AD7946BCPWP - Analog Devices

14-Bit, 500 kSPS PulSAR™

ADC in MSOP/QFN

Preliminary Technical Data

AD7946

Rev Pr D

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FEATURES

14-bit resolution with no missing codes

Throughput: 500 kSPS

INL: ±0.4 LSB typ, ±1 LSB max (±0.0061 % of FSR)

S/(N + D): 85 dB @ 20 kHz

THD: −100 dB @ 20 kHz

Pseudo-differential analog input range

0 V to VREF with VREF up to VDD

No pipeline delay

Single-supply 5V operation with

1.8 V/2.5 V/3 V/5 V logic interface

Serial interface SPI®/QSPI™/µWire/DSP compatible

Daisy chain multiple ADCs and BUSY indicator

Power dissipation

3.3 mW @ 5 V/100 kSPS,

3.3 µW @ 5 V/100 SPS

Stand-by current: 1 nA

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

QFN (LFCSP), 3 mm × 3 mm same space as SOT-23

Pin-for-pin compatible with the 16-Bit AD7686

APPLICATIONS

Battery-powered equipment

Data acquisition

Instrumentation

Medical instruments

Process control

Table 1. MSOP, QFN (LFCSP)/SOT-23 14 and16-Bit ADC

Type 100 kSPS 250 kSPS 500 kSPS

16-Bit True

Differential

AD7684 AD7687 AD7688

16-Bit Pseudo

Differential/Unipolar

AD7683 AD7685

AD7694

AD7686

16-Bit Unipolar AD7680

14-Bit Pseudo

Differential/Unipolar

AD7942

AD7946

14-Bit Unipolar AD7940

APPLICATION DIAGRAM

AD7946

REF

GND

VDD

IN+

IN–

VIO

SDI

SCK

SDO

CNV

1.8 TO VDD

3- OR 4-WIRE INTERFACE

(SPI, DAISY CHAIN, CS)

0.5 TO 5V 5V

0 TO VREF

Figure 1.

GENERAL DESCRIPTION

The AD7946 is a 14-bit, charge redistribution successive

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

from a single 5V power supply, VDD. It contains a low power,

high speed, 14-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.

Its power 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 and 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 AD7946 is housed in a 10-lead MSOP or a 10-lead QFN

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

AD7946 Preliminary Technical Data

Rev Pr D | Page 2 of 27

TABLE OF CONTENTS

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

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

Absolute Maximum Ratings............................................................ 6

ESD Caution.................................................................................. 6

Pin Configuration and Function Descriptions............................. 7

Terminology ...................................................................................... 8

Typical Performance Characteristics ............................................. 9

Circuit Information.................................................................... 12

Converter Operation.................................................................. 12

Typical Connection Diagram ................................................... 13

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

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

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

Evaluating the AD7946’s Performance.................................... 24

Outline Dimensions....................................................................... 25

Ordering Guide .......................................................................... 26

REVISION HISTORY

5/04—Revision D: Preliminary

Preliminary Technical Data AD7946

Rev Pr D | Page 3 of 27

SPECIFICATIONS

VDD = 4.5 V to 5.5 V, VIO = 2.3 V to VDD, VREF = VDD, TA = –40°C to +85°C, unless otherwise noted.

Table 2.

Parameter Conditions Min Typ Max Unit

RESOLUTION 14 Bits

ANALOG INPUT

Voltage Range IN+ − IN− 0 VREF V

Absolute Input Voltage IN+ −0.1 VDD + 0.1 V

IN− −0.1 0.1 V

Analog Input CMRR fIN = 250 kHz 65 dB

Leakage Current at 25°C Acquisition Phase 1 nA

Input Impedance

ACCURACY

No Missing Codes 14 Bits

Differential Linearity Error −0.7 ±0.25 +0.7 LSB1

Integral Linearity Error −1 ±0.4 +1 LSB

Transition Noise REF = VDD = 5 V 0.33 LSB

Gain Error2, TMIN to TMAX ±TBD ±TBD LSB

Gain Error Temperature Drift ±TBD ppm/°C

Offset Error2, TMIN to TMAX ±TBD ±TBD mV

Offset Temperature Drift ±TBD ppm/°C

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

THROUGHPUT

Conversion Rate 0 500 kSPS

Transient Response Full-Scale Step 400 ns

AC ACCURACY

Signal-to-Noise fIN = 20 kHz, VREF = 5 V 83 85 dB3

Spurious-Free Dynamic Range fIN = 20 kHz −100 dB

Total Harmonic Distortion fIN = 20 kHz −100 dB

Signal-to-(Noise + Distortion) fIN = 20 kHz, VREF = 5 V 83 85 dB

fIN = 20 kHz, VREF = 5 V, −60 dB

Input

25 dB

Intermodulation Distortion4 TBD dB

1 LSB means least significant bit. With the 5 V input range, one LSB is 305.2 µV.

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

3All 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.

AD7946 Preliminary Technical Data

Rev Pr D | Page 4 of 27

VDD = 4.5 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 500 kSPS, REF = 5 V TBD µA

SAMPLING DYNAMICS

−3 dB Input Bandwidth 9 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 14 Bits Straight Binary

Pipeline Delay Conversion Results Available Immediately

after Completed Conversion

VOL I

SINK= +500 µA 0.4 V

VOH I

SOURCE= −500 µA VIO − 0.3 V

POWER SUPPLIES

VDD Specified Performance 4.5 5.5 V

VIO Specified Performance 2.3 VDD + 0.3 V

VIO Range 1.8 VDD + 0.3 V

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

Power Dissipation VDD = 5 V, 100 SPS Throughput 3.3 µW

VDD = 5 V, 100 kSPS Throughput 3.3 5 mW

VDD = 5 V, 500 kSPS Throughput 25 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 Analog Devices for extended temperature range.

Preliminary Technical Data AD7946

Rev Pr D | Page 5 of 27

TIMING SPECIFICATIONS

−40°C to +85°C, VDD = 4.5 V to 5.5 V, VIO = 2.3 V to 5.5 V or VDD + 0.3 V, whichever is the lowest, unless otherwise stated.

Table 4. 1

Symbol Min Typ Max Unit

Conversion Time: CNV Rising Edge to Data Available tCONV 0.5 1.6 µs

Acquisition Time tACQ 400 ns

Time between Conversions tCYC 2 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 19 ns

VIO above 3 V 20 ns

VIO above 2.7 V 21 ns

VIO above 2.3 V 22 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 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

VIO above 4.5 V 15 ns

VIO above 2.3 V 26 ns

1 See Figure 2 and Figure 3 for load conditions.

AD7946 Preliminary Technical Data

Rev Pr D | Page 6 of 27

ABSOLUTE MAXIMUM RATINGS

Table 5.

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 Range

Vapor Phase (60 sec) 215°C

Infrared (15 sec) 220°C

1 See the Analog Input 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

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.

500µAI

1.4V

TO SDO C

50pF

02968-PrH-002

Figure 2. 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

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.

02968-PrH-003

Figure 3. Voltage Reference Levels for Timing

Preliminary Technical Data AD7946

Rev Pr D | Page 7 of 27

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

AD7946

REF

VDD

IN+

IN–

GND

VIO

SDI

SCK

SDO

CNV

Figure 4.10-Lead MSOP and QFN (LFCSP) Pin Configuration

Table 6. Pin Function Descriptions

Pin No. Mnemonic Type1 Function

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 IN−. The voltage range, i.e., the difference between IN+ and IN−, is 0 V to

VREF.

4 IN− AI Analog Input Ground Sense. To be connected 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 14 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

AD7946 Preliminary Technical Data

Rev Pr D | Page 8 of 27

TERMINOLOGY

Integral Nonlinearity Error (INL)

Linearity error 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 1/2 LSB

before the first code transition. Positive full scale is defined as a

level 1 1/2 LSB beyond the last code transition. The deviation is

measured from the middle of each code to the true straight line

(Figure 21).

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 1/2 LSB above analog

ground (152.6 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 1 1/2 LSB below the nominal full scale

(4.999542 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 has been 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 S/(N+D) by the following formula

[

]

(

)

)02.6/76.1/

−

DNSENOB

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) Ratio (S/[N+D])

S/(N+D) 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 S/(N+D) 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 was applied.

Preliminary Technical Data AD7946

Rev Pr D | Page 9 of 27

TYPICAL PERFORMANCE CHARACTERISTICS

Figure 5. Integral Nonlinearity vs. Code

Figure 6. Histogram of a DC Input at the Code Center

Figure 7. FFT Plot

Figure 8. Differential Nonlinearity vs. Code

Figure 9. Histogram of a DC Input at the Code Center

Figure 10. S/[N + D] vs. Frequency

AD7946 Preliminary Technical Data

Rev Pr D | Page 10 of 27

Figure 11. SNR vs. Temperature

Figure 12. THD vs. Frequency

Figure 13. THD, SFDR vs. Temperature

Figure 14. SNR and THD vs. Input Level

Figure 15. Operating Currents vs. Supply

Figure 16. Power-Down Currents vs. Temperature

Preliminary Technical Data AD7946

Rev Pr D | Page 11 of 27

Figure 17. Operating Currents vs. Temperature

Figure 18. Offset and Gain Error vs. Temperature

Figure 19. tDSDO vs. Capacitance Load and Supply

AD7946 Preliminary Technical Data

Rev Pr D | Page 12 of 27

SW+MSB

4,096C

IN+

LSB

COMP CONTROL

LOGIC

SWITCHES CONTROL

BUSY

OUTPUT CODE

CNV

REF

GND

IN–

4C 2C C C8,192C

SW–MSB

4,096C

LSB

4C 2C C C8,192C

Figure 20. ADC Simplified Schematic

CIRCUIT INFORMATION

The AD7946 is a fast, low power, single-supply, precise 14-bit

ADC using a successive approximation architecture.

The AD7946 is capable of converting 500,000 samples per

second (500 kSPS) and powers down between conversions.

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

3.3µW, ideal for battery-powered applications.

The AD7946 provides the user with an on-chip track-and-hold

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

ideal for multiple multiplexed channel applications.

The AD7946 is specified from 4.5 V to 5.5 V, and can be

interfaced to either 5 V, 3.3 V, 2.5 V, or 1.8 V digital logic. 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 16 Bit ADC AD7686.

CONVERTER OPERATION

The AD7946 is a successive approximation ADC based on a

charge redistribution DAC. Figure 20 shows the simplified

schematic of the ADC. The capacitive DAC consists of two

identical arrays of 14 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.

Thus, 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/16384).

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

in order to bring the comparator back into a balanced

condition. After the completion of this process, the part returns

to the acquisition phase and the control logic generates the

ADC output code and a BUSY signal indicator.

Because the AD7946 has an on-board conversion clock, the

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

Preliminary Technical Data AD7946

Rev Pr D | Page 13 of 27

Transfer Functions

The ideal transfer characteristic for the AD7946 is shown in

Figure 21 and Table 7.

000...000

000...001

000...010

111...101

111...110

111...111

ADC CODE (STRAIGHT BINARY)

ANALOG INPUT

+FS – 1.5 LSB

+FS – 1 LSB

–FS + 1 LSB

–FS

–FS + 0.5 LSB

02968-PrH-006

Figure 21. ADC Ideal Transfer Function

Table 7. Output Codes and Ideal Input Voltages

Description

Analog Input

VREF = 5 V Digital Output Code Hexa

FSR – 1 LSB 4.999695 V 3FFF1

Midscale + 1 LSB 2.500305 V 2001

Midscale 2.5 V 2000

Midscale – 1 LSB 2.499695 V 1FFF

–FSR + 1 LSB 305.2 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).

TYPICAL CONNECTION DIAGRAM

Figure 22 shows an example of the recommended connection

diagram for the AD7946 when multiple supplies are available.

AD7946

REF

GND

VDD

IN–

IN+ VIOSDI

SCK

SDO

CNV

3- OR 4-WIRE INTERFACE (NOTE 5)

100nF

100nF 5V

10µF

(NOTE 2)

≥7V

≤–2V

1.8V TO VDD

REF

0 TO VREF

33Ω

2.7nF

(NOTE 3)

(NOTE 4)

(NOTE 1)

NOTE 1: SEE REFERENCE SECTION FOR REFERENCE SELECTION.

NOTE 2: C

REF

IS USUALLY

NOTE 3: SEE DRIVER AMPLIFIER CHOICE SECTION.

NOTE 4: OPTIONAL FILTER. SEE ANALOG INPUT SECTION.

NOTE 5: SEE DIGITAL INTERFACE FOR MOST CONVENIENT INTERFACE MODE.

A 10µF CERAMIC CAPACITOR (X5R).

Figure 22. Typical Application Diagram with multiple supplies

AD7946 Preliminary Technical Data

Rev Pr D | Page 14 of 27

Analog Input

Figure 23 shows an equivalent circuit of the input structure of

the AD7946.

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 become

forward-biased and start conducting current. However, 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

VDD

Figure 23. 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 24, 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.

Figure 24. Analog Input CMRR vs. Frequency

During the acquisition phase, the impedance of the analog

input IN+ can be modeled as a parallel combination of

capacitor C1 and the network formed by the series connection

of R1 and C2. C1 is primarily the pin capacitance. R1 is typically

600 Ω and is a lumped component made up of some serial

resistors and the on resistance of the switches. C2 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 C1. R1 and C2 make a 1-pole, low-pass

filter that reduces undesirable aliasing effect and limits the

noise.

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

AD7946 can be driven directly. Large source impedances

significantly affect the ac performance, especially total

harmonic distortion (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 25.

Figure 25. THD vs. Analog Input Frequency and Source Resistance

Preliminary Technical Data AD7946

Rev Pr D | Page 15 of 27

Driver Amplifier Choice

Although the AD7946 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 in order to preserve the SNR and

transition noise performance of the AD7946. Note that the

AD7946 has a noise much lower than most of the other 14-

bit ADCs and, therefore, can be driven by a noisier op amp

while preserving the same or better system performance.

The noise coming from the driver is filtered by the AD7946

analog input circuit 1-pole, low-pass filter made by R1 and

C2 or by the external filter, if one is used.

• For ac applications, the driver needs to have a THD

performance suitable to that of the AD7946. Figure 12

gives the THD versus frequency that the driver should

exceed.

• For multichannel multiplexed applications, the driver

amplifier and the AD7946 analog input circuit must be

able to settle for a full-scale step of the capacitor array at a

14-bit level (0.006%). 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 14-bit level

and should be verified prior to driver selection.

Table 8. Recommended Driver Amplifiers.

Amplifier Typical Application

AD8021 Very low noise and high frequency

AD8022 Low noise and high frequency

OP184 Low power, low noise, and low frequency

AD8605, AD8615 5 V single-supply, low power

AD8519 Small, low power and low frequency

AD8031 High frequency and low power

Voltage Reference Input

The AD7946 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, e.g., 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.

AD7946 Preliminary Technical Data

Rev Pr D | Page 16 of 27

Power Supply

The AD7946 is specified 4.5 V to 5.5 V. 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 supplies needed, the

VIO and VDD can be tied together. The AD7946 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 26,

which represents PSRR over frequency.

Figure 26. PSRR vs. Frequency

The AD7946 powers down automatically at the end of each

conversion phase and, therefore, the power scales linearly with

the sampling rate as shown in see Figure 27. This makes the

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

battery-powered applications.

Figure 27. Operating Currents vs. Sampling Rate

Supplying the ADC from the Reference

For simplified applications, the AD7946, with its low operating

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

shown in Figure 28. 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 28.

AD8031

AD7946

VIOREF VDD

10µF 1µF

10Ω

10kΩ

1µF

(NOTE 1)

NOTE 1: OPTIONAL REFERENCE BUFFER AND FILTER

Figure 28. Example of Application Circuit

Preliminary Technical Data AD7946

Rev Pr D | Page 17 of 27

DIGITAL INTERFACE

Though the AD7946 has a reduced number of pins, it offers

flexibility in its serial interface modes.

The AD7946, when in CS mode, is compatible with SPI, QSPI,

digital hosts, and DSPs, e.g., 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 AD7946, 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 mode, the AD7946 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 (Figure 32 and Figure 36).

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

(Figure 40).

AD7946 Preliminary Technical Data

Rev Pr D | Page 18 of 27

CS MODE 3-Wire, No BUSY Indicator

This mode is usually used when a single AD7946 is connected

to an SPI compatible digital host. The connection diagram is

shown in Figure 29 and the corresponding timing is given in

Figure 30.

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 the conversion is complete, the AD7946

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 also using the SCK falling edge

will allow a faster reading rate provided it has an acceptable

hold time. After the 14th SCK falling edge or when CNV goes

high, whichever is earlier, SDO returns to high impedance.

CNV

SCK

SDOSDI DATA IN

CLK

CONVERT

IO DIGITAL HOST

AD7946

Figure 29. CS Mode 3-Wire, No BUSY Indicator

Connection Diagram (SDI High)

04656-PrC-008

SDO D13 D12 D11 D1 D0

tDIS

SCK 1 2 3 12 13 14

tSCK

tSCKL

tSCKH

tHSDO

tDSDO

CNV

CONVERSIONACQUISITION

tCONV

tCYC

ACQUISITION

DI = 1

tCNVH

tACQ

tEN

Figure 30. CS Mode 3-Wire, No BUSY Indicator Serial Interface Timing (SDI High)

Preliminary Technical Data AD7946

Rev Pr D | Page 19 of 27

CS Mode 3-Wire with BUSY Indicator

This mode is usually used when a single AD7946 is connected

to an SPI compatible digital host having an interrupt input.

The connection diagram is shown in Figure 31 and the

corresponding timing is given in Figure 32.

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 AD7946 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 also using the SCK falling edge will allow a faster

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

optional 15th SCK falling edge, or when CNV goes high,

whichever is earlier, SDO returns to high impedance.

CNV

SCK

SDOSDI DATA IN

IRQ

CLK

CONVERT

VIO DIGITAL HOST

AD7946

47kΩ

Figure 31. CS Mode 3-Wire with BUSY Indicator

Connection Diagram (SDI High)

04656-PrC-010

SDO

D13 D12 D1 D0

tDIS

SCK

1 2 3 13 14 15

tSCK

tSCKL

tSCKH

tHSDO

tDSDO

CNV

CONVERSIONACQUISITION

tCONV

tCYC

tCNVH

tACQ

ACQUISITION

SDI = 1

Figure 32. CS Mode 3-Wire with BUSY Indicator Serial Interface Timing (SDI High)

AD7946 Preliminary Technical Data

Rev Pr D | Page 20 of 27

CS Mode 4-Wire, No BUSY Indicator

This mode is usually used when multiple AD7946s are

connected to an SPI compatible digital host.

A connection diagram example using two AD7946s is shown in

Figure 33 and the corresponding timing is given in Figure 34.

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 AD7946 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 driving edges. The data is valid on both SCK

edges. Although the nondriving edge can be used to capture the

data, a digital host also using the SCK falling edge will allow a

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

the 14th SCK falling edge, or when SDI goes high, whichever is

earlier, SDO returns to high impedance and another AD7946

can be read.

CNV

SCK

SDOSDI

DATA IN

CLK

CS1

CONVERT

CS2

DIGITAL HOST

AD7946

CNV

SCK

SDOSDI

AD7946

Figure 33. CS Mode 4-Wire, No BUSY Indicator Connection Diagram

04656-PrC-012

SDO D13 D12 D11 D1 D0

DIS

SCK 123 262728

HSDO

DSDO

CONVERSIONACQUISITION

CONV

CYC

ACQ

ACQUISITION

SDI(CS1)

CNV

SSDICNV

HSDICNV

12 13

SCK

SCKL

SCKH

D0 D13 D12

15 1614

SDI(CS2)

Figure 34. CS Mode 4-Wire, No BUSY Indicator Serial Interface Timing

Preliminary Technical Data AD7946

Rev Pr D | Page 21 of 27

CS Mode 4-Wire with BUSY Indicator

This mode is usually used when a single AD7946 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 35 and the

corresponding timing is given in Figure 36.

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 AD7946 then enters the acquisition phase

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

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

SCK edges. Although the rising edge can be used to capture the

data, a digital host also using the SCK falling edge will allow a

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

the optional 15th SCK falling edge, or SDI going high,

whichever is earlier, the SDO returns to high impedance.

CNV

SCK

SDOSDI DATA IN

IRQ

CLK

CONVERT

CS1

VIO DIGITAL HOST

AD7946

47kΩ

Figure 35. CS Mode 4-Wire with BUSY Indicator Connection Diagram

04656-PrC-014

SDO D13 D12 D1 D0

DIS

SCK 1 2 3 13 14 15

SCK

SCKL

SCKH

HSDO

DSDO

CONVERSION

CQUISITION

CONV

CYC

ACQ

ACQUISITION

SDI

CNV

SSDICNV

HSDICNV

Figure 36. CS Mode 4-Wire with BUSY Indicator Serial Interface Timing

AD7946 Preliminary Technical Data

Rev Pr D | Page 22 of 27

Chain Mode, No BUSY Indicator

This mode can be used to daisy chain multiple AD7946s on a 3-

wire serial interface. This feature is useful for reducing

component count and wiring connections, e.g., 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 AD7946s is shown in

Figure 37 and the corresponding timing is given in Figure 38.

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

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

mode, and disables the BUSY indicator. 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 AD7946 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 14 × 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 also using the SCK falling edge will allow a faster

reading rate and, consequently more AD7946s 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 set-up time

and 3 V interface, up to five AD7946s running at a conversion

rate of 333 kSPS can be daisy-chained on a 3-wire port.

CNV

SCK

SDOSDI

CLK

CONVERT

DATA IN

DIGITAL HOST

AD7946

CNV

SCK

SDOSDI

AD7946

Figure 37. Chain Mode, No BUSY Indicator Connection Diagram

04656-PrC-016

SDO

= SDI

13 D

12 D

SCK 123 262728

SSDISCK

HSDISC

CONVERSIONACQUISITION

CONV

CYC

ACQ

ACQUISITION

CNV

12 13

SCK

SCKL

SCKH

15 1614

SDI

= 0

SDO

13 D

12 D

11 D

13 D

HSDO

DSDO

SSCKCNV

HSCKCNV

Figure 38. Chain Mode, No BUSY Indicator Serial Interface Timing

Preliminary Technical Data AD7946

Rev Pr D | Page 23 of 27

Chain Mode with BUSY Indicator

This mode can also be used to daisy chain multiple AD7946s on

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

feature is useful for reducing component count and wiring

connections, e.g., 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 AD7946s is shown

in Figure 39 and the corresponding timing is given in Figure 40.

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 39) SDO will be driven high. This transition on SDO can

be used as a BUSY indicator to trigger the data readback

controlled by the digital host. The AD7946 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 14 × 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 will allow a faster

reading rate and, consequently more AD7946s in the chain,

provided the digital host has an acceptable hold time. For

instance, with a 5 ns digital host set-up time and 3 V interface,

up to five AD7946s running at a conversion rate of 333 kSPS

can be daisy-chained to a single 3-wire port.

CNV

SCK

SDOSDI

CLK

CONVERT

DATA IN

IRQ

DIGITAL HOST

AD7946

CNV

SCK

SDOSDI

AD7946

CNV

SCK

SDOSDI

AD7946

Figure 39. Chain Mode with BUSY Indicator Connection Diagram

04656-PrC-018

SDO

= SDI

13 D

12 D

SCK 123 35 41 42

CONVERSION

ACQUISITION

CONV

CYC

ACQ

ACQUISITION

CNV = SDI

413

SCK

SCKH

SCKL

15 3113

SDO

= SDI

13 D

12 D

11 D

13 D

SSDISCK

HSDISC

HSDO

DSDO

SDO

13 D

12 D

11 D

17 27 2816 29

13D

13 D

DSDOSDI

SSCKCNV

HSCKCNV

Figure 40. Chain Mode with BUSY Indicator Serial Interface Timing

AD7946 Preliminary Technical Data

Rev Pr D | Page 24 of 27

APPLICATION HINTS

LAYOUT

The printed circuit board that houses the AD7946 should be

designed so that the analog and digital sections are separated

and confined to certain areas of the board. The pinout of the

AD7946 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

AD7946 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 such a case, it

should be joined underneath the AD7946s.

The AD7946 voltage reference input REF has a dynamic input

impedance and should be decoupled with minimal parasitic

inductances. That is done by placing the reference decoupling

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

and GND pins and connect these pins with wide, low

impedance traces.

Finally, the power supply VDD and VIO of the AD7946 should

be decoupled with ceramic capacitors, typically 100 nF, placed

close to the AD7946 and connected using short and large traces

to provide low impedance paths and reduce the effect of glitches

on the power supply lines.

An example of layout following these rules is shown in Figure

41 and Figure 42.

EVALUATING THE AD7946’S PERFORMANCE

Other recommended layouts for the AD7946 are outlined in the

evaluation board for the AD7946 (EVAL-AD7946). The

evaluation board package includes a fully assembled and tested

evaluation board, documentation, and software for controlling

the board from a PC via the EVAL-CONTROL BRD2.

Figure 41. Example of Layout of the AD7946 (Top Layer)

Figure 42. Example of Layout of the AD7946 (Bottom Layer)

Preliminary Technical Data AD7946

Rev Pr D | Page 25 of 27

OUTLINE DIMENSIONS

0.23

0.08

0.80

0.60

0.40

8°

0°

0.15

0.00 0.27

0.17

0.95

0.85

0.75

SEATING

PLANE

1.10 MAX

10 6

0.50 BSC

3.00 BSC

4.90 BSC

PIN 1

COPLANARITY

0.10

COMPLIANT TO JEDEC STANDARDS MO-187BA

Figure 43.10-Lead Micro Small Outline Package [MSOP]

(RM-10)

Dimensions shown in millimeters

3.00

BSC SQ

INDEX

AREA

TOP VIEW

1.50

BCS SQ

EXPOSED PAD

(BOTTOM VIEW)

1.74

1.64

1.49

2.48

2.38

2.23

10 6

0.50 BSC 0.50

0.40

0.30

0.80

0.75

0.70 0.05 MAX

0.02 NOM

EATING

PLANE 0.30

0.23

0.18

0.20 REF

0.80 MAX

0.55 TYP

PIN 1

INDICATOR

PADDLE CONNECTED TO GND.

THIS CONNECTION IS NOT

REQUIRED TO MEET THE

ELECTRICAL PERFORMANCES

Figure 44. 10-Lead Lead Frame Chip Scale Package [ QFN (LFCSP)]

3 mm × 3 mm Body

(CP-10)

Dimensions shown in millimeters

AD7946 Preliminary Technical Data

Rev Pr D | Page 26 of 27

ORDERING GUIDE

Models Temperature Range Package (Option) Transport Media, Quantity Brand

AD7946BRM –40°C to +85°C MSOP (RM-10) Tube, 50 C1E

AD7946BRMRL7 –40°C to +85°C MSOP (RM-10) Reel, 1,000 C1E

AD7946BCPWP –40°C to +85°C QFN [LFCSP] (CP-10) Waffle pack, 50 C1E

AD7946BCPRL7 –40°C to +85°C QFN [LFCSP] (CP-10) Reel, 1,500 C1E

EVAL-AD7946CB1 Evaluation Board

EVAL-CONTROL BRD22 Controller Board

EVAL-CONTROL BRD32 Controller Board

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

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

Preliminary Technical Data AD7946

Rev Pr D | Page 27 of 27

NOTES

registered trademarks are the property of their respective owners.

PR04656-0-8/04(PrD)