+

±

GND

+2.5 V

-2.5 V

Sine Wave

OPAx325

ADC

C

VREF

VSS

VDD

5 V

3.3 V

R

OPAx325

Input

+

20

OPA325, OPA2325, OPA4325

SBOS637D – OCTOBER 2016 – REVISED JUNE 2019

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Product Folder Links: OPA325 OPA2325 OPA4325

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Copyright © 2016–2019, Texas Instruments Incorporated

Application Information (continued)

8.1.3 Driving an Analog-to-Digital Converter

The low-noise and wide-gain bandwidth of the OPAx325, combined with rail-to-rail input/output and zero-

crossover distortion, make these devices an excellent input driver for ADCs. Figure 45 shows the OPAx325

driving an ADC. The amplifier is connected as a unity-gain, noninverting buffer.

Figure 45. The OPAx325 as an Input Driver for ADCs

8.2 Typical Application

Operational amplifiers are commonly used as unity-gain buffers. Figure 46 shows the schematic for an amplifier

configured as a unity-gain buffer. If the input signal range to the amplifier is very close to the rails or includes the

rails, a rail-to-rail amplifier must be used. However, regular rail-to-rail amplifiers introduce significant distortion to

the signal. This design compares the distortion introduced by a typical CMOS input amplifier with that of the

OPAx325 (a zero-crossover amplifier).

Figure 46. The OPAx325 Configured as a Unity-Gain Buffer Amplifier

8.2.1 Design Requirements

The following parameters are used for this design example:

•

Gain = +1 V/V (inverting gain)

•

V+ = 2.5 V, V– = –2.5 V

•