Figure 8-9 shows the schematic for a 16-bit, precision, 1MSPS, successive approximation register (SAR), analog-to-digital converter (ADC). This circuit is used to measure the driving capability of the PGA849 with the ADS8860 single-ended input ADC.

Figure 8-9 Driving the ADS8860 SAR ADC

The circuit accepts single-ended or differential input signals. The PGA849 operates with independent input and output power supplies. In this example, ±15V power supplies are used to power the input-stage, and a unipolar 5.3V supply powers the output-stage. The PGA849 output stage supply is powered by the same 5.3V ADC supply. The 5.3V output supply operation prevents overloading the ADC inputs during PGA overdrive conditions. The REF6250 is selected as the ADC voltage reference. The REF6250 is a low-noise, low-drift, precision, 5V reference connected to the ADS8860 reference input ADC REFIN pin.

The PGA849 output voltage is developed with respect to the REF pin. The REF pin is set to the SAR ADC midscale voltage by dividing the REF6250 ADC reference with a precision resistive voltage divider. The OPA192 buffer drives the PGA849 REF pin. The OPA192 is a precision amplifier with low offset, low drift and 10MHz bandwidth.