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

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