Figure 9-5 shows the OPA855 configured as a TIA, with the optical sensor reverse biased so that the diode

cathode is tied to the positive bias voltage. A RC filter can be used at the reverse bias node as a low-pass filter

to eliminate high-frequency noise. The internal capacitance of photodetectors vary based on sensor type and the

value of the applied reverse voltage. The setups between each sensor type slightly differs, but the connection to

the amplifier is consistent throughout.

The difference between each optical design comprise choosing the feedback resistor to set the transimpedance

gain, and the feedback capacitance to compensate for the additional input capacitance. With an 8-GHz GBWP,

the OPA855 can accommodate very fast rise times to pair with emerging optical sensors to meet industry

demands for faster optical detections.

The dc voltage bias at the noninverting input of the OPA855 shown in Figure 9-5 sets the common-mode voltage

which maximizes the output swing of the system in mismatched power supply configurations. The dc bias is

critical to avoid clipping or saturating the output stage of the amplifier. For the later stages, a fully differential

amplifier (FDA) can be used to convert single-ended signal to a differential input to drive an analog-to-digital

converter (ADC) as shown in Figure 9-1. Higher order filters can be added between the FDA and ADC for

system noise reduction.

Figure 9-7 shows the performance that results from the design parameters provided in Table 9-1, and Figure 9-8

shows the general trends. Both figures depict the closed-loop bandwidth performance of the OPA855 configured

as a TIA using different sensor types and gain configurations. Figure 9-7 shows the amplifier performance based

on the chosen photodetector from the values provided in Table 9-1. PMTs and MPPCs have higher intrinsic

gains, but requires a wide bandwidth to compensate for the higher internal capacitance. Whereas, PDs and

APDs require higher gain configurations to achieve similar output voltage levels. The OPA855 can provide

the bandwidth to accommodate for both optical challenges. Figure 9-7 shows a generic view of the amplifier

performance as a function of sensor capacitance and transimpedance gain. Increasing the feedback resistance

and input capacitance, decreases the closed-loop bandwidth. Throughout the trends, the amount of change in

closed-loop bandwidth is consistent in relationship of the changes in both terms. A photodiode capacitance of 1

pF and a feedback resistance of 1 kΩ results in a very high closed-loop system bandwidth of 1.1 GHz.

OPA855

SBOS622D – JULY 2018 – REVISED MAY 2025

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