Figure 9-4 shows the use of the OPAx863 as a SAR ADC input driver and reference buffer driving the ADS7945. sensors, which are used for interface with the physical environment, exhibit high output impedance, and cannot drive SAR ADC inputs directly. A wide-GBW amplifier, such as the OPAx863, is needed to charge the switching capacitors at the SAR ADC input, and quickly settle to the required accuracy within the given acquisition time. The ADC core draws transient current from the reference input during the conversion (digitization) phase, which must be driven with a wide-GBW amplifier to offer fast settling and maintain a stable reference voltage for excellent digitization performance. The OPAx863 reference buffer is used in a composite loop with the OPA378 precision amplifier because of limitations in precision performance of wide-GBW amplifiers. The precision amplifier maintains low-offset output, whereas the OPAx863 devices provide the output drive and fast-settling performance.

Figure 9-4 OPAx863 as Low-Power SAR ADC Driver