OPA863DBVR datasheet(27/52 Pages) TI

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9.4 Low-Power SAR ADC Driver and Reference Buffer

Figure 9-4 shows the use of the OPAx863 devices 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 like the OPAx863 devices are needed to

charge the switching capacitors at the SAR ADC input and to settle fast 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 needs to be driven with a wide-GBW amplifier to offer fast settling and maintain a

stable reference voltage for superior digitization performance. The OPAx863 devices reference buffer is used in

a composite loop with the OPA378 precision amplifier due to 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.

ADS7945

14-bit

2 MSPS

–

6 V

AVDD

DVDD

5 V

OPAx863

–

6 V

OPAx863

VREF

FILT

–

5 V

OPA378

REF5050

5.0 V

Reference

12 V

OUT

Sensor

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

9.5 Variable Reference Generator Using MDAC

High-speed amplifiers may be used as a voltage buffer at MDAC output to generate a fast settling variable

reference voltage. Figure 9-5 shows a representative circuit using DAC8801 and OPAx863.

DAC8801

–

OUT

REF

+6 V

6 V

OPAx863

OUT

Figure 9-5. Variable Reference Generator Using MDAC and OPAx863

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OPA863, OPA2863, OPA4863

SBOS982I – JUNE 2020 – REVISED DECEMBER 2022

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