+

±

+

±

High-Gain Band Pass Filt er

High-Speed Peak Detector

ADS794 5

14-bit

2 MSPS

R

S

+

±

6V

AV DD

DVDD

5V

5V

OPA x863

+

±

C

CB

6V

OPA x863

VREF

R

FIL T

C

FIL T

+

±

5V

OPA 378

REF505 0

5.0 V

Reference

12V

R

G

Z

OUT

Sen sor

16

OPA2863

SBOS982A – JUNE 2020 – REVISED JUNE 2020

www.ti.com

Product Folder Links: OPA2863

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Copyright © 2020, Texas Instruments Incorporated

9.4 Low-Power SAR ADC Driver and Reference Buffer

Figure 15 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 real-world, 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.

During the conversion (digitization) phase, the ADC core draws transient current from the reference input which

needs to be driven with a wide-GBW amplifier to offer fast settling and maintain a stable reference voltage for

superior digitization performance. Due to limitations in precision performance of wide-GBW amplifiers, the

OPAx863 devices reference buffer is used in a composite loop with the OPA378 precision amplifier. The

precision amplifier maintains low-offset output whereas the OPAx863 devices provide the output drive and fast-

settling performance.

Figure 15. OPAx863 as Low-Power SAR ADC Driver

9.5 Front-End Gain and Filtering

Figure 16. High-Gain Narrow Bandpass Filter and Peak Detector Circuit

Ultrasonic signaling is used for proximity and obstacle detection, level sensing, sonars, and so forth. Such signal

chains detect the amplitude of received ultrasonic signal at a particular center frequency. Figure 16 shows a

high-gain narrow bandpass filter and peak detector circuit using any of the OPAx863 devices. The signal at the

frequency of interest is filtered out, gained and peak detected to report the amplitude at the output of this circuit.

The phase information is lost in this circuit. The OPAx863 devices are used with its 50-MHz GBW to add a

single-stage gain, and the peak detection capability is easily made with the RRIO capability of these amplifiers.