The circuit described in this document provides single-ended, low-distortion sampling of an industrial level, dc-coupled signal. The driver circuit shown in Figure 1 is optimized for applications requiring best distortion performance. Maximum AD7366/AD7367 performance is achieved by providing adequate settling time and low impedance in the circuit.
Figure 1: Typical Connection Diagram with the AD8021 for Driving the Analog Inputs of the AD7366/AD7367
(Simplified Schematic, Decoupling and All Connections Not Shown)
The AD7366 and the AD7367 are, respectively, 12-bit and 14-bit, 1 MSPS, 2-channel, simultaneous sampling SAR ADCs. These devices have a total of four analog multiplexed inputs (two per channel), which operate in single-ended mode. The analog input range on the AD7366/AD7367 is programmable and can support ±10 V, ±5 V, and 0 V to 10 V using the internal 2.5 V reference. An analog input range of ±12 V requires a 3 V external reference.
The AD7366/AD7367 are fabricated on the Analog Devices, Inc., industrial CMOS process (iCMOS), which is a technology platform combining the advantages of low and high voltage CMOS. The input circuits of the AD7366/AD7367 operate on VDD and VSS voltages of ±12 V nominal, while the rest of the ADC operates on an AVCC, a DVCC, and a VDRIVE of +5 V. The iCMOS process, therefore, allows the AD7366/AD7367 to accept high voltage bipolar signals in addition to reducing power consumption and package size.
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 because large source impedances significantly affect the ac performance of the ADC. The choice of the 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 140 ns.
The AD8021 high performance voltage feedback 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 performance. 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. The AD8021 is connected in the noninverting mode with a gain of 2. The AD7366/AD7367 programmable bipolar input voltage ranges (referenced to the input of the AD8021) are ±5 V and ±2.5 V.
The circuit must be constructed on a multilayer PC board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see MT-031 Tutorial, MT-101 Tutorial, and the AD7366/AD7367 evaluation board layout).