power-down state, if the differential voltage between the input pins of the amplifier exceeds a diode voltage drop,

an additional low-impedance path is created between the noninverting input pin and the output pin.

9 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.

9.1 Application Information

9.1.1 Using the OPA858 as a Transimpedance Amplifier

The OPA858 design has been optimized to meet the industry's growing demand for wideband, low-noise

photodiode amplifiers. The closed-loop bandwidth of a transimpedance amplifier is a function of the following:

1. The total input capacitance. This includes the photodiode capacitance, input capacitance of the amplifier

(common-mode and differential capacitance) and any stray capacitance from the PCB.

2. The op amp gain bandwidth product (GBWP).

3. The transimpedance gain RF.

Figure 9-1 shows the OPA858 configured as a TIA with the avalanche photodiode (APD) reverse biased such

that the APD cathode is tied to a large positive bias voltage. In this configuration the APD sources current into

the op amp feedback loop so that the output swings in a negative direction relative to the input common-mode

voltage. To maximize the output swing in the negative direction, the OPA858 common-mode is set close to the

positive limit, 1.6 V from the positive supply rail.

+

±

GND

+

±

R

F

C

F

OPA858

5 V

GND

3.4 V

100 V

Figure 9-1. Transimpedance Amplifier Circuit

The feedback resistance RF and the input capacitance form a zero in the noise gain that results in instability

if left unchecked. To counteract the effect of the zero, a pole is inserted by adding the feedback capacitor

(CF.) into the noise gain transfer function. The Transimpedance Considerations for High-Speed Amplifiers

application report discusses theories and equations that show how to compensate a transimpedance amplifier

for a particular gain and input capacitance. The bandwidth and compensation equations from the application

report are available in an Excel® calculator. What You Need To Know About Transimpedance Amplifiers – Part 1

provides a link to the calculator.

The equations and calculators in the application report and blog posts referenced above are used to model the

bandwidth (f-3dB) and noise (IRN) performance of the OPA858 configured as a TIA. The resultant performance is

shown in Figure 9-2 and Figure 9-3. The left side Y-axis shows the closed-loop bandwidth performance, while

the right side of the graph shows the integrated input referred noise. The noise bandwidth to calculate IRN, for a

fixed RF and CPD is set equal to the f–3dB frequency.

www.ti.com

OPA858

SBOS629B – APRIL 2018 – REVISED MAY 2025

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