CN0042| Application Note
Circuit Note
CN0042
A
Verified Circuit Designs Using Analog Devices Signal Processing Devices
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Driving the AD7366 bipolar SAR ADC in Low-Distortion DC-Coupled applications
Contributed November 7, 2008 Devices connected in this Circuit Note: AD7366/7 Bipolar Input, Dual 12/14-Bit, 2-Channel, SAR ADC AD8021 Low Noise, High Speed Amplifier
Circuit Function and Benefits
The circuit described in this document provides single-ended, low-distortion sampling of a DC-coupled signal. The driver circuit shown here is optimized for applications requiring best distortion performance by ensuring that the maximum AD7366/AD7367 performance is achieved, by providing adequate settling time and low impedance in the driver circuit.
Circuit Description
The AD7366 and the AD7367 are 12-bit and 14-bit, 2-channel, simultaneous sampling SAR ADCs. These devices have a total of Figure 1 Typical Connection Diagram with the AD8021 for Driving the four analog multiplexed inputs, which operate Analog Inputs of the AD7366/AD7367 in single-ended mode. The analog input range on the AD7366/AD7367 is programmable and can support 10 V, 5 V, 0 V to 10 V and 12 V (with a 3 V external reference). In applications where the signal source has high impedance, analog input signals should be buffered before being applied to the inputs of the AD7366/AD7367 as large source impedances significantly affect the AC performance of the ADC. The choice of op-amp used to drive the inputs is a function of the particular application and depends on the analog input voltage range selected. The driver amplifier must be able to settle for a full-scale step to a 14-bit level (0.0061%) for the AD7367 or a 12-bit level (0.024%) for the AD7366 in less than the specified acquisition time of 140ns.
The AD8021 high-performance op-amp is an ideal choice as a single-ended input buffer/driver for the AD7366/AD7367 due to its exceptionally high performance, high speed, low noise and low-distortion voltage feedback. It also meets the above stated requirement when operating in single-ended mode. Figure 1 shows the configuration of the AD7366/AD7367 with the AD8021 in a single-ended configuration. The AD8021 needs an external compensating NPO type capacitor (CCOMP) as indicated in Figure 1.
Common Variations
The AD8022 is a suitable replacement for the AD8021 in high frequency applications where a dual version is required. For lower frequency applications, recommended op-amps are the AD797, AD845, and AD8610.
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