shows an equivalent circuit of the analog input
structure of the AD7908/AD7918/AD7928. The two diodes (D1
and D2) provide ESD protection for the analog inputs. Care
must be taken to ensure that the analog input signal never
exceeds the supply rails by more than 300 mV. This causes these
diodes to become forward biased and start conducting current
into the substrate. 10 mA is the maximum current these diodes
can conduct without causing irreversible damage to the part.
The Capacitor C1 in Figure 16 is typically about 4 pF and can
primarily be attributed to pin capacitance. The Resistor R1 is a
lumped component made up of the on resistance of the track-
and-hold switch and also includes the on resistance of the input
multiplexer. The total resistance is typically about 400 Ω. The
Capacitor C2 is the ADC sampling capacitor and has a
capacitance of 30 pF typically. For ac applications, removing
high frequency components from the analog input signal is
recommended by use of an RC lowpass filter on the relevant
analog input pin. In applications where harmonic distortion
AD7908/AD7918/AD7928
Rev. C | Page 19 of 28
ADC CODE
100…000
011…111
100…001
100…010
011…110
•
•
000…001
111…111
•
•
000…000
ANALOG INPUT
1LSB = 2 × V
REF
/256 AD7908
1LSB = 2 × V
REF
/4096 AD7928
1LSB = 2 × V
REF
/1024 AD7918
+V
REF
– 1 LSB
V
REF
– 1 LSB
–V
REF
+ 1 LSB
03089-018
and signal-to-noise ratio are critical, the analog input should be
driven from a low impedance source. Large source impedances
significantly affect the ac performance of the ADC. This can
necessitate the use of an input buffer amplifier. The choice of
the op amp is a function of the particular application.
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance depends on the amount of total harmonic
distortion (THD) that can be tolerated. The THD increases as
the source impedance increases, and performance degrades (see
Figure 8).
Figure 18. Twos Complement Transfer Characteristic
with REFIN ± REFIN Input Range
V
IN
C1
4pF
C2
30pF
R1
D1
D2
AV
DD
CONVERSION PHASE: SWITCH OPEN
TRACK PHASE: SWITCH CLOSED
03089-016
HANDLING BIPOLAR INPUT SIGNALS
Figure 19 shows how useful the combination of the 2 × REFIN
input range and the twos complement output coding scheme is
for handling bipolar input signals. If the bipolar input signal is
biased about REFIN and twos complement output coding is
selected, then REFIN becomes the zero code point, −REFIN is
negative full scale and +REFIN becomes positive full scale, with
a dynamic range of 2 × REFIN.
Figure 16. Equivalent Analog Input Circuit
ADC TRANSFER FUNCTION
TYPICAL CONNECTION DIAGRAM
The output coding of the AD7908/AD7918/AD7928 is either
straight binary or twos complement, depending on the status of
the LSB in the control register. The designed code transitions
occur at successive LSB values (that is, 1 LSB, 2 LSBs, and so
on). The LSB size is REFIN/256 for the AD7908, REFIN/1024 for
the AD7918, and REFIN/4096 for the AD7928. The ideal transfer
characteristic for the AD7908/AD7918/AD7928 when straight
binary coding is selected is shown in Figure 17, and the ideal
transfer characteristic for the AD7908/AD7918/AD7928 when
twos complement coding is selected is shown in Figure 18.
Figure 20 shows a typical connection diagram for the
AD7908/AD7918/AD7928. In this setup, the AGND pin is
connected to the analog ground plane of the system. In Figure 20,
REFIN is connected to a decoupled 2.5 V supply from a reference
source, the AD780, to provide an analog input range of 0 V to
2.5 V (if RANGE bit is 1) or 0 V to 5 V (if RANGE bit is 0).
Although the AD7908/AD7918/AD7928 is connected to a VDD
of 5 V, the serial interface is connected to a 3 V microprocessor.
The VDRIVE pin of the AD7908/AD7918/ AD7928 is connected
to the same 3 V supply of the microprocessor to allow a 3 V
logic interface (see the Digital Inputs section). The conversion
result is output in a 16-bit word. This 16-bit data stream
consists of a leading zero, three address bits indicating which
channel the conversion result corresponds to, followed by the 12
bits of conversion data for the AD7928 (10 bits of data for the
AD7918 and 8 bits of data for the AD7908, each followed by
two and four trailing zeros, respectively). For applications
where power consumption is of concern, the power-down
modes should be used between conversions or bursts of several
conversions to improve power performance (see the Modes of
Operation section).
000…000
0V
ANALOG INPUT
111…111
000…001
000…010
111…110
•
•
111…000
•
011…111
•
•
1 LSB
NOTE
1. V
REF
IS EITHER REF
IN
OR 2 × REF
IN.
1LSB = V
REF
/256 AD7908
1LSB = V
REF
/4096 AD7928
1LSB = V
REF
/1024 AD7918
+V
REF
– 1 LSB
ADC CODE
03089-017
Figure 17. Straight Binary Transfer Characteristic
AD7908/AD7918/AD7928
Rev. C | Page 20 of 28
R3
R2
R4
REF
IN
+REF
IN
(= 2 × REF
IN)
REF
IN
–REF
IN
V
DRIVE
AD7908/
AD7918/
AD7928
DSP/µP
V
DD
0.1µF
V
AV
DD
V
DD
DOUT
TWOS COMPLEMENT
011…111
000…000
100…000
(= 0V)
0
V
V
R1
R1 = R2 = R3 = R4
V
REF
V
IN
0
V
IN
7
•
•
03089-019
Figure 19. Handling Bipolar Signals
5
V
SUPPLY
AV
DD
V
IN
0
V
IN
7
AD7908/
AD7918/
AD7928
3V
SUPPLY
AGND
•
•
0V TO REF
IN
SCLK
DOUT
CS
DIN
V
DRIVE
NOTE
1. ALL UNUSED INPUT CHANNELS SHOULD BE CONNECTED TO AGND.
2.5V
AD780
REF
IN
SERIAL
INTERFACE
µC/µP
0.1µF 10µF
0.1µF 0.1µF 10µF
03089-020
Figure 20. Typical Connection Diagram
Analog Input Selection
Any one of eight analog input channels can be selected for
conversion by programming the multiplexer with the Address
Bit ADD2 to Address Bit ADD0 in the control register. The
channel configurations are shown in Table 8. The AD7908/
AD7918/AD7928 can also be configured to automatically cycle
through a number of channels as selected. The sequencer
feature is accessed via the SEQ and SHADOW bits in the
control register (see Table 10).
The AD7908/AD7918/AD7928 can be programmed to
continuously convert on a selection of channels in ascending
order. The analog input channels to be converted on are
selected through programming the relevant bits in the
SHADOW register (see the SHADOW Register section). The
next serial transfer then acts on the sequence programmed by
executing a conversion on the lowest channel in the selection.
The next serial transfer results in a conversion on the next
highest channel in the sequence, and so on.
It is not necessary to write to the control register once a
sequencer operation has been initiated. The WRITE bit must be
set to zero or the DIN line tied low to ensure the control register
is not accidentally overwritten, or the sequence operation
interrupted. If the control register is written to at any time
during the sequence, then it must be ensured that the SEQ and
SHADOW bits are set to 1, 0 to avoid interrupting the
automatic conversion sequence. This pattern continues until
such time as the AD7908/AD7918/AD7928 is written to and the
SEQ and SHADOW bits are configured with any bit combination
except 1, 0. On completion of the sequence, the AD7908/
AD7918/AD7928 sequencer returns to the first selected channel
in the SHADOW register and commence the sequence again.
Rather than selecting a particular sequence of channels, a
number of consecutive channels beginning with Channel 0 can
also be programmed via the control register alone, without
needing to write to the SHADOW register. This is possible if the
SEQ and SHADOW bits are set to 1,1. The channel address bits
ADD2 through ADD0 then determine the final channel in the
consecutive sequence. The next conversion is on Channel 0,
then Channel 1, and so on until the channel selected via the
address bits ADD2 through ADD0 is reached. The cycle begins
again on the next serial transfer, provided the WRITE bit is set
to low, or if high, that the SEQ and SHADOW bits are set to
1, 0; then the ADC continues its preprogrammed automatic
sequence uninterrupted.
Regardless of which channel selection method is used, the 16-bit
word output from the AD7928 during each conversion always
contains a leading zero, three channel address bits that the
conversion result corresponds to, followed by the 12-bit
conversion result. The AD7918 outputs a leading zero, three
channel address bits that the conversion result corresponds to,
followed by the 10-bit conversion result and two trailing zeros;
the AD7908 outputs a leading zero, three channel address bits
that the conversion result corresponds to, followed by the 8-bit
conversion result and four trailing zeros. (See the Serial
Interface section.)
Digital Inputs
The digital inputs applied to the AD7908/AD7918/AD7928 are
not limited by the maximum ratings that limit the analog
AD7908/AD7918/AD7928
Rev. C | Page 21 of 28
inputs. Instead, the digital inputs applied can go to 7 V and are
not restricted by the AVDD + 0.3 V limit as on the analog inputs.
Another advantage of SCLK, DIN, and CS not being restricted
by the AVDD + 0.3 V limit is the fact that power supply
sequencing issues are avoided. If CS, DIN, or SCLK are applied
before AVDD, there is no risk of latch-up as there would be on
the analog inputs if a signal greater than 0.3 V was applied prior
to AVDD.
VDRIVE
The AD7908/AD7918/AD7928 also have the VDRIVE feature.
VDRIVE controls the voltage at which the serial interface operates.
VDRIVE allows the ADC to easily interface to both 3 V and 5 V
processors. For example, if the AD7908/AD7918/AD7928 were
operated with an AVDD of 5 V, the VDRIVE pin could be powered
from a 3 V supply. The AD7908/AD7918/AD7928 have better
dynamic performance with an AVDD of 5 V while still being able
to interface to 3 V processors. Care should be taken to ensure
VDRIVE does not exceed AVDD by more than 0.3 V. See the
Absolute Maximum Ratings section.
Reference
An external reference source should be used to supply the 2.5 V
reference to the AD7908/AD7918/AD7928. Errors in the
reference source results in gain errors in the AD7908/
AD7918/AD7928 transfer function and adds to the specified
full-scale errors of the part. A capacitor of at least 0.1 μF should
be placed on the REFIN pin. Suitable reference sources for the
AD7908/AD7918/AD7928 include the AD780, REF192,
AD1582, ADR03, ADR381, ADR391, and ADR421.
If 2.5 V is applied to the REFIN pin, the analog input range can
either be 0 V to 2.5 V or 0 V to 5 V, depending on the setting of
the RANGE bit in the control register.
MODES OF OPERATION
The AD7908/AD7918/AD7928 have a number of different
modes of operation. These modes are designed to provide
flexible power management options. These options can be
chosen to optimize the power dissipation/throughput rate ratio
for differing application requirements. The mode of operation
of the AD7908/AD7918/AD7928 is controlled by the power
management bits, PM1 and PM0, in the control register, as
detailed in Table 9. When power supplies are first applied to the
AD7908/AD7918/AD7928, care should be taken to ensure that
the part is placed in the required mode of operation (see
Powering Up the AD7908/AD7918/AD7928 section).
Normal Mode (PM1 = PM0 = 1)
This mode is intended for the fastest throughput rate
performance, as the user does not have to worry about any
power-up times with the AD7908/AD7918/AD7928 remaining
fully powered at all times. Figure 21 shows the general diagram
of the operation of the AD7908/AD7918/AD7928 in this mode.
The conversion is initiated on the falling edge of CS and the
track-and-hold enters hold mode as described in the
section. The data presented to the AD7908/AD7918/
AD7928 on the DIN line during the first 12 clock cycles of the
data transfer are loaded into the control register (provided
WRITE bit is set to 1). If data is to be written to the SHADOW
register (SEQ = 0, SHADOW = 1 on previous write), data
presented on the DIN line during the first 16 SCLK cycles is
loaded into the SHADOW register. The part remains fully
powered up in normal mode at the end of the conversion as
long as PM1 and PM0 are both loaded with 1 on every data
transfer.
Serial
Interface
Sixteen serial clock cycles are required to complete the
conversion and access the conversion result. The track-and-
hold goes back into track on the 14th SCLK falling edge. CS can
then idle high until the next conversion or can idle low until
sometime prior to the next conversion, effectively idling CS low.
Once a data transfer is complete (DOUT has returned to three-
state), another conversion can be initiated after the quiet time,
tQUIET, has elapsed by bringing CS low again.
112
CS
SCLK
DOUT
DIN
16
1 LEADING ZERO + 3 CHANNEL
IDENTIFIER BITS + CONVERSION RESULT
NOTES
1. CONTROL REGISTER DATA IS LOADED ON FIRST 12 SCLK CYCLES.
2. SHADOW REGISTER DATA IS LOADED ON FIRST 16 SCLK CYCLES.
DATA IN TO CONTROL/SHADOW REGISTER
03089-021
Figure 21. Normal Mode Operation
Full Shutdown Mode (PM1 = 1, PM0 = 0)
In this mode, all internal circuitry on the AD7908/AD7918/
AD7928 is powered down. The part retains information in the
control register during full shutdown. The AD7908/AD7918/
AD7928 remains in full shutdown until the power management
bits in the control register, PM1 and PM0, are changed.
If a write to the control register occurs while the part is in full
shutdown, with the power management bits changed to PM0 =
PM1 = 1, normal mode, the part begins to power up on the CS
rising edge. The track-and-hold that was in hold while the part was
in full shutdown returns to track on the 14th SCLK falling edge.
To ensure that the part is fully powered up, tPOWER UP, should
have elapsed before the next CS falling edge. shows
the general diagram for this sequence.
Figure 22
Auto Shutdown Mode (PM1 = 0, PM0 = 1)
In this mode, the AD7908/AD7918/AD7928 automatically
enters shutdown at the end of each conversion when the control
register is updated. When the part is in shutdown, the track and
hold is in hold mode. Figure 23 shows the general diagram of
AD7908/AD7918/AD7928
Rev. C | Page 22 of 28
the operation of the AD7908/AD7918/AD7928 in this mode. In
shutdown mode, all internal circuitry on the AD7908/AD7918/
AD7928 is powered down. The part retains information in the
control register during shutdown. The AD7908/AD7918/
AD7928 remains in shutdown until the next CS falling edge it
receives. On this CS falling edge, the track-and-hold that was in
hold while the part was in shutdown returns to track. Wakeup
time from auto shutdown is 1 μs, and the user should ensure
that 1 μs has elapsed before attempting a valid conversion.
When running the AD7908/AD7918/AD7928 with a 20 MHz
clock, one dummy cycle should be sufficient to ensure the part
is fully powered up. During this dummy cycle the contents of
the control register should remain unchanged; therefore the
WRITE bit should be 0 on the DIN line. This dummy cycle
effectively halves the throughput rate of the part, with every
other conversion result being valid. In this mode, the power
consumption of the part is greatly reduced with the part
entering shutdown at the end of each conversion. When the
control register is programmed to move into auto shutdown, it
does so at the end of the conversion. The user can move the
ADC in and out of the low power state by controlling the CS
signal.
Powering Up the AD7908/AD7918/AD7928
When supplies are first applied to the AD7908/AD7918/
AD7928, the ADC can power up in any of the operating modes
of the part. To ensure the part is placed into the required
operating mode, the user should perform a dummy cycle
operation as outlined in Figure 24.
CS
SCLK
DOUT
DIN
1
1614
1
PART IS IN
FULL SHUTDOWN
PA R T B E G I N
S
TO P
O
WER UP ON
CS RISING EDGE AS PM1 = PM0 = 1
THE P
A
RT IS FULL
Y
P
O
WERED UP
ONCE
t
POWER UP
HAS ELAPSED
CONTROL REGISTER IS LOADED ON THE FIRST 12 CLOCKS.
PM1 = 1, PM0 = 1
TO KEEP THE PART IN NORMAL MODE,
LOAD PM1 = PM0 = 1 IN CONTROL REGISTER
CHANNEL IDENTIFIER BITS + CONVERSION RESULT
DATA IN TO CONTROL REGISTER DATA IN TO CONTROL/SHADOW REGISTER
1614
t
12
0
3089-022
Figure 22. Full Shutdown Mode Operation
1
CS
SCLK
DOUT
DIN
16 1611 16
DUMMY CONVERSION
CHANNEL IDENTIFIER
BITS + CONVERSION RESULT
CHANNEL IDENTIFIER
BITS + CONVERSION RESULT
INVALID DATA
DATA IN TO CONTROL/SHADOW REGISTER
DATA IN TO CONTROL/SHADOW REGISTER
TO KEEP PART IN THIS MODE, LOAD PM1 = 0,
PM0 = 1 IN CONTROL REGISTER OR SET WRITE BIT = 0
PART IS FULLY
POWERED UP
1212
12
PART ENTERS SHUTDOWN ON CS
RISING EDGE AS PM1 = 0, PM0 = 1
PART BEGINS TO POWER
UP ON CS FALLING EDGE
PART ENTERS SHUTDOWN
ON CS RISING EDGE
AS PM1 = 0, PM0 = 1
CONTROL REGISTER IS LOADED ON THE
FIRST 12 CLOCKS, PM1 = 0, PM0 = 1 CONTROL REGISTER CONTENTS SHOULD
NOT CHANGE. WRITE BIT = 0
03089-023
Figure 23. Auto Shutdown Mode Operation
AD7908/AD7918/AD7928
Rev. C | Page 23 of 28
1
CS
SCLK
DOUT
DIN
16 116116
DUMMY CONVERSION
INVALID DATA
KEEP DIN LINE TIED HIGH FOR FIRST TWO DUMMY CONVERSIONS CONTROL REGISTER IS LOADED ON THE FIRST
12 CLOCK EDGES
121212
DUMMY CONVERSION
INVALID DATA INVALID DATA
DATA IN TO CONTROL REGISTER
CORRECT VALUE IN CONTROL
REGISTER, VALID DATA FROM
NEXT CONVERSION, USER CAN
WRITE TO SHADOW REGISTER
IN NEXT CONVERSION
0
3089-024
Figure 24. Placing AD7928 into the Required Operating Mode After Supplies are Applied
POWER VS. THROUGHPUT RATE
By operating in auto shutdown mode on the AD7908/AD7918/
AD7928, the average power consumption of the ADC decreases
at lower throughput rates. Figure 25 shows how as the
throughput rate is reduced, the part remains in its shutdown
state longer and the average power consumption over time
drops accordingly.
For example, if the AD7928 is operated in a continuous
sampling mode with a throughput rate of 100 kSPS and an
SCLK of 20 MHz (AVDD = 5 V), and the device is placed in auto
shutdown mode, that is, if PM1 = 0 and PM0 = 1, then the
power consumption is calculated as follows:
The maximum power dissipation during normal operation is
13.5 mW (AVDD = 5 V). If the power-up time from auto
shutdown is one dummy cycle, that is, 1 μs, and the remaining
conversion time is another cycle, that is, 1 μs, then the AD7928
can be said to dissipate 13.5 mW for 2 μs during each
conversion cycle. For the remainder of the conversion cycle,
8 μs, the part remains in auto shutdown mode. The AD7928 can
be said to dissipate 2.5 μW for the remaining 8 μs of the
conversion cycle. If the throughput rate is 100 kSPS, the cycle
time is 10 μs and the average power dissipated during each cycle is
(2/10) × (13.5 mW) + (8 / 10) × (2.5 μW) = 2.702 mW
Figure 25 shows the maximum power vs. throughput rate when
using the auto shutdown mode with 3 V and 5 V supplies.
AV
DD
= 5V AV
DD
= 3V
10
1
0.1
0.01 0 50 100 150 200 250 300 350
POWER (mW)
THROUGHPUT (kSPS)
03089-025
Figure 25. AD7928 Power vs. Throughput Rate
SERIAL INTERFACE
Figure 26, Figure 27, and Figure 28 show the detailed timing
diagrams for serial interfacing to the AD7908, AD7918, and
AD7928, respectively. The serial clock provides the conversion
clock and also controls the transfer of information to and from
the AD7908/AD7918/AD7928 during each conversion.
The CS signal initiates the data transfer and conversion process.
The falling edge of CS puts the track-and-hold into hold mode,
takes the bus out of three-state; the analog input is sampled at
this point. The conversion is also initiated at this point and
requires 16 SCLK cycles to complete. The track-and-hold goes
back into track on the 14th SCLK falling edge as shown in
, , and at Point B, except when the
write is to the SHADOW register, in which case the track-and-
hold does not return to track until the rising edge of
Figure 26 Figure 27 Figure 28
CS, that is,
Point C in . On the 16th SCLK falling edge, the DOUT
line goes back into three-state. If the rising edge of
Figure 29
CS occurs
before 16 SCLKs have elapsed, the conversion is terminated, the
DOUT line goes back into three-state, and the control register is
not updated; otherwise DOUT returns to three-state on the
16th SCLK falling edge as shown in , , and
. Sixteen serial clock cycles are required to perform the
conversion process and to access data from the AD7908/
AD7918/AD7928. For the AD7908/AD7918/AD7928, the
Figure 26 Figure 27
Figure 28
AD7908/AD7918/AD7928
Rev. C | Page 24 of 28
8/10/12 bits of data are preceded by a leading zero and the
3-channel address bits (ADD2 to ADD0) identify which
channel the result corresponds to. CS going low provides the
leading zero to be read in by the microcontroller or DSP. The
three remaining address bits and data bits are then clocked out
by subsequent SCLK falling edges beginning with the first
address bit (ADD2). Thus the first falling clock edge on the
serial clock has a leading zero provided and also clocks out
Address Bit ADD2. The final bit in the data transfer is valid on
the 16th falling edge, having been clocked out on the previous
(15th) falling edge.
Writing of information to the control register takes place on the
first 12 falling edges of SCLK in a data transfer, assuming the
MSB, that is, the WRITE bit, has been set to 1. If the control
register is programmed to use the SHADOW register, then
writing of information to the SHADOW register takes place on
all 16 SCLK falling edges in the next serial transfer, as shown for
example on the AD7928 in Figure 29. Two sequence options
can be programmed in the SHADOW register. If the user does
not want to program a second sequence, then the eight LSBs
should be filled with zeros. The SHADOW register is updated
upon the rising edge of CS and the track-and-hold begins to
track the first channel selected in the sequence.
The AD7908 outputs a leading zero and three channel address
bits that the conversion result corresponds to, followed by the
8-bit conversion result and four trailing zeros. The AD7918
outputs a leading zero and three channel address bits that the
conversion result corresponds to, followed by the 10-bit
conversion result and two trailing zeros. The 16-bit word read
from the AD7928 always contains a leading zero and three
channel address bits that the conversion result corresponds to,
followed by the 12-bit conversion result.
MICROPROCESSOR INTERFACING
The serial interface on the AD7908/AD7918/AD7928 allows the
part to be directly connected to a range of many different
microprocessors. This section explains how to interface the
AD7908/AD7918/AD7928 with some of the more common
microcontroller and DSP serial interface protocols.
AD7908/AD7918/AD7928 to TMS320C541
The serial interface on the TMS320C541 uses a continuous
serial clock and frame synchronization signals to synchronize
the data transfer operations with peripheral devices like the
AD7908/AD7918/AD7928. The CS input allows easy
interfacing between the TMS320C541 and the AD7908/
AD7918/AD7928 without any glue logic required. The serial
port of the TMS320C541 is set up to operate in burst mode with
internal CLKX0 (Tx serial clock on Serial Port 0) and FSX0 (Tx
frame sync from Serial Port 0). The serial port control register
(SPC) must have the following setup: FO = 0, FSM = 1, MCM = 1,
and TXM = 1. The connection diagram is shown in . It
should be noted that for signal processing applications, it is
imperative that the frame synchronization signal from the
TMS320C541 provides equidistant sampling. The VDRIVE pin of
the AD7908/AD7918/AD7928 takes the same supply voltage as
that of the TMS320C541. This allows the ADC to operate at a
higher voltage than the serial interface, that is, TMS320C541, if
necessary.
Figure 30
AD7908/AD7918/AD7928
Rev. C | Page 25 of 28
CS
SCLK
DOUT
DIN
THREE-STATE THREE-STATE
FOUR TRAILING ZEROS
ZERO
B
12345611 12 13 14 15 16
WRITE SEQ1 DONTC ADD2 ADD1 ADD0
ADD2 ADD1 ADD0 DB7 DB6 DB0 ZERO ZERO ZERO ZERO
CODING DONTC DONTC DONTC DONTC
t
5
t
8
t
11
t
QUIET
t
9
t
3
t
2
t
4
t
7
t
6
t
CONVERT
t
10
0
3089-026
THREE IDENTIFICATION BITS
Figure 26. AD7908 Serial Interface Timing Diagram
CS
SCLK
DOUT
DIN
THREE-STATE THREE-STATE
TWO TRAILING ZEROS
ZERO
B
12345611 12 13 14 15 16
WRITE SEQ DONTC ADD2 ADD1 ADD0
ADD2 ADD1 ADD0 DB7 DB6 DB2 DB1 DB0 ZERO ZERO
CODING DONTC DONTC DONTC DONTC
t
5
t
8
t
11
t
QUIET
t
9
t
3
t
2
t
4
t
7
t
6
t
CONVERT
t
10
03089-027
THREE IDENTIFICATION BITS
Figure 27. AD7918 Serial Interface Timing Diagram
B
ZERO
CS
SCLK
DOUT
DIN
THREE-STATE THREE-STATE
123456
13 14 15 16
WRITE SEQ DONTC ADD2 ADD1 ADD0
DB2 DB1 DB0
DONTCDONTC DONTC
t
5
t
8
t
11
t
QUIET
t
9
t
3
t
2
t
7
t
4
t
6
t
CONVERT
t
10
ADD2 ADD1 ADD0 DB11 DB10
0
3089-028
THREE IDENTIFICATION BITS
Figure 28. AD7928 Serial Interface Timing Diagram
C
ZERO
CS
SCLK
DOUT
DIN
THREE-STATE THREE-STATE
THREE IDENTIFICATION BITS
t
5
t
8
t
11
t
9
t
3
t
2
t
7
t
4
t
6
t
CONVERT
t
10
ADD2 ADD1 ADD0 DB11 DB10 DB2 DB1 DB0
12345613 14 15 16
SEQUENCE 1 SEQUENCE 2
V
IN
0V
IN
1V
IN
2V
IN
3V
IN
4V
IN
5V
IN
6V
IN
7
0
3089-029
Figure 29. AD7928 Writing to SHADOW Register Timing Diagram
AD7908/AD7918/AD7928
Rev. C | Page 26 of 28
TMS320C541
1
AD7908/
AD7918/
AD7928
1
CLKX
CLKR
DR
DT
FSX
FSR
V
DD
SCLK
DOUT
DIN
CS
1
ADDITIONAL PINS REMOVED FOR CLARITY.
V
DRIVE
03089-030
Figure 30. Interfacing to the TMS320C541
AD7908/AD7918/AD7928 to ADSP-21xx
The ADSP-21xx family of DSPs are interfaced directly to the
AD7908/AD7918/AD7928 without any glue logic required. The
VDRIVE pin of the AD7908/AD7918/AD7928 takes the same
supply voltage as that of the ADSP-21xx. This allows the ADC
to operate at a higher voltage than the serial interface, that is,
ADSP-21xx, if necessary.
The SPORT0 control register should be set up as follows:
TFSW = RFSW = 1, alternate framing
INVRFS = INVTFS = 1, active low frame signal
DTYPE = 00, right justify data
SLEN = 1111, 16-bit data-words
ISCLK = 1, internal serial clock
TFSR = RFSR = 1, frame every word
IRFS = 0
ITFS = 1
The connection diagram is shown in Figure 31. The ADSP-21xx
has the TFS and RFS of the SPORT tied together, with TFS set
as an output and RFS set as an input. The DSP operates in
alternate framing mode and the SPORT control register is set
up as described. The frame synchronization signal generated on
the TFS is tied to CS, as with all signal processing applications
where equidistant sampling is necessary. However, in this
example the timer interrupt is used to control the sampling rate
of the ADC, and under certain conditions equidistant sampling
cannot be achieved.
AD7908/
AD7918/
AD7928
1
ADSP-21xx
1
SCLK
DR
RFS
TFS
DT
SCLK
DOUT
CS
DIN
V
DRIVE
V
DD
1
ADDITIONAL PINS REMOVED FOR CLARITY.
03089-031
Figure 31. Interfacing to the ADSP-21xx
The timer register, for example, is loaded with a value that
provides an interrupt at the required sample interval. When an
interrupt is received, a value is transmitted with TFS/DT (ADC
control word). The TFS is used to control the RFS and thus the
reading of data. The frequency of the serial clock is set in the
SCLKDIV register. When the instruction to transmit with TFS
is given (that is, AX0 = TX0), the state of the SCLK is checked.
The DSP waits until the SCLK has gone high, low, and high
before transmission can start. If the timer and SCLK values are
chosen, such that the instruction to transmit occurs on or near
the rising edge of SCLK, then the data can be transmitted or it
can wait until the next clock edge.
For example, if the ADSP-2189 had a 20 MHz crystal such that
it had a master clock frequency of 40 MHz, then the master
cycle time would be 25 ns. If the SCLKDIV register was loaded
with the value 3, then an SCLK of 5 MHz is obtained, and eight
master clock periods elapse for every one SCLK period.
Depending on the throughput rate selected, if the timer register
is loaded with the value, say 803 (803 + 1 = 804), 100.5 SCLKs
occur between interrupts and subsequently between transmit
instructions. This situation results in nonequidistant sampling
as the transmit instruction is occurring on a SCLK edge. If the
number of SCLKs between interrupts is a whole integer figure
of N, then equidistant sampling is implemented by the DSP.
AD7908/AD7918/AD7928 to DSP563xx
The connection diagram in Figure 32 shows how the
AD7908/AD7918/AD7928 can be connected to the synchronous
serial interface (ESSI) of the DSP563xx family of DSPs from
Motorola. Each ESSI (two on board) is operated in synchronous
mode (SYN bit in CRB = 1) with internally generated word
length frame sync for both Tx and Rx (Bit FSL1 = 0 and
Bit FSL0 = 0 in CRB). Normal operation of the ESSI is selected
by making MOD = 0 in the CRB. Set the word length to 16 by
setting Bit WL1 = 1 and Bit WL0 = 0 in CRA. The FSP bit in the
CRB should be set to 1 so the frame sync is negative. It should
be noted that for signal processing applications, it is imperative
that the frame synchronization signal from the DSP563xx
provides equidistant sampling.
In the example shown in Figure 32, the serial clock is taken
from the ESSI so the SCK0 pin must be set as an output, SCKD
= 1. The VDRIVE pin of the AD7908/AD7918/AD7928 takes the
same supply voltage as that of the DSP563xx. This allows the
ADC to operate at a higher voltage than the serial interface, that
is, DSP563xx, if necessary.
AD7908/AD7918/AD7928
Rev. C | Page 27 of 28
AD7908/
AD7918/
AD79281
DSP563xx1
SCK
SRD
STD
SC2
SCLK
DOUT
DIN
VDRIVE
VDD
1ADDITIONAL PINS REMOVED FOR CLARITY.
CS
03089-032
Figure 32. Interfacing to the DSP563xx
APPLICATION HINTS
Grounding and Layout
The AD7908/AD7918/AD7928 have very good immunity to
noise on the power supplies, as can be seen by the PSRR vs.
Supply Ripple Frequency plot, Figure 6. However, care should
still be taken with regard to grounding and layout.
The printed circuit board that houses the AD7908/AD7918/
AD7928 should be designed so that the analog and digital
sections are separated and confined to certain areas of the
board. This facilitates the use of ground planes that can be
separated easily. A minimum etch technique is generally best
for ground planes as it gives the best shielding.
All four AGND pins of the AD7908/AD7918/AD7928 should
be sunk in the AGND plane. If the AD7908/AD7918/AD7928
is in a system where multiple devices require an AGND to
DGND connection, the connection should be made at only one
point in the plane. Using a star ground point, the connection
should be established as close as possible to the AD7908/
AD7918/AD7928.
Avoid running digital lines under the device, as these couple
noise onto the die. The analog ground plane should be allowed
to run under the AD7908/AD7918/AD7928 to avoid noise
coupling. The power supply lines to the AD7908/AD7918/
AD7928 should use as large a trace as possible to provide low
impedance paths and reduce the effects of glitches on the power
supply line. Fast switching signals, like clocks, should be
shielded with digital ground to avoid radiating noise to other
sections of the board, and clock signals should never be run
near the analog inputs. Avoid crossover of digital and analog
signals. Traces on opposite sides of the board should run at
right angles to each other. This reduces the effects of
feedthrough through the board. A microstrip technique is by far
the best, but is not always possible with a double sided board. In
this technique, the component side of the board is dedicated to
ground planes while signals are placed on the solder side.
Good decoupling is also important. All analog supplies should
be decoupled with 10 μF tantalum in parallel with 0.1 μF
capacitors to AGND. To achieve the best performance from
these decoupling components, they must be placed as close as
possible to the device, ideally right up against the device. The
0.1 μF capacitors should have low effective series resistance
(ESR) and effective series inductance (ESI), such as the
common ceramic types or surface mount types, which provide a
low impedance path to ground at high frequencies to handle
transient currents due to internal logic switching.
Evaluating the AD7908/AD7918/AD7928 Performance
The recommended layout for the AD7908/AD7918/AD7928 is
outlined in the AD7908/AD7918/AD7928 evaluation board.
The evaluation board package includes a fully assembled and
tested evaluation board, documentation, and software for
controlling the board from the PC via the eval-board controller.
The eval-board controller can be used in conjunction with the
AD7908/AD7918/AD7928 evaluation board, as well as many
other Analog Devices evaluation boards ending in the CB
designator, to demonstrate/evaluate the ac and dc performance
of the AD7908/AD7918/AD7928.
The software allows the user to perform ac (fast Fourier
transform) and dc (histogram of codes) tests on the
AD7908/AD7918/AD7928. The software and documentation
are on a CD shipped with the evaluation board.
AD7908/AD7918/AD7928
Rev. C | Page 28 of 28
OUTLINE DIMENSIONS
COMPLIANT TO JEDEC STANDARDS MO-153-AC
20
1
11
10
6.40 BSC
4.50
4.40
4.30
PIN 1
6.60
6.50
6.40
SEATING
PLANE
0.15
0.05
0.30
0.19
0.65
BSC
1.20 MAX 0.20
0.09 0.75
0.60
0.45
8°
0°
COPLANARIT
Y
0.10
Figure 33. 20-Lead Thin Shrink Small Outline Package [TSSOP]
(RU-20)
Dimensions shown in millimeters
ORDERING GUIDE
Model Temperature Range Linearity Error (LSB)1 Package Description Package Option
AD7908BRU −40°C to +85°C ±0.2 20-Lead TSSOP RU-20
AD7908BRU-REEL −40°C to +85°C ±0.2 20-Lead TSSOP RU-20
AD7908BRU-REEL7 −40°C to +85°C ±0.2 20-Lead TSSOP RU-20
AD7908BRUZ2 −40°C to +85°C ±0.2 20-Lead TSSOP RU-20
AD7908BRUZ-REEL2 −40°C to +85°C ±0.2 20-Lead TSSOP RU-20
AD7908BRUZ-REEL72 −40°C to +85°C ±0.2 20-Lead TSSOP RU-20
AD7918BRU −40°C to +85°C ±0.5 20-Lead TSSOP RU-20
AD7918BRU-REEL −40°C to +85°C ±0.5 20-Lead TSSOP RU-20
AD7918BRU-REEL7 −40°C to +85°C ±0.5 20-Lead TSSOP RU-20
AD7918BRUZ2 −40°C to +85°C ±0.5 20-Lead TSSOP RU-20
AD7918BRUZ-REEL2 −40°C to +85°C ±0.5 20-Lead TSSOP RU-20
AD7918BRUZ-REEL72 −40°C to +85°C ±0.5 20-Lead TSSOP RU-20
AD7928BRU −40°C to +85°C ±1 20-Lead TSSOP RU-20
AD7928BRU-REEL −40°C to +85°C ±1 20-Lead TSSOP RU-20
AD7928BRU-REEL7 −40°C to +85°C ±1 20-Lead TSSOP RU-20
AD7928BRUZ2 −40°C to +85°C ±1 20-Lead TSSOP RU-20
AD7928BRUZ-REEL2 −40°C to +85°C ±1 20-Lead TSSOP RU-20
AD7928BRUZ-REEL72 −40°C to +85°C ±1 20-Lead TSSOP RU-20
EVAL-AD79x8CBZ2, 3 Evaluation Board
EVAL-CONTROL BRD4 Controller Board
1 Linearity error here refers to integral linearity error.
2 Z = RoHS Compliant Part.
3 This can be used as a standalone evaluation board or in conjunction with the evaluation controller board for evaluation/demonstration purposes. The board comes
with one chip of each the AD7908, AD7918, and AD7928.
4 This board is a complete unit allowing a PC to control and communicate with all Analog Devices evaluation boards ending in the CB designators. To order a complete
evaluation kit, order the particular ADC evaluation board, such as the EVAL-AD79x8CB, the EVAL-CONTROL BRD2, and a 12 V ac transformer. See the relevant
evaluation board technical note for more information.
©2006–2008 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D03089-0-11/08(C)
