Analog Devices: CN0261 Reference Design for Optimizing AC Performance in an 18-bit, 250 kSPS, PulSAR Measurement Circuit

atomar — Sun, 27 Oct 2013 06:01:52 GMT

Current Revision posted to Documents by atomar on 10/27/2013 6:01:52 AM

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Kit Overview

Choosing complementary products for high performance ADCs can be a challenge. The EVAL-CN0261-SDPZEVAL-CN0261-SDPZ circuit shows a complete front end solution for the 18-bit, 250 kSPS PulSAR® ADC, which is optimized for ac performance.

The circuit centers on the AD7691AD7691, which is a low power ADC (1.35 mW @ 2.5 V and 100 kSPS) from the PulSAR family. The ADC is driven directly from the AD8597AD8597 ultralow distortion, ultralow noise amplifier, and the ADC’s reference is the ultralow noise 5 V ADR435ADR435. The circuit achieves 101 dB SNR and 118 dB THD with a 1 kHz input tone.

The heart of this circuit is the AD7691AD7691, an 18-bit, 250 kSPS charge redistribution, successive approximation, analog-todigital converter (ADC) that operates from a single power supply.

It contains a low power, high speed, 18-bit sampling ADC with no missing codes, an internal conversion clock, and a versatile serial interface port. On the CNV rising edge, it samples the voltage difference between the IN+ and IN- pins. The voltages on these pins swing in opposite phases between 0 V and REF. The reference voltage, REF, is applied externally and can be set up to the supply voltage. The AD7691AD7691 power scales linearly with throughput.

The ADC is driven from the AD8597AD8597 (4.8 mA/amplifier), which is a low noise, low distortion operational amplifier ideal for use as an input buffer. The low noise of 1.1 nV/√Hz and low harmonic distortion of less than −120 dB at audio frequencies give the AD8597ARZAD8597ARZ the wide dynamic range necessary for preamplifiers in audio, medical, and instrumentation applications. The excellent slew rate of 14 V/μs and 10 MHz gain bandwidth product make it highly suitable for medical applications.

The voltage reference used in this application is the ADR435ADR435, which is one of a family of XFET® voltage references featuring low noise, high accuracy, and low temperature drift performance.

The AD7691 was interfaced to the SDP-B System Demonstration Platform board and the ADC SPIcompatible serial interface was connected to the DSP SPORT interface VIO supply. The SDP Board does not come as part of the evaluation kit and can be ordered separately (EVAL-SDP-CB1ZEVAL-SDP-CB1Z).

Key Applications: Electronic Test & Measurement

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Development Tools

EVAL-CN0261-SDPZEVAL-CN0261-SDPZ circuit evaluation board
System Demonstration Board (EVAL-SDP-CB1ZEVAL-SDP-CB1Z)

- Blackfin Processor ADSP-BF527
- USB 2.0 Interface to PC
- JTAG header footprint
- 32MB SDRAM
- 2 x Small Footprint 120 Pin Connectors
- Configurable Peripheral Interfaces - SPI - SPORT - I2C - GPIOs - Parallel - Timers

Function generator, Audio Precision SYS-2522
External 10 nF ceramic filter capacitor
Power supplies: +8 V @ 50 mA, -2 V @ 50 mA, +5 V @ 500 mA.
PC with a USB port and Windows® XP or Windows Vista® (32-bit), or Windows® 7 (32-bit)

Technical Documents

Learning Center

Type	Description
User Manual	ADI: User Manual for CN-0261 Optimizing AC Performance in an 18-bit, 250 kSPS, PulSAR Measurement Circuit
User Guide	ADI: User Guide for EVAL-SDP-CB1Z
Datasheet	ADI: Datasheet for AD7691: 18-Bit, 1.5 LSB INL, 250 kSPS PulSAR Differential ADC in MSOP/QFN (Rev C)
Datasheet	ADI: Datasheet for AD8597/85899: Single and Dual, Ultralow Distortion, Ultralow Noise Op Amps (Rev C)
Datasheet	ADI: Datasheet for ADR430/ADR431/ADR433/ADR434/ADR435/ADR439: Ultralow Noise XFET Voltage References with Current Sink and Source Capability (Rev J)

Design Elements

Type	Description
Schematics	ADI: Schematics for EVAL-CN0261-SDPZ Evaluation Board
Layout	ADI: Layout for EVAL-CN0261-SDPZ
BOM	ADI: Bill Of Material (BOM) for EVAL-CN0261-SDPZ Evaluation Board

Video

www.youtube.com/watch

Kit Features

Key Features for EVAL-CN0261-SDPZ Evaluation Board are as below:

The circuit centers on the AD7691AD7691 PulSAR Differential ADC
- 18-bit resolution with no missing codes, Throughput: 250 kSPS
- INL: ±0.75 LSB typical, ±1.5 LSB maximum (±6 ppm of FSR)
- Dynamic range: 102 dB typical @ 250 kSPS
- Oversampled dynamic range: 125 dB @1 kSPS
- Noise-free code resolution: 20 bits @ 1 kSPS
- Effective resolution: 22.7 bits @ 1 kSPS
- SINAD: 101.5 dB typical @ 1 kHz
- THD: −125 dB typical @ 1 kHz
- True differential analog input range: ±VREF
  - 0 V to VREF with VREF up to VDD on both inputs
- No pipeline delay
The ADC is driven from the AD8597AD8597 (4.8 mA/amplifier)
- Low noise: 1.1 nV/√Hz at 1 kHz
- Low distortion: −120 dB THD @ 1 kHz
- Input noise, 0.1 Hz to 10 Hz: <76 nV p-p
- Slew rate: 14 V/μs, Wide bandwidth: 10 MHz
- Supply current: 4.8 mA/amp typical
- Low offset voltage: 10 μV typical, CMRR: 120 dB
- Unity-gain stable, ±15 V operation
ADC’s reference is the ultralow noise 5 V ADR435ADR435
- Low noise (0.1 Hz to 10.0 Hz): 3.5 µV p-p @ 2.5 V output
- No external capacitor required
- Low temperature coefficient A Grade: 10 ppm/°C maximum, B Grade: 3 ppm/°C maximum
- Load regulation: 15 ppm/mA
- Line regulation: 20 ppm/V
- Wide operating range: ADR435: 7.0 V to 18 V
- High output source and sink current: +30 mA and −20 mA

Kit Contents

The Analog Devices EVAL-CN0261-SDPZEVAL-CN0261-SDPZ kit is supplied with below contents:

Evaluation Board

Tags: adr435, cn0261, eval-sdp-cb1z, ad8597, ad7691, eval-cn0261-sdpz