STAC9205X5TAEB2X - INTEGRATED DEVICE TECHNOLOGY

16-Bit, 250 kSPS PulSAR

ADC in MSOP/QFN

AD7685

Rev. B

Information furnished by Analog Devices is believed to be accurate and reliable. However, no

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

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Tel: 781.329.4700 www.analog.com

FEATURES

16-bit resolution with no missing codes

Throughput: 250 kSPS

INL: ±0.6 LSB typical, ±2 LSB maximum (±0.003% of FSR)

SINAD: 93.5 dB @ 20 kHz

THD: −110 dB @ 20 kHz

Pseudo differential analog input range

0 V to VREF with VREF up to VDD

No pipeline delay

Single-supply operation 2.3 V to 5.5 V with

1.8 V to 5 V logic interface

Serial interface SPI®-/QSPI™-/MICROWIRE™-/DSP-compatible

Daisy-chain multiple ADCs, BUSY indicator

Power dissipation

1.4 μW @ 2.5 V/100 SPS

1.35 mW @ 2.5 V/100 kSPS, 4 mW @ 5 V/100 kSPS

Standby current: 1 nA

10-lead package: MSOP (MSOP-8 size) and

3 mm × 3 mm QFN (LFCSP) (SOT-23 size)

Pin-for-pin-compatible with 10-lead MSOP/QFN PulSAR® ADCs

APPLICATIONS

Battery-powered equipment

Medical instruments

Mobile communications

Personal digital assistants (PDAs)

Data acquisition

Instrumentation

Process controls

02968-005

CODE

655360 16384 32768 49152

INL (LSB)

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

POSITIVE INL = +0.33LSB

NEGATIVE INL = –0.50LSB

Figure 1. Integral Nonlinearity vs. Code.

APPLICATION DIAGRAM

AD7685

REF

GND

VDD

IN+

IN–

VIO

SDI

SCK

SDO

CNV

1.8V TO VDD

3- OR 4-WIRE INTERFACE

(SPI, DAISY CHAIN, CS)

0.5

TO VDD 2.5

TO 5V

0 TO VREF

02968-001

Figure 2.

Table 1. MSOP, QFN (LFCSP)/SOT-23

14-/16-/18-Bit PulSAR ADC

Type

100

kSPS

250

kSPS

400 kSPS

500 kSPS

1000

kSPS

ADC

Driver

18-Bit True AD7691 AD7690 AD7982 ADA4941

Differential AD7982 ADA4841

16-Bit True AD7684 AD7687 AD7688 ADA4941

Differential AD7693 ADA4841

16-Bit Pseudo AD7680 AD7685 AD7686 AD7980 ADA4841

Differential AD7683 AD7694

14-Bit Pseudo AD7940 AD7942 AD7946 ADA4841

Differential

GENERAL DESCRIPTION

The AD7685 is a 16-bit, charge redistribution successive

approximation, analog-to-digital converter (ADC) that operates

from a single power supply, VDD, between 2.3 V to 5.5 V. It

contains a low power, high speed, 16-bit sampling ADC with no

missing codes, an internal conversion clock, and a versatile serial

interface port. The part also contains a low noise, wide bandwidth,

short aperture delay, track-and-hold circuit. On the CNV rising

edge, it samples an analog input IN+ between 0 V to REF with

respect to a ground sense IN−. The reference voltage, REF, is

applied externally and can be set up to the supply voltage.

Power dissipation scales linearly with throughput.

The SPI-compatible serial interface also features the ability,

using the SDI input, to daisy chain several ADCs on a single

3-wire bus or provides an optional BUSY indicator. It is

compatible with 1.8 V, 2.5 V, 3 V, or 5 V logic using the

separate supply VIO.

The AD7685 is housed in a 10-lead MSOP or a 10-lead QFN

(LFCSP) with operation specified from −40°C to +85°C.

AD7685

Rev. B | Page 2 of 28

TABLE OF CONTENTS

Features .............................................................................................. 1

Applications....................................................................................... 1

Application Diagram........................................................................ 1

General Description......................................................................... 1

Revision History ............................................................................... 2

Specifications..................................................................................... 3

Timing Specifications....................................................................... 5

Absolute Maximum Ratings............................................................ 7

ESD Caution.................................................................................. 7

Pin Configuration and Function Descriptions............................. 8

Terminology ...................................................................................... 9

Typical Performance Characteristics ........................................... 10

Theory of Operation ...................................................................... 13

Circuit Information.................................................................... 13

Converter Operation.................................................................. 13

Typical Connection Diagram ................................................... 14

Analog Inputs.............................................................................. 15

Driver Amplifier Choice ........................................................... 16

Voltage Reference Input ............................................................ 16

Power Supply............................................................................... 16

Supplying the ADC from the Reference.................................. 17

Digital Interface.......................................................................... 17

CS Mode 3-Wire, No BUSY Indicator..................................... 18

CS Mode 3-Wire with BUSY Indicator ................................... 19

CS Mode 4-Wire, No BUSY Indicator..................................... 20

CS Mode 4-Wire with BUSY Indicator ................................... 21

Chain Mode, No BUSY Indicator ............................................ 22

Chain Mode with BUSY Indicator........................................... 23

Application Hints ........................................................................... 24

Layout .......................................................................................... 24

Evaluating the Performance of the AD7685............................... 24

True 16-Bit Isolated Application Example.............................. 25

Outline Dimensions....................................................................... 26

Ordering Guide .......................................................................... 27

REVISION HISTORY

3/07—Rev. A to Rev. B

Changes to Features and Table 1 .................................................... 1

Changes to Table 3............................................................................ 4

Moved Figure 3 and Figure 4 to Page............................................. 6

Inserted Figure 6; Renumbered Sequentially................................ 8

Changes to Figure 13 and Figure 14............................................. 11

Changes to Figure 27...................................................................... 14

Changes to Table 9.......................................................................... 16

Changes to Figure 32...................................................................... 17

Changes to Figure 43...................................................................... 22

Changes to Figure 45...................................................................... 23

Updated Outline Dimensions....................................................... 26

Changes to Ordering Guide .......................................................... 27

12/04—Rev. 0 to Rev. A

Changes to Specifications................................................................ 3

Changes to Figure 17 Captions..................................................... 11

Changes to Power Supply Section ................................................ 17

Changes to Digital Interface Section............................................ 18

Changes to CS Mode 4-Wire No Busy Indicator Section ......... 21

Changes to CS Mode 4-Wire with Busy Indicator Section....... 22

Changes to Chain Mode, No Busy Indicator Section ................ 23

Changes to Chain Mode with Busy Indicator Section............... 24

Added True 16-Bit Isolated Application Example Section ....... 26

Added Figure 47.............................................................................. 26

Changes to Ordering Guide .......................................................... 28

4/04—Revision 0: Initial Revision

AD7685

Rev. B | Page 3 of 28

SPECIFICATIONS

VDD = 2.3 V to 5.5 V, VIO = 2.3 V to VDD, VREF = VDD, TA = –40°C to +85°C, unless otherwise noted.

Table 2.

A Grade B Grade C Grade

Parameter Conditions Min Typ Max Min Typ Max Min Typ Max Unit

RESOLUTION 16 16 16 Bits

ANALOG INPUT

Voltage Range IN+ − IN− 0 VREF 0 VREF 0 VREF V

Absolute Input Voltage IN+ −0.1 VDD +

0.1

−0.1 VDD +

0.1

−0.1 VDD +

0.1

IN− −0.1 +0.1 −0.1 +0.1 −0.1 +0.1 V

Analog Input CMRR fIN = 250 kHz 65 65 65 dB

Leakage Current at 25°C Acquisition phase 1 1 1 nA

Input Impedance See the

Analog Inputs section

See the

Analog Inputs section

See the

Analog Inputs section

ACCURACY

No Missing Codes 15 16 16 Bits

Differential Linearity Error −1 ±0.7 −1 ±0.5 +1.5 LSB1

Integral Linearity Error −6 +6 −3 ±1 +3 −2 ±0.6 +2 LSB

Transition Noise REF = VDD = 5 V 0.5 0.5 0.45 LSB

Gain Error2, TMIN to TMAX ±2 ±30 ±2 ±30 ±2 ±15 LSB

Gain Error Temperature Drift ±0.3 ±0.3 ±0.3 ppm/°C

Offset Error2, TMIN to TMAX VDD = 4.5 V to 5.5 V ±0.1 ±1.6 ±0.1 ±1.6 ±0.1 ±1.6 mV

VDD = 2.3 V to 4.5 V ±0.7 ±3.5 ±0.7 ±3.5 ±0.7 ±3.5 mV

Offset Temperature Drift ±0.3 ±0.3 ±0.3 ppm/°C

Power Supply Sensitivity VDD = 5 V ± 5% ±0.05 ±0.05 ±0.05 LSB

THROUGHPUT

Conversion Rate VDD = 4.5 V to 5.5 V 0 250 0 250 0 250 kSPS

VDD = 2.3 V to 4.5 V 0 200 0 200 0 200 kSPS

Transient Response Full-scale step 1.8 1.8 1.8 μs

AC ACCURACY

Signal-to-Noise Ratio fIN = 20 kHz,

VREF = 5 V

90 90 92 91.5 93.5 dB3

IN = 20 kHz,

VREF = 2.5 V

86 86 88 87.5 88.5 dB

Spurious-Free Dynamic

Range

fIN = 20 kHz −100 −106 −110 dB

Total Harmonic Distortion fIN = 20 kHz −100 −106 −110 dB

Signal-to-(Noise + Distortion) fIN = 20 kHz,

VREF = 5 V

89 90 92 91.5 93.5 dB

IN = 20 kHz,

VREF = 5 V,

−60 dB input

32 33.5 dB

IN = 20 kHz,

VREF = 2.5 V

86 85.5 87.5 87 88.5 dB

Intermodulation Distortion4 −110 −115 dB

1 LSB means least significant bit. With the 5 V input range, 1 LSB is 76.3 μV.

2 See Terminology section. These specifications do include full temperature range variation but do not include the error contribution from the external reference.

3 All specifications in dB are referred to a full-scale input FS. Tested with an input signal at 0.5 dB below full-scale, unless otherwise specified.

4 fIN1 = 21.4 kHz, fIN2 = 18.9 kHz, each tone at −7 dB below full scale.

AD7685

Rev. B | Page 4 of 28

VDD = 2.3 V to 5.5 V, VIO = 2.3 V to VDD, VREF = VDD, TA = –40°C to +85°C, unless otherwise noted.

Table 3.

Parameter Conditions Min Typ Max Unit

REFERENCE

Voltage Range 0.5 VDD + 0.3 V

Load Current 250 kSPS, REF = 5 V 50 μA

SAMPLING DYNAMICS

−3 dB Input Bandwidth 2 MHz

Aperture Delay VDD = 5 V 2.5 ns

DIGITAL INPUTS

Logic Levels

VIL –0.3 0.3 × VIO V

VIH 0.7 × VIO VIO + 0.3 V

IIL −1 +1 μA

IIH −1 +1 μA

DIGITAL OUTPUTS

Data Format Serial 16 bits straight binary

Pipeline Delay Conversion results available immediately

after completed conversion

VOL ISINK = +500 μA 0.4 V

VOH ISOURCE = −500 μA VIO − 0.3 V

POWER SUPPLIES

VDD Specified performance 2.3 5.5 V

VIO Specified performance 2.3 VDD + 0.3 V

VIO Range 1.8 VDD + 0.3 V

Standby Current1, 2VDD and VIO = 5 V, 25°C 1 50 nA

Power Dissipation VDD = 2.5 V, 100 SPS throughput 1.4 μW

VDD = 2.5 V, 100 kSPS throughput 1.35 2.4 mW

VDD = 2.5 V, 200 kSPS throughput 2.7 4.8 mW

VDD = 5 V, 100 kSPS throughput 4 6 mW

VDD = 5 V, 250 kSPS throughput 10 15 mW

TEMPERATURE RANGE3

Specified Performance TMIN to TMAX −40 +85 °C

1 With all digital inputs forced to VIO or GND as required.

2 During acquisition phase.

3 Contact sales for extended temperature range.

AD7685

Rev. B | Page 5 of 28

TIMING SPECIFICATIONS

−40°C to +85°C, VIO = 2.3 V to 5.5 V or VDD + 0.3 V, whichever is the lowest, unless otherwise stated.

Table 4. VDD = 4.5 V to 5.5 V1

Parameter Symbol Min Typ Max Unit

Conversion Time: CNV Rising Edge To Data Available tCONV 0.5 2.2 μs

Acquisition Time tACQ 1.8 μs

Time Between Conversions tCYC 4 μs

CNV Pulse Width (CS Mode) tCNVH 10 ns

SCK Period (CS Mode) tSCK 15 ns

SCK Period (Chain Mode) tSCK

VIO Above 4.5 V 17 ns

VIO Above 3 V 18 ns

VIO Above 2.7 V 19 ns

VIO Above 2.3 V 20 ns

SCK Low Time tSCKL 7 ns

SCK High Time tSCKH 7 ns

SCK Falling Edge to Data Remains Valid tHSDO 5 ns

SCK Falling Edge to Data Valid Delay tDSDO

VIO Above 4.5 V 14 ns

VIO Above 3 V 15 ns

VIO Above 2.7 V 16 ns

VIO Above 2.3 V 17 ns

CNV or SDI Low to SDO D15 MSB Valid (CS Mode) tEN

VIO Above 4.5 V 15 ns

VIO Above 2.7 V 18 ns

VIO Above 2.3 V 22 ns

CNV or SDI High or Last SCK Falling Edge to SDO High Impedance (CS Mode) tDIS 25 ns

SDI Valid Setup Time from CNV Rising Edge (CS Mode) tSSDICNV 15 ns

SDI Valid Hold Time from CNV Rising Edge (CS Mode) tHSDICNV 0 ns

SCK Valid Setup Time from CNV Rising Edge (Chain Mode) tSSCKCNV 5 ns

SCK Valid Hold Time from CNV Rising Edge (Chain Mode) tHSCKCNV 5 ns

SDI Valid Setup Time from SCK Falling Edge (Chain Mode) tSSDISCK 3 ns

SDI Valid Hold Time from SCK Falling Edge (Chain Mode) tHSDISCK 4 ns

SDI High to SDO High (Chain Mode with Busy Indicator) tDSDOSDI

VIO Above 4.5 V 15 ns

VIO Above 2.3 V 26 ns

1 See Figure 3 and Figure 4 for load conditions.

AD7685

Rev. B | Page 6 of 28

−40°C to +85°C, VIO = 2.3 V to 4.5 V or VDD + 0.3 V, whichever is the lowest, unless otherwise stated.

Table 5. VDD = 2.3V to 4.5 V1

Parameter Symbol Min Typ Max Unit

Conversion Time: CNV Rising Edge to Data Available tCONV 0.7 3.2 μs

Acquisition Time tACQ 1.8 μs

Time Between Conversions tCYC 5 μs

CNV Pulse Width (CS Mode) tCNVH 10 ns

SCK Period (CS Mode) tSCK 25 ns

SCK Period (Chain Mode) tSCK

VIO Above 3 V 29 ns

VIO Above 2.7 V 35 ns

VIO Above 2.3 V 40 ns

SCK Low Time tSCKL 12 ns

SCK High Time tSCKH 12 ns

SCK Falling Edge to Data Remains Valid tHSDO 5 ns

SCK Falling Edge to Data Valid Delay tDSDO

VIO Above 3 V 24 ns

VIO Above 2.7 V 30 ns

VIO Above 2.3 V 35 ns

CNV or SDI Low to SDO D15 MSB Valid (CS Mode) tEN

VIO Above 2.7 V 18 ns

VIO Above 2.3 V 22 ns

CNV or SDI High or Last SCK Falling Edge to SDO High Impedance (CS Mode) tDIS 25 ns

SDI Valid Setup Time from CNV Rising Edge (CS Mode) tSSDICNV 30 ns

SDI Valid Hold Time from CNV Rising Edge (CS Mode) tHSDICNV 0 ns

SCK Valid Setup Time from CNV Rising Edge (Chain Mode) tSSCKCNV 5 ns

SCK Valid Hold Time from CNV Rising Edge (Chain Mode) tHSCKCNV 8 ns

SDI Valid Setup Time from SCK Falling Edge (Chain Mode) tSSDISCK 5 ns

SDI Valid Hold Time from SCK Falling Edge (Chain Mode) tHSDISCK 4 ns

SDI High to SDO High (Chain Mode with Busy Indicator) tDSDOSDI 36 ns

1 See Figure 3 and Figure 4 for load conditions.

500µA I

1.4V

TO SDO

50pF

02968-002

Figure 3. Load Circuit for Digital Interface Timing

30% VIO

70% VIO

2V OR VIO – 0.5V

0.8V OR 0.5V

2V OR VIO – 0.5V

DELAY

02968-003

NOTES

1. 2V IF VIO ABOVE 2.5V, VIO – 0.5V IF VIO BELOW 2.5V.

2. 0.8V IF VIO ABOVE 2.5V, 0.5V IF VIO BELOW 2.5V.

Figure 4. Voltage Levels for Timing

AD7685

Rev. B | Page 7 of 28

ABSOLUTE MAXIMUM RATINGS

Table 6.

Parameter Rating

Analog Inputs

IN+1, IN−1, REF GND − 0.3 V to VDD + 0.3 V

or ±130 mA

Supply Voltages

VDD, VIO to GND −0.3 V to +7 V

VDD to VIO ±7 V

Digital Inputs to GND −0.3 V to VIO + 0.3 V

Digital Outputs to GND −0.3 V to VIO + 0.3 V

Storage Temperature Range −65°C to +150°C

Junction Temperature 150°C

θJA Thermal Impedance 200°C/W (MSOP-10)

θJC Thermal Impedance 44°C/W (MSOP-10)

Lead Temperature

Vapor Phase (60 sec) 215°C

Infrared (15 sec) 220°C

1 See the Analog Inputs section.

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

AD7685

Rev. B | Page 8 of 28

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

REF

VDD

IN+

IN–

GND

VIO

SDI

SCK

SDO

CNV

AD7685

TOP VIEW

(Not to Scale)

02968-004

Figure 5. 10-Lead MSOP Pin Configuration

1REF

2VDD

3IN+

4IN–

5GND

10 VIO

9SDI

8SCK

7SDO

6 CNV

TOP VIEW

(Not to Scale)

AD7685

02968-005

Figure 6. 10-Lead QFN (LFCSP) Pin Configuration

Table 7. Pin Function Descriptions

Pin No Mnemonic Type1Description

1 REF AI

Reference Input Voltage. The REF range is from 0.5 V to VDD. It is referred to the GND pin. This pin should

be decoupled closely to the pin with a 10 μF capacitor.

2 VDD P Power Supply.

3 IN+ AI Analog Input. It is referred to IN−. The voltage range, that is, the difference between IN+ and IN−, is 0 V to VREF.

4 IN− AI Analog Input Ground Sense. Connect to the analog ground plane or to a remote sense ground.

5 GND P Power Supply Ground.

6 CNV DI

Convert Input. This input has multiple functions. On its leading edge, it initiates the conversions and

selects the interface mode of the part, chain, or CS mode. In CS mode, it enables the SDO pin when low.

In chain mode, the data should be read when CNV is high.

7 SDO DO Serial Data Output. The conversion result is output on this pin. It is synchronized to SCK.

8 SCK DI Serial Data Clock Input. When the part is selected, the conversion result is shifted out by this clock.

9 SDI DI Serial Data Input. This input provides multiple features. It selects the interface mode of the ADC as follows:

Chain mode is selected if SDI is low during the CNV rising edge. In this mode, SDI is used as a data input

to daisy chain the conversion results of two or more ADCs onto a single SDO line. The digital data level

on SDI is output on SDO with a delay of 16 SCK cycles.

CS mode is selected if SDI is high during the CNV rising edge. In this mode, either SDI or CNV can enable

the serial output signals when low, and if SDI or CNV is low when the conversion is complete, the BUSY

indicator feature is enabled.

10 VIO P Input/Output Interface Digital Power. Nominally at the same supply as the host interface (1.8 V, 2.5 V, 3 V, or 5 V).

1AI = analog input, DI = digital input, DO = digital output, and P = power.

AD7685

Rev. B | Page 9 of 28

TERMINOLOGY

Integral Nonlinearity Error (INL)

INL refers to the deviation of each individual code from a line

drawn from negative full scale through positive full scale. The

point used as negative full scale occurs ½ LSB before the first

code transition. Positive full scale is defined as a level 1½ LSB

beyond the last code transition. The deviation is measured from

the middle of each code to the true straight line (see Figure 26).

Differential Nonlinearity Error (DNL)

In an ideal ADC, code transitions are 1 LSB apart. DNL is the

maximum deviation from this ideal value. It is often specified in

terms of resolution for which no missing codes are guaranteed.

Offset Error

The first transition should occur at a level ½ LSB above analog

ground (38.1 μV for the 0 V to 5 V range). The offset error is

the deviation of the actual transition from that point.

Gain Error

The last transition (from 111 . . . 10 to 111 . . . 11) should

occur for an analog voltage 1½ LSB below the nominal full

scale (4.999886 V for the 0 V to 5 V range). The gain error is the

deviation of the actual level of the last transition from the ideal

level after the offset is adjusted out.

Spurious-Free Dynamic Range (SFDR)

The difference, in decibels (dB), between the rms amplitude of

the input signal and the peak spurious signal.

Effective Number of Bits (ENOB)

ENOB is a measurement of the resolution with a sine wave

input. It is related to SINAD by

ENOB = (SINADdB − 1.76)/6.02

and is expressed in bits.

Total Harmonic Distortion (THD)

THD is the ratio of the rms sum of the first five harmonic

components to the rms value of a full-scale input signal and is

expressed in dB.

Signal-to-Noise Ratio (SNR)

SNR is the ratio of the rms value of the actual input signal to the

rms sum of all other spectral components below the Nyquist

frequency, excluding harmonics and dc. The value for SNR is

expressed in dB.

Signal-to-(Noise + Distortion), SINAD

SINAD is the ratio of the rms value of the actual input signal to

the rms sum of all other spectral components below the Nyquist

frequency, including harmonics but excluding dc. The value for

SINAD is expressed in dB.

Aperture Delay

Aperture delay is a measure of the acquisition performance and

is the time between the rising edge of the CNV input and when

the input signal is held for a conversion.

Transient Response

The time required for the ADC to accurately acquire its input

after a full-scale step function is applied.

AD7685

Rev. B | Page 10 of 28

TYPICAL PERFORMANCE CHARACTERISTICS

02968-047

CODE

655360 16384 32768 49152

INL (LSB)

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

POSITIVE INL = +0.33LSB

NEGATIVE INL = –0.50LSB

Figure 7. Integral Nonlinearity vs. Code

02968-006

CODE IN HEX

80EA 80EB 80EC 80ED80E5 80E6 80E7 80E8 80E9

COUNTS

250000

200000

150000

100000

50000

0012 00

204292

27755

29041

VDD = REF = 5V

Figure 8. Histogram of a DC Input at the Code Center

02968-007

FREQUENCY (kHz)

1200 20406080100

AMPLITUDE (dB OF FULL SCALE)

–20

–40

–60

–80

–100

–120

–160

–140

–180

8192 POINT FFT

VDD = REF = 5V

fS = 250kSPS

fIN = 20.45kHz

SNR = 93.3dB

THD = –111.6dB

SFDR = –113.7dB

SECOND HARMONIC = –113.7dB

THIRD HARMONIC = –117.6dB

Figure 9. FFT Plot

02968-008

CODE

655360 16384 32768 49152

DNL (LSB)

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

POSITIVE DNL = +0.21LSB

NEGATIVE DNL = –0.30LSB

Figure 10. Differential Nonlinearity vs. Code

02968-009

CODE IN HEX

8058804E 804F 8050 8051 8052 8053 8054 8055 8056 8057

COUNTS

140000

120000

100000

80000

60000

40000

20000

213

6956

179 0 0

60966

125055

8667

59082

VDD = REF = 2.5V

Figure 11. Histogram of a DC Input at the Code Center

02968-010

FREQUENCY (kHz)

1200 20406080100

AMPLITUDE (dB OF FULL SCALE)

–20

–40

–60

–80

–100

–120

–140

–160

–180

16384 POINT FFT

VDD = REF = 2.5V

S = 250kSPS

IN = 20.45kHz

SNR = 88.8dB

THD = –103.5dB

SFDR = –104.5dB

SECOND HARMONIC = –112.4dB

THIRD HARMONIC = –105.4dB

Figure 12. FFT Plot

AD7685

Rev. B | Page 11 of 28

02968-011

REFERENCE VOLTAGE (V)

5.52.3 2.7 3.1 3.5 3.9 4.3 4.7 5.1

ENOB (Bits)

SINAD (dB)

100

SNR

SINAD

ENOB

Figure 13. SNR, SINAD, and ENOB vs. Reference Voltage

02968-012

SINAD (dB)

FREQUENCY (kHz)

2000 50 100 150

100

VREF = 5V, –1dB

VREF = 2.5V, –1dB

VREF = 5V, –10dB

Figure 14. SINAD vs. Frequency

02968-013

TEMPERATURE (°C)

125–55 –35 –15 5 25 45 65 85 105

SNR (dB)

100

VREF = 5V

VREF = 2.5V

Figure 15. SNR vs. Temperature

02968-014

REFERENCE VOLTAGE (V)

5.52.3 2.7 3.1 3.5 3.9 4.3 4.7 5.1

THD, SFDR (dB)

–

–95

–100

–105

–110

–115

–120

–125

–130

SFDR

THD

Figure 16. THD, SFDR vs. Reference Voltage

02968-015

FREQUENCY (kHz)

2000 50 100 150

THD (dB)

–

–70

–80

–90

–100

–110

–120

VREF = 2.5V, –1dB

VREF = 5V, –1dB

VREF = 5V, –10dB

Figure 17. THD vs. Frequency

02968-016

TEMPERATURE (°C)

125–55 –35 –15 5 25 45 65 85 105

THD (dB)

–

–100

–110

–120

–130

VREF = 2.5V

VREF = 5V

Figure 18. THD vs. Temperature

AD7685

Rev. B | Page 12 of 28

02968-017

INPUT LEVEL (dB)

0–10 –8 –6 –4 –2

THD (dB)

–120

–

105

–110

–115

SNR REFERENCE TO FULL SCALE (dB)

SNR

THD

Figure 19. SNR and THD vs. Input Level

02968-018

SUPPLY (V)

5.52.3 2.7 3.1 3.5 3.9 4.3 4.7 5.1

OPER

TING CURRENTS (µA)

1000

750

500

250

VDD

VIO

fS = 100kSPS

Figure 20. Operating Currents vs. Supply

02968-019

TEMPERATURE (°C)

125–55 –35 –15 5 25 45 65 85 105

POWER-DOWN CURRENTS (nA)

1000

750

500

250

VDD + VIO

Figure 21. Power-Down Currents vs. Temperature

02968-020

TEMPERATURE (°C)

125–55 –35 –15 5 25 45 65 85 105

OPER

TING CURRENTS (µA)

1000

750

500

250

VDD = 5V

VIO

fS = 100kSPS

VDD = 2.5V

Figure 22. Operating Currents vs. Temperature

02968-021

TEMPERATURE (°C)

125–55 –35 –15 5 25 45 65 85 105

OFFSET, GAIN ERROR (LSB)

–2

–1

–4

–3

–6

–5

GAIN ERROR

OFFSET ERROR

Figure 23. Offset and Gain Error vs. Temperature

02968-022

SDO CAPACITIVE LOAD (pF)

120020406080100

DSDO

DELAY (ns)

VDD = 2.5V, 85°C

VDD = 3.3V, 25°C

VDD = 3.3V, 85°C

VDD = 5V, 85°C

VDD = 5V, 25°C

VDD = 2.5V, 25°C

Figure 24. tDSDO Delay vs. Capacitance Load and Supply

AD7685

Rev. B | Page 13 of 28

THEORY OF OPERATION

SW+MSB

16,384C

IN+

LSB

COMP CONTROL

LOGIC

SWITCHES CONTROL

BUSY

OUTPUT CODE

CNV

REF

GND

IN–

4C 2C C C32,768C

SW–MSB

16,384C

LSB

4C 2C C C32,768C

02968-023

Figure 25. ADC Simplified Schematic

CIRCUIT INFORMATION

The AD7685 is a fast, low power, single-supply, precise 16-bit

ADC using a successive approximation architecture.

The AD7685 is capable of converting 250,000 samples per

second (250 kSPS) and powers down between conversions.

When operating at 100 SPS, for example, it consumes typically

1.35 μW with a 2.5 V supply, ideal for battery-powered

applications.

The AD7685 provides the user with on-chip, track-and-hold

and does not exhibit any pipeline delay or latency, making it

ideal for multiple multiplexed channel applications.

The AD7685 is specified from 2.3 V to 5.5 V and can be

interfaced to any 1.8 V to 5 V digital logic family. It is housed in

a 10-lead MSOP or a tiny 10-lead QFN (LFCSP) that combines

space savings and allows flexible configurations.

It is pin-for-pin-compatible with the AD7686, AD7687, and

AD7688.

CONVERTER OPERATION

The AD7685 is a successive approximation ADC based on a

charge redistribution DAC. Figure 25 shows the simplified

schematic of the ADC. The capacitive DAC consists of two

identical arrays of 16 binary weighted capacitors, which are

connected to the two comparator inputs.

During the acquisition phase, terminals of the array tied to the

comparator’s input are connected to GND via SW+ and SW−.

All independent switches are connected to the analog inputs.

Therefore, the capacitor arrays are used as sampling capacitors

and acquire the analog signal on the IN+ and IN− inputs. When

the acquisition phase is complete and the CNV input goes high,

a conversion phase is initiated. When the conversion phase

begins, SW+ and SW− are opened first. The two capacitor

arrays are then disconnected from the inputs and connected to

the GND input. Therefore, the differential voltage between the

inputs IN+ and IN− captured at the end of the acquisition phase

is applied to the comparator inputs, causing the comparator to

become unbalanced. By switching each element of the capacitor

array between GND and REF, the comparator input varies by

binary weighted voltage steps (VREF/2, VREF/4 . . . VREF/65536).

The control logic toggles these switches, starting with the MSB,

to bring the comparator back into a balanced condition. After

the completion of this process, the part powers down and

returns to the acquisition phase, and the control logic generates

the ADC output code and a BUSY signal indicator.

Because the AD7685 has an on-board conversion clock, the

serial clock, SCK, is not required for the conversion process.

AD7685

Rev. B | Page 14 of 28

Transfer Functions

The ideal transfer characteristic for the AD7685 is shown in

Figure 26 and Table 8.

000...000

000...001

000...010

111...101

111...110

111...111

ADC CODE (STRAIGHT BIN

ANALOG INPUT

+FS – 1.5 LSB

+FS – 1 LSB

–FS + 1 LSB

–FS

–FS + 0.5 LSB

02968-024

Figure 26. ADC Ideal Transfer Function

TYPICAL CONNECTION DIAGRAM

Figure 27 shows an example of the recommended connection

diagram for the AD7685 when multiple supplies are available.

AD7685

REF

GND

VDD

IN–

IN+

VIO

SDI

SCK

SDO

CNV

3- OR 4-WIRE INTERFACE

100nF

10µF

≥7V

≤–2V

1.8V TO VDD

REF

0TO VREF

33Ω

2.7nF

02968-025

NOTES

1. SEE VOLTAGE REFERENCE INPUT SECTION FOR REFERENCE SELECTION.

2. C

REF

IS USUALLY A 10µF CERAMIC CAPACITOR (X5R).

3. SEE DRIVER AMPLIFIER CHOICE SECTION.

4. OPTIONAL FILTER. SEE ANALOG INPUT SECTION.

5. SEE DIGITAL INTERFACE FOR MOST CONVENIENT INTERFACE MODE.

Figure 27. Typical Application Diagram with Multiple Supplies

Table 8. Output Codes and Ideal Input Voltages

Description Analog Input VREF = 5 V Digital Output Code Hexa

FSR – 1 LSB 4.999924 V FFFF1

Midscale + 1 LSB 2.500076 V 8001

Midscale 2.5 V 8000

Midscale – 1 LSB 2.499924 V 7FFF

–FSR + 1 LSB 76.3 μV 0001

–FSR 0 V 00002

1 This is also the code for an overranged analog input (VIN+ − VIN− above VREF − VGND).

2 This is also the code for an underranged analog input (VIN+ − VIN− below VGND).

AD7685

Rev. B | Page 15 of 28

ANALOG INPUTS

Figure 28 shows an equivalent circuit of the input structure of

the AD7685.

The two diodes, D1 and D2, provide ESD protection for the

analog inputs IN+ and IN−. Care must be taken to ensure that

the analog input signal never exceeds the supply rails by more

than 0.3 V because this will cause these diodes to begin to

forward-bias and start conducting current. These diodes can

handle a forward-biased current of 130 mA maximum. For

instance, these conditions could eventually occur when the

input buffer’s (U1) supplies are different from VDD. In such a

case, an input buffer with a short-circuit current limitation can

be used to protect the part.

IN+

OR IN–

GND

02968-026

Figure 28. Equivalent Analog Input Circuit

This analog input structure allows the sampling of the

differential signal between IN+ and IN−. By using this

differential input, small signals common to both inputs are

rejected, as shown in Figure 29, which represents the typical

CMRR over frequency. For instance, by using IN− to sense a

remote signal ground, ground potential differences between

the sensor and the local ADC ground are eliminated.

02968-027

FREQUENCY (kHz)

100001 10 100 1000

CMRR (dB)

= 5V

= 2.5V

Figure 29. Analog Input CMRR vs. Frequency

During the acquisition phase, the impedance of the analog

inputs (IN+ or IN−) can be modeled as a parallel combination

of capacitor CPIN and the network formed by the series connection

of RIN and CIN. CPIN is primarily the pin capacitance. RIN is

typically 3 kΩ and is a lumped component made up of some

serial resistors and the on resistance of the switches. CIN is

typically 30 pF and is mainly the ADC sampling capacitor.

During the conversion phase, where the switches are opened,

the input impedance is limited to CPIN. RIN and CIN make a

1-pole, low-pass filter that reduces undesirable aliasing effects

and limits the noise.

When the source impedance of the driving circuit is low, the

AD7685 can be driven directly. Large source impedances

significantly affect the ac performance, especially THD. The dc

performances are less sensitive to the input impedance. The

maximum source impedance depends on the amount of THD

that can be tolerated. The THD degrades as a function of the

source impedance and the maximum input frequency, as shown

in Figure 30.

02968-028

FREQUENCY (kHz)

1000 255075

THD (dB)

–

–70

–80

–90

–100

–110

–120

= 33Ω

= 50Ω

= 100Ω

= 250Ω

Figure 30. THD vs. Analog Input Frequency and Source Resistance

AD7685

Rev. B | Page 16 of 28

DRIVER AMPLIFIER CHOICE

Although the AD7685 is easy to drive, the driver amplifier

needs to meet the following requirements:

• The noise generated by the driver amplifier needs to be kept

as low as possible to preserve the SNR and transition noise

performance of the AD7685. Note that the AD7685 has a

noise much lower than most of the other 16-bit ADCs and,

therefore, can be driven by a noisier amplifier to meet a given

system noise specification. The noise coming from the

amplifier is filtered by the AD7685 analog input circuit low-

pass filter made by RIN and CIN or by an external filter, if one

is used. Because the typical noise of the AD7685 is 35 μV

rms, the SNR degradation due to the amplifier is

⎟

⎠

⎞

⎜

⎝

⎛

−

3dB

2)(f

20log

LOSS

SNR

where:

f–3dB is the input bandwidth in MHz of the AD7685

(2 MHz) or the cutoff frequency of the input filter, if one is

used.

N is the noise gain of the amplifier (for example, 1 in buffer

configuration).

eN is the equivalent input noise voltage of the op amp, in

nV/√Hz.

• For ac applications, the driver should have a THD

performance commensurate with the AD7685. Figure 17

shows the AD7685’s THD vs. frequency.

• For multichannel, multiplexed applications, the driver

amplifier and the AD7685 analog input circuit must settle a

full-scale step onto the capacitor array at a 16-bit level

(0.0015%). In the amplifier’s data sheet, settling at 0.1% to

0.01% is more commonly specified. This could differ

significantly from the settling time at a 16-bit level and

should be verified prior to driver selection.

Table 9. Recommended Driver Amplifiers

Amplifier Typical Application

ADA4841-x Very low noise and low power

AD8605, AD8615 5 V single-supply, low power

AD8655 5 V single-supply, low power

OP184 Low power, low noise, and low frequency

AD8021 Very low noise and high frequency

AD8022 Very low noise and high frequency

AD8519 Small, low power and low frequency

AD8031 High frequency and low power

VOLTAGE REFERENCE INPUT

The AD7685 voltage reference input, REF, has a dynamic input

impedance and should therefore be driven by a low impedance

source with efficient decoupling between the REF and GND

pins, as explained in the Layout section.

When REF is driven by a very low impedance source, for

example, a reference buffer using the AD8031 or the AD8605, a

10 μF (X5R, 0805 size) ceramic chip capacitor is appropriate for

optimum performance.

If an unbuffered reference voltage is used, the decoupling value

depends on the reference used. For instance, a 22 μF (X5R,

1206 size) ceramic chip capacitor is appropriate for optimum

performance using a low temperature drift ADR43x reference.

If desired, smaller reference decoupling capacitor values down

to 2.2 μF can be used with a minimal impact on performance,

especially DNL.

POWER SUPPLY

The AD7685 is specified over a wide operating range from

2.3 V to 5.5 V. It has, unlike other low voltage converters, a

noise low enough to design a 16-bit resolution system with low

supply and respectable performance. It uses two power supply

pins: a core supply VDD and a digital input/output interface

supply VIO. VIO allows direct interface with any logic between

1.8 V and VDD. To reduce the number of supplies needed, the

VIO and VDD can be tied together. The AD7685 is independent of

power supply sequencing between VIO and VDD. Additionally,

it is very insensitive to power supply variations over a wide

frequency range, as shown in Figure 31, which represents PSRR

over frequency.

02968-029

FREQUENCY (kHz)

100001 10 100 1000

PSRR (dB)

110

100

VDD = 5V

VDD = 2.5V

Figure 31. PSRR vs. Frequency

AD7685

Rev. B | Page 17 of 28

The AD7685 powers down automatically at the end of each

conversion phase and, therefore, the power scales linearly with

the sampling rate, as shown in Figure 32. This makes the part

ideal for low sampling rate (even a few Hz) and low battery-

powered applications.

02968-030

SAMPLING RATE (SPS)

100000010 100 1000 10000 100000

OPER

TING CURRENTS (µA)

1000

0.1

0.001

10000

100

0.01

VDD = 5V

VIO

VDD = 2.5V

Figure 32. Operating Currents vs. Sampling Rate

SUPPLYING THE ADC FROM THE REFERENCE

For simplified applications, the AD7685, with its low operating

current, can be supplied directly using the reference circuit, as

shown in Figure 33. The reference line can be driven by either:

• The system power supply directly.

• A reference voltage with enough current output capability,

such as the ADR43x.

• A reference buffer, such as the AD8031, that can also filter

the system power supply, as shown in Figure 33.

AD8031

AD7685

VIOREF VDD

10µF 1µF

10Ω

10kΩ

1µF

02968-031

OPTIONAL REFERENCE BUFFER AND FILTER.

Figure 33. Example of Application Circuit

DIGITAL INTERFACE

Though the AD7685 has a reduced number of pins, it offers

substantial flexibility in its serial interface modes.

The AD7685, when in CS mode, is compatible with SPI, QSPI,

digital hosts, and DSPs, for example, Blackfin® ADSP-BF53x or

ADSP-219x. This interface can use either 3-wire or 4-wire. A

3-wire interface using the CNV, SCK, and SDO signals minimizes

wiring connections, useful, for instance, in isolated applications.

A 4-wire interface using the SDI, CNV, SCK, and SDO signals

allows CNV, which initiates the conversions, to be independent

of the readback timing (SDI). This is useful in low jitter

sampling or simultaneous sampling applications.

The AD7685, when in chain mode, provides a daisy-chain

feature using the SDI input for cascading multiple ADCs on a

single data line similar to a shift register.

The mode in which the part operates depends on the SDI level

when the CNV rising edge occurs. The CS mode is selected if

SDI is high and the chain mode is selected if SDI is low. The

SDI hold time is such that when SDI and CNV are connected

together, the chain mode is always selected.

In either the CS mode or the chain mode, the AD7685 offers the

flexibility to optionally force a start bit in front of the data bits.

This start bit can be used as a BUSY signal indicator to

interrupt the digital host and trigger the data reading.

Otherwise, without a BUSY indicator, the user must time out

the maximum conversion time prior to readback.

The BUSY indicator feature is enabled as follows:

• In the CS mode, if CNV or SDI is low when the ADC

conversion ends (see Figure 37 and Figure 41).

• In the chain mode, if SCK is high during the CNV rising edge

(see Figure 45).

AD7685

Rev. B | Page 18 of 28

CS MODE 3-WIRE, NO BUSY INDICATOR

This mode is usually used when a single AD7685 is connected

to an SPI-compatible digital host.

The connection diagram is shown in Figure 34, and the

corresponding timing is given in Figure 35.

With SDI tied to VIO, a rising edge on CNV initiates a

conversion, selects the CS mode, and forces SDO to high

impedance. Once a conversion is initiated, it will continue to

completion irrespective of the state of CNV. For instance, it

could be useful to bring CNV low to select other SPI devices,

such as analog multiplexers, but CNV must be returned high

before the minimum conversion time and held high until the

maximum conversion time to avoid the generation of the BUSY

signal indicator. When conversion is completed, the AD7685

enters the acquisition phase and powers down. When CNV

goes low, the MSB is output onto SDO. The remaining data bits

are then clocked by subsequent SCK falling edges. The data is

valid on both SCK edges. Although the rising edge can be used

to capture the data, a digital host using the SCK falling edge will

allow a faster reading rate provided it has an acceptable hold

time. After the 16th SCK falling edge or when CNV goes high,

whichever is earlier, SDO returns to high impedance.

CNV

SCK

SDOSDI DATA IN

CLK

CONVERT

DIGITAL HOST

AD7685

02968-032

Figure 34. CS Mode 3-Wire, No BUSY Indicator

Connection Diagram (SDI High)

SDO D15 D14 D13 D1 D0

DIS

SCK

123 141516

SCK

SCKL

SCKH

HSDO

DSDO

CNV

CONVERSIONACQUISITION

CONV

CYC

ACQUISITION

SDI = 1

CNVH

ACQ

02968-033

Figure 35. CS Mode 3-Wire, No BUSY Indicator Serial Interface Timing (SDI High)

AD7685

Rev. B | Page 19 of 28

CS MODE 3-WIRE WITH BUSY INDICATOR

This mode is usually used when a single AD7685 is connected

to an SPI-compatible digital host having an interrupt input.

The connection diagram is shown in Figure 36, and the

corresponding timing is given in Figure 37.

With SDI tied to VIO, a rising edge on CNV initiates a

conversion, selects the CS mode, and forces SDO to high

impedance. SDO is maintained in high impedance until the

completion of the conversion irrespective of the state of CNV.

Prior to the minimum conversion time, CNV could be used to

select other SPI devices, such as analog multiplexers, but CNV

must be returned low before the minimum conversion time and

held low until the maximum conversion time to guarantee the

generation of the BUSY signal indicator. When the conversion

is complete, SDO goes from high impedance to low. With a

pull-up on the SDO line, this transition can be used as an

interrupt signal to initiate the data reading controlled by the

digital host. The AD7685 then enters the acquisition phase and

powers down. The data bits are then clocked out, MSB first, by

subsequent SCK falling edges. The data is valid on both SCK

edges. Although the rising edge can be used to capture the data,

a digital host using the SCK falling edge will allow a faster reading

rate provided it has an acceptable hold time. After the optional

17th SCK falling edge, or when CNV goes high, whichever is

earlier, SDO returns to high impedance.

DATA IN

IRQ

CLK

CONVERT

VIO DIGITAL HOST

02968-034

47kΩ

CNV

SCK

SDOSDI

AD7685

Figure 36. CS Mode 3-Wire with BUSY Indicator

Connection Diagram (SDI High)

SDO

D15 D14 D1 D0

DIS

SCK

1 2 3 15 16 17

SCK

SCKL

SCKH

HSDO

DSDO

CNV

CONVERSIONACQUISITION

CONV

CYC

CNVH

ACQ

ACQUISITION

SDI = 1

2968-035

Figure 37. CS Mode 3-Wire with BUSY Indicator Serial Interface Timing (SDI High)

AD7685

Rev. B | Page 20 of 28

CS MODE 4-WIRE, NO BUSY INDICATOR

This mode is usually used when multiple AD7685s are

connected to an SPI-compatible digital host.

A connection diagram example using two AD7685s is shown in

Figure 38, and the corresponding timing is given in Figure 39.

With SDI high, a rising edge on CNV initiates a conversion,

selects the CS mode, and forces SDO to high impedance. In this

mode, CNV must be held high during the conversion phase and

the subsequent data readback (if SDI and CNV are low, SDO is

driven low). Prior to the minimum conversion time, SDI could

be used to select other SPI devices, such as analog multiplexers,

but SDI must be returned high before the minimum conversion

time and held high until the maximum conversion time to

avoid the generation of the BUSY signal indicator. When the

conversion is complete, the AD7685 enters the acquisition

phase and powers down. Each ADC result can be read by

bringing low its SDI input, which consequently outputs the

MSB onto SDO. The remaining data bits are then clocked by

subsequent SCK falling edges. The data is valid on both SCK

edges. Although the rising edge can be used to capture the data,

a digital host using the SCK falling edge will allow a faster

reading rate, provided it has an acceptable hold time. After the

16th SCK falling edge, or when SDI goes high, whichever is

earlier, SDO returns to high impedance and another AD7685

can be read.

If multiple AD7685s are selected at the same time, the SDO

output pin handles this contention without damage or induced

latch-up. Meanwhile, it is recommended to keep this contention

as short as possible to limit extra power dissipation.

DATA IN

CLK

CS1

CONVERT

CS2

DIGITAL HOST

02968-036

CNV

SCK

SDOSDI

AD7685

CNV

SCK

SDOSDI

AD7685

Figure 38. CS Mode 4-Wire, No BUSY Indicator Connection Diagram

SDO D15 D14 D13 D1 D0

DIS

SCK 123 303132

HSDO

DSDO

CONVERSIONACQUISITION

CONV

CYC

ACQ

ACQUISITION

SDI(CS1)

CNV

SSDICNV

HSDICNV

14 15

SCK

SCKL

SCKH

D0 D15 D14

17 1816

SDI(CS2)

02968-037

Figure 39. CS Mode 4-Wire, No BUSY Indicator Serial Interface Timing

AD7685

Rev. B | Page 21 of 28

CS MODE 4-WIRE WITH BUSY INDICATOR

This mode is usually used when a single AD7685 is connected

to an SPI-compatible digital host, which has an interrupt input,

and it is desired to keep CNV, which is used to sample the

analog input, independent of the signal used to select the data

reading. This requirement is particularly important in

applications where low jitter on CNV is desired.

The connection diagram is shown in Figure 40, and the

corresponding timing is given in Figure 41.

With SDI high, a rising edge on CNV initiates a conversion,

selects the CS mode, and forces SDO to high impedance. In this

mode, CNV must be held high during the conversion phase and

the subsequent data readback (if SDI and CNV are low, SDO is

driven low). Prior to the minimum conversion time, SDI could

be used to select other SPI devices, such as analog multiplexers,

but SDI must be returned low before the minimum conversion

time and held low until the maximum conversion time to

guarantee the generation of the BUSY signal indicator. When

the conversion is complete, SDO goes from high impedance to

low. With a pull-up on the SDO line, this transition can be used

as an interrupt signal to initiate the data readback controlled by

the digital host. The AD7685 then enters the acquisition phase

and powers down. The data bits are then clocked out, MSB first,

by subsequent SCK falling edges. The data is valid on both SCK

edges. Although the rising edge can be used to capture the data,

a digital host using the SCK falling edge will allow a faster

reading rate provided it has an acceptable hold time. After the

optional 17th SCK falling edge, or SDI going high, whichever is

earlier, the SDO returns to high impedance.

DATA IN

IRQ

CLK

CONVERT

CS1

VIO DIGITAL HOST

02968-038

47kΩ

CNV

SCK

SDOSDI

AD7685

Figure 40. CS Mode 4-Wire with BUSY Indicator Connection Diagram

SDO D15 D14 D1 D0

DIS

SCK 1 2 3 151617

SCK

SCKL

SCKH

HSDO

DSDO

CONVERSIONACQUISITION

CONV

CYC

ACQ

ACQUISITION

SDI

CNV

SSDICNV

HSDICNV

02968-039

Figure 41. CS Mode 4-Wire with BUSY Indicator Serial Interface Timing

AD7685

Rev. B | Page 22 of 28

CHAIN MODE, NO BUSY INDICATOR

This mode can be used to daisy-chain multiple AD7685s on a

3-wire serial interface. This feature is useful for reducing

component count and wiring connections, for example, in

isolated multiconverter applications or for systems with a

limited interfacing capacity. Data readback is analogous to

clocking a shift register.

A connection diagram example using two AD7685s is shown in

Figure 42, and the corresponding timing is given in Figure 43.

When SDI and CNV are low, SDO is driven low. With SCK low,

a rising edge on CNV initiates a conversion and selects the

chain mode. In this mode, CNV is held high during the

conversion phase and the subsequent data readback. When the

conversion is complete, the MSB is output onto SDO and the

AD7685 enters the acquisition phase and powers down. The

remaining data bits stored in the internal shift register are then

clocked by subsequent SCK falling edges. For each ADC, SDI

feeds the input of the internal shift register and is clocked by the

SCK falling edge. Each ADC in the chain outputs its data MSB

first, and 16 × N clocks are required to readback the N ADCs.

The data is valid on both SCK edges. Although the rising edge

can be used to capture the data, a digital host using the SCK

falling edge will allow a faster reading rate and, consequently,

more AD7685s in the chain, provided the digital host has an

acceptable hold time. The maximum conversion rate may be

reduced due to the total readback time. For instance, with a 5 ns

digital host setup time and 3 V interface, up to eight AD7685s

running at a conversion rate of 220 kSPS can be daisy-chained

on a 3-wire port.

CLK

CONVERT

DATA IN

DIGITAL HOST

02968-040

CNV

SCK

SDOSDI

AD7685

CNV

SCK

SDOSDI

AD7685

Figure 42. Chain Mode Connection Diagram

SDO

= SDI

15 D

14 D

SCK 1 2 3 303132

SSDISCK

HSDISCK

CONVERSIONACQUISITION

CONV

CYC

ACQ

ACQUISITION

CNV

14 15

SCK

SCKL

SCKH

17 1816

SDI

= 0

SDO

15 D

14 D

13 D

15 D

HSDO

DSDO

SSCKCNV

HSCKCNV

02968-041

Figure 43. Chain Mode Serial Interface Timing

AD7685

Rev. B | Page 23 of 28

CHAIN MODE WITH BUSY INDICATOR

This mode can also be used to daisy chain multiple AD7685s on

a 3-wire serial interface while providing a BUSY indicator. This

feature is useful for reducing component count and wiring

connections, for example, in isolated multiconverter applications or

for systems with a limited interfacing capacity. Data readback is

analogous to clocking a shift register.

A connection diagram example using three AD7685s is shown

in Figure 44, and the corresponding timing is given in Figure 45.

When SDI and CNV are low, SDO is driven low. With SCK

high, a rising edge on CNV initiates a conversion, selects the

chain mode, and enables the BUSY indicator feature. In this

mode, CNV is held high during the conversion phase and the

subsequent data readback. When all ADCs in the chain have

completed their conversions, the nearend ADC (ADC C in

Figure 44) SDO is driven high. This transition on SDO can be

used as a BUSY indicator to trigger the data readback controlled

by the digital host. The AD7685 then enters the acquisition

phase and powers down. The data bits stored in the internal

shift register are then clocked out, MSB first, by subsequent

SCK falling edges. For each ADC, SDI feeds the input of the

internal shift register and is clocked by the SCK falling edge.

Each ADC in the chain outputs its data MSB first, and 16 × N + 1

clocks are required to readback the N ADCs. Although the

rising edge can be used to capture the data, a digital host also

using the SCK falling edge allows a faster reading rate and,

consequently, more AD7685s in the chain, provided the digital

host has an acceptable hold time. For instance, with a 5 ns

digital host setup time and 3 V interface, up to eight AD7685s

running at a conversion rate of 220 kSPS can be daisy-chained

to a single 3-wire port.

CLK

CONVERT

DATA IN

IRQ

DIGITAL HOST

02968-042

CNV

SCK

SDOSDI

AD7685

CNV

SCK

SDOSDI

AD7685

CNV

SCK

SDOSDI

AD7685

Figure 44. Chain Mode with BUSY Indicator Connection Diagram

SDO

= SDI

15 D

14 D

SCK 123 35 47 48

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

CNV = SDI

415

tSCK

tSCKH

tSCKL

17 3416

SDO

= SDI

15 D

14 D

13 D

15 D

tSSDISCK tHSDISCK

tHSDO

tDSDO

SDO

15 D

14 D

13 D

19 31 3218 33

15D

15 D

tDSDOSDI

tSSCKCNV

tHSCKCNV

2968-043

tDSDOSDI

Figure 45. Chain Mode with BUSY Indicator Serial Interface Timing

AD7685

Rev. B | Page 24 of 28

APPLICATION HINTS

LAYOUT

The printed circuit board (PCB) that houses the AD7685

should be designed so that the analog and digital sections are

separated and confined to certain areas of the board. The

pinout of the AD7685 with all its analog signals on the left side

and all its digital signals on the right side eases this task.

Avoid running digital lines under the device because these

couple noise onto the die, unless a ground plane under the

AD7685 is used as a shield. Fast switching signals, such as CNV

or clocks, should never run near analog signal paths. Crossover

of digital and analog signals should be avoided

At least one ground plane should be used. It could be common

or split between the digital and analog section. In the latter case,

the planes should be joined underneath the AD7685.

The AD7685 voltage reference input REF has a dynamic input

impedance and should be decoupled with minimal parasitic

inductances. This is done by placing the reference decoupling

ceramic capacitor close to, and ideally right up against, the REF

and GND pins and connected with wide, low impedance traces.

Finally, the power supplies VDD and VIO should be decoupled

with ceramic capacitors, typically 100 nF, placed close to the

AD7685 and connected using short and wide traces to provide

low impedance paths and to reduce the effect of glitches on the

power supply lines.

An example layout following these rules is shown in Figure 46

and Figure 47.

EVALUATING THE PERFORMANCE OF THE AD7685

Other recommended layouts for the AD7690 are outlined in

the documentation of the evaluation board (EVAL-AD7685CB).

The evaluation board package includes a fully assembled and

tested evaluation board, documentation, and software for

controlling the board from a PC via the universal evaluation

control board (EVAL-CONTROL BRD3).

02968-044

Figure 46. Example of Layout of the AD7685 (Top Layer)

2968-045

Figure 47. Example of Layout of the AD7685 (Bottom Layer)

AD7685

Rev. B | Page 25 of 28

TRUE 16-BIT ISOLATED APPLICATION EXAMPLE

In applications where high accuracy and isolation are required,

for example, power monitoring, motor control, and some

medical equipment, the circuit given in Figure 48, using the

AD7685 and the ADuM1402C digital isolator, provides a

compact and high performance solution.

Multiple AD7685s are daisy-chained to reduce the number of

signals to isolate. Note that the SCKOUT, which is a readback of

the AD7685’s clock, has a very short skew with the DATA

signal. This skew is the channel-to-channel matching

propagation delay of the digital isolator (tPSKCD). This allows

running the serial interface at the maximum speed of the digital

isolator (45 Mbits/s for the ADuM1402C), which would have

been otherwise limited by the cascade of the propagation delays

of the digital isolator.

The complete analog chain runs on a 5 V single supply using

the ADR391 low dropout reference voltage and the rail-to-rail

CMOS AD8618 amplifier while offering true bipolar input range.

IN– GND

REF VDD VIOSDO

VDD1, VE1

GND1

VDD2, VE2

GND2

SCK

CNV

SDI

AD7685

1/4 AD8618

1kΩ

2V REF

±10V INPUT

5V REF 5

10µF 100nF 100nF

IN– GND

REF VDD VIOSDO

SCK

CNV

SDI

AD7685

1/4 AD8618

1kΩ

2V REF

±10V INPUT

5V REF 5V

10µF 100nF

IN– GND

REF VDD VIOSDO

SCK

CNV

SDI

AD7685

1/4 AD8618

1kΩ

2V REF

±10V INPUT

5V REF 5V

10µF 100nF

IN–

IN+

GND

REF VDD VIOSDO

SCK

CNV

SDI

AD7685

1/4 AD8618

1kΩ4kΩ

4kΩ

2V REF

1kΩ1kΩ

1kΩ

100nF10µF

5V REF

2V REF

4kΩ

±10V INPUT

5V REF 5V

10µF 100nF

VIA

VIB

VOC

VOD

VOA

VOB

VIC

VID

2.7V TO 5V

100nF

DATA

SCKOUT

SCKIN

CONVERT

ADuM1402C

OUT

GND

ADR391

02968-046

Figure 48. A True 16-Bit Isolated Simultaneous Sampling Acquisition System

AD7685

Rev. B | Page 26 of 28

OUTLINE DIMENSIONS

COMPLIANT TO JEDEC STANDARDS MO-187-BA

0.23

0.08

0.80

0.60

0.40

8°

0°

0.15

0.05

0.33

0.17

0.95

0.85

0.75

SEATING

PLANE

1.10 MAX

10 6

0.50 BSC

PIN 1

COPLANARITY

0.10

3.10

3.00

2.90

3.10

3.00

2.90

5.15

4.90

4.65

Figure 49.10-Lead Micro Small Outline Package [MSOP]

(RM-10)

Dimensions shown in millimeters

022207-A

3.00

BSC SQ

INDE

ARE A

TOP VIEW

1.50

BSC SQ

EXPOSED

PAD

(BOT TOM VIEW)

1.74

1.64

1.49

2.48

2.38

2.23

0.50

BSC

0.50

0.40

0.30

PIN 1

INDICATOR

0.80

0.75

0.70 0.05 MAX

0.02 NOM

SEATING

PLANE 0.30

0.23

0.18

0.20 REF

0.80 MAX

0.55 TYP

SIDE VIEW

PADDLE CONNECTED TO GND.

THIS CONNECTION IS NOT

REQUIRED TO MEET THE

ELECTRICAL PERFORMANCES

Figure 50. 10-Terminal Quad Flat No Lead Package [QFN (LFCSP_WD)]

3 mm × 3 mm Body

(CP-10-9)

Dimensions shown in millimeters

AD7685

Rev. B | Page 27 of 28

ORDERING GUIDE

Model

Integral

Nonlinearity

No Missing

Code

Temperature

Range Package Description

Package

Option Branding

Ordering

Quantity

AD7685ACPZRL1 ±6 LSB max 15 Bits –40°C to +85°C 10-Lead QFN (LFCSP_WD) CP-10-9 C4H Tube, 50

AD7685ACPZRL71±6 LSB max 15 Bits –40°C to +85°C 10-Lead QFN (LFCSP_WD) CP-10-9 C4H Reel, 1,000

AD7685ARM ±6 LSB max 15 Bits –40°C to +85°C 10-Lead MSOP RM-10 C37 Tube, 50

AD7685ARMRL7 ±6 LSB max 15 Bits –40°C to +85°C 10-Lead MSOP RM-10 C37 Reel, 1,000

AD7685ARMZ1±6 LSB max 15 Bits –40°C to +85°C 10-Lead MSOP RM-10 C4H Tube, 50

AD7685ARMZRL71±6 LSB max 15 Bits –40°C to +85°C 10-Lead MSOP RM-10 C4H Reel, 1,000

AD7685BCPZRL1±3 LSB max 16 Bits –40°C to +85°C 10-Lead QFN (LFCSP_WD) CP-10-9 C3D Tube, 50

AD7685BCPZRL71±3 LSB max 16 Bits –40°C to +85°C 10-Lead QFN (LFCSP_WD) CP-10-9 C3D Reel, 1,000

AD7685BRM ±3 LSB max 16 Bits –40°C to +85°C 10-Lead MSOP RM-10 C01 Tube, 50

AD7685BRMRL7 ±3 LSB max 16 Bits –40°C to +85°C 10-Lead MSOP RM-10 C01 Reel, 1,000

AD7685BRMZ1±3 LSB max 16 Bits –40°C to +85°C 10-Lead MSOP RM-10 C3D Tube, 50

AD7685BRMZRL71±3 LSB max 16 Bits –40°C to +85°C 10-Lead MSOP RM-10 C3D Reel, 1,000

AD7685CCPZRL1±2 LSB max 16 Bits –40°C to +85°C 10-Lead QFN (LFCSP_WD) CP-10-9 C4J Tube, 50

AD7685CCPZRL71±2 LSB max 16 Bits –40°C to +85°C 10-Lead QFN (LFCSP_WD) CP-10-9 C4J Reel, 1,000

AD7685CRM ±2 LSB max 16 Bits –40°C to +85°C 10-Lead MSOP RM-10 C00 Tube, 50

AD7685CRMRL7 ±2 LSB max 16 Bits –40°C to +85°C 10-Lead MSOP RM-10 C00 Reel, 1,000

AD7685CRMZ1±2 LSB max 16 Bits –40°C to +85°C 10-Lead MSOP RM-10 C4J Tube, 50

AD7685CRMZRL71±2 LSB max 16 Bits –40°C to +85°C 10-Lead MSOP RM-10 C4J Reel, 1,000

EVAL-AD7685CB2 Evaluation Board

EVAL-AD7685CBZ1, 2 Evaluation Board

EVAL-CONTROL BRD23 Controller Board

EVAL-CONTROL BRD33 Controller Board

1 Z = RoHS Compliant Part.

2 This board can be used as a standalone evaluation board or in conjunction with the EVAL-CONTROL BRDx for evaluation/demonstration purposes.

3 These boards allow a PC to control and communicate with all Analog Devices evaluation boards ending in the CB designators.

AD7685

Rev. B | Page 28 of 28

NOTES

registered trademarks are the property of their respective owners.

D02968-0-3/07(B)