18-Bit, 250 kSPS Data Acquisition System Optimized for AC Performance
The circuit centers on the AD7691, 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 AD8597 ultralow distortion, ultralow noise amplifier, and the ADC’s reference is
the ultralow noise 5 V ADR435. The circuit achieves 101 dB SNR and 118 dB THD with a 1 kHz input tone.
Figure 1. High Performance, 18-Bit ADC Front End (Simplified Schematic: All Connections and Decoupling Not Shown)
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 AD7691 power scales linearly with throughput.
For the experiments carried out for this circuit note, the
AD7691was interfaced to the SDP (System Demonstration
Platform board, EVAL-SDP-CB1Z), and the ADC SPIcompatible
serial interface was connected to the DSP SPORT
interface VIO supply.
The AD7691 is housed in a 10-lead MSOP or a 10-lead QFN (LFCSP).
The ADC is driven from the AD8597 (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 AD8597 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 AD8597 can be operated on supply voltages up to ±15 V. In
the circuit, supply voltages of +8 V and −2 V were chosen in
order to minimize power dissipation.
The AD8597 is available in 8-lead SOIC and LFCSP packages. The 180 Ω resistors and the 2.7 nF capacitors form a single-pole 327 kHz low-pass filter to further reduce noise.
The voltage reference used in this application is the ADR435, which is one of a family of XFET® voltage references featuring low noise, high accuracy, and low temperature drift performance. Using patented temperature drift curvature correction and XFET (eXtra implanted junction FET) technology, voltage change vs. temperature is minimized.
The ADR43x family can source up to 30 mA of output current and sink up to 20 mA. It also has a trim terminal to adjust the output voltage over a 0.5% range without compromising performance.
The ADR435 is available in either an 8-lead MSOP or an 8-lead narrow SOIC package.
The dynamic performance of the above configuration is shown in Figure 2 and Figure 3 and is summarized below:
SNR = 101.02dB
THD = 118.44 dB
SINAD = 100.94 dB
Dynamic Range = 101.5 dB
Figure 2. Evaluation Board Software Output Screen Capture
Figure 3. FFT Details for 1 kHz Tone Sampling at 250 kSPS
The AD7986 (2 MSPS) is available in 20-lead 4 mm × 4 mm LFCSP (QFN).
The AD8599 op amp is a dual version of the AD8597 and can be used in the circuit, if desired.
The ADA4941-1 is optimized for driving differential input 18-bit ADCs and has 2.2 mA/amplifier and 10.2 nV/√Hz noise.
A summary of PulSAR ADCs and recommended drivers can be found at PulSAR® Analog-to-Digital Converter Evaluation Kit.
Other reference voltages within the ADR43x family or from other reference families are available from the Analog Devices portfolio.
The inclusion of a buffer to create the VCM signal to bias the
input would be normal, however, specified performance was
achieved without the need of a buffer in this circuit.
- EVAL-CN0261-SDPZ circuit evaluation board
- System Demonstration Board (EVAL-SDP-CB1Z)
- Function generator, Audio Precision SYS-2522
- External 10 nF ceramic filter capacitor as shown in Figure 4
- 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)
Setup and Test
The block diagram of ac performance measurement setup is shown in Figure 4. The EVAL-CN0261-SDPZ board is driven with bench supplies as shown. Complete documentation for the board can be found at www.analog.com/CN0261DesignSupport.
To measure the frequency response, the equipment was connected as shown in Figure 5. The Audio Precision SYS-2522 was set to output a 1 kHz tone at an input signal level of 0.5 dB below full-scale. The external 10 nF capacitor acts as a low-pass noise reduction filter for the output of the signal generator. Using the evaluation board software, the FFT data was then captured and analyzed.
The software analysis is part of the evaluation board software, which allows the user to capture and analyze ac or dc performance.
In addition to ac performance, the evaluation board software also allows users to analyze the waveform data and create a histogram for the measured input signal.
Figure 4. Test Setup for Measuring AC Performance
|AD7691||18-Bit, 1.5 LSB INL, 250 kSPS PulSAR® Differential ADC in MSOP/QFN||
|ADR435||Ultralow Noise XFET® 5.0V Voltage Reference w/Current Sink and Source Capability||
|AD8597||Ultralow Distortion, Ultralow Noise Op Amp (single)||