OPA656 datasheet(13/25 Pages) TI

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9.1.2 Wideband, Transimpedance Design Using OPA817

The OPA817 design is optimized for wideband, low-noise transimpedance applications with high GBWP, low-

input voltage, current noise, and low input capacitance. The high voltage capability allows greater flexibility

of supply voltages along with wider output voltage swings. Figure 9-3 shows an example circuit of a typical

photodiode amplifier circuit. As Figure 9-3 shows, the photodiode is generally reverse biased in a TIA application

so that the photodiode current in the circuit of flows into the op amp feedback path. This results in an output

voltage that reduces from VREF with increasing photodiode current. In this type of configuration and depending

on the application needs, VREF can be biased closer to VS+ to achieve the desired output swing. Consider the

common-mode input range when VREF bias is used so that the common-mode input voltage stays within the

valid range of the OPA817.

The key design elements that determine the closed-loop bandwidth, f–3dB, of the circuit are as follows:

1. The op amp GBWP.

2. The transimpedance gain, RF.

3. The total input capacitance, CTOT, that includes photodiode capacitance, input capacitance of the amplifier

(common-mode and differential capacitance), and PCB parasitic capacitance.

+5 V

–5 V

–

BIAS

OPA817

REF

TOT

Figure 9-3. Wideband, Low-Noise, Transimpedance Amplifier

Equation 1 shows the relationship between the above mentioned three elements for a Butterworth response.

3dB

TOT

GBWP

2 R C

(1)

The feedback resistance (RF) and the total input capacitance (CTOT) form a zero in the noise gain and results

in instability if left uncompensated. To counteract the effect of the zero, a pole is inserted in the noise gain by

adding the feedback capacitor (CF). 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 a Microsoft Excel™ calculator. What You Need To Know About Transimpedance Amplifiers – Part

1 provides a link to the calculator. As shown in Figure 9-3, the details of maximizing the dynamic range of TIA

front-ends are provided in the Maximizing the Dynamic Range of Analog TIA Front-End application note.

www.ti.com

OPA817

SBOS847 – JULY 2022

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