8 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

8.1 Application Information

The OPAx328 offer outstanding dc and ac performance. These devices operate with up to a 5.5-V power supply,

and offer an ultra-low input bias current and a 40-MHz bandwidth. These features make the OPAx328 family of

robust operational amplifiers great for both communication and industrial applications.

8.1.1 Capacitive Load and Stability

The OPAx328 are designed for use in high-speed applications for transimpedance amplifiers (TIA) and ADC

input-driving amplifiers. As with all op amps, there may be specific instances where the OPAx328 become

unstable. The particular op-amp circuit configuration, layout, gain, and output loading are some of the factors

to consider when establishing whether an amplifier is stable in operation. An op amp in the unity-gain (1-V/V)

buffer configuration driving a capacitive load exhibits a greater tendency to become unstable compared to an

amplifier operating at a higher noise gain (see Figure 6-28). The capacitive load, in conjunction with the op-amp

output impedance, creates a pole within the loop gain that degrades the phase margin. The degradation of the

phase margin increases as the capacitive loading increases. When operating in the unity-gain configuration, the

OPAx328 remain stable with a pure capacitive load up to 100 pF.

Figure 8-1 shows one technique to increase the capacitive load drive capability of an amplifier operating in a

unity-gain configuration is to insert a small resistor (RS), typically 10 Ω to 50 Ω, in series with the output. This

resistor significantly reduces the overshoot and ringing associated with large capacitive loads.

+

–

R

S

C

L

GND

R

L

V

IN

V

OUT

Figure 8-1. Improving Capacitive Load Drive

8.2 Typical Applications

8.2.1 Bidirectional Current-Sensing

This single-supply, low-side, bidirectional current-sensing design example detects load currents from –1 A to

+1 A. The single-ended output spans from 110 mV to 3.19 V. This design uses the OPAx328 because of the low

offset voltage and rail-to-rail input and output. One of the amplifiers is configured as a difference amplifier and

the other amplifier provides the reference voltage.

Figure 8-2 shows the schematic.

OPA328, OPA2328

SBOS957B – FEBRUARY 2022 – REVISED NOVEMBER 2022

www.ti.com

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