OPA656 datasheet(14/25 Pages) TI

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9.2 Typical Applications

9.2.1 High Bandwidth, 100-kΩ Gain Transimpedance Design

The high GBWP, low input voltage, and current noise of the OPA817 make it an excellent wideband

transimpedance amplifier for moderate to high transimpedance gains.

+5 V

–5 V

–

10 k

0.8 pF

VBIAS

OPA817

Supply decoupling

not shown

5 pF

CDIFF

0.1 pF

CCM

2.6 pF

OPA817's input differential and

common-mode capacitance

Figure 9-4. Wideband, High-Sensitivity, Transimpedance Amplifier

9.2.2 Design Requirements

Table 9-1 lists the design requirements for a high-bandwidth, high-transimpedance-gain amplifier.

Table 9-1. Design Requirements

TARGET BANDWIDTH (MHz) TRANSIMPEDANCE GAIN (kΩ) PHOTODIODE CAPACITANCE (pF)

9.2.3 Detailed Design Procedure

Designs that require high bandwidth from a large area detector with relatively high transimpedance gain benefit

from the low-input voltage noise of the OPA817. This input voltage noise is peaked up over frequency by the

diode source capacitance, and can, in many cases, become the limiting factor to input sensitivity. Figure 9-4

shows the transimpedance circuit with the parameters as defined in Table 9-1. With these three variables set

(including the parasitic input capacitance for the OPA817 and the printed circuit board (PCB) added to CD), the

feedback capacitor value (CF) can be set to control the frequency response. For a discussion about the theories

and equations that show how to compensate a transimpedance amplifier for a particular transimpedance gain

and input capacitance, see the Transimpedance Considerations for High-Speed Amplifiers application report.

The bandwidth and compensation equations from the application report are available in a Microsoft Excel™

calculator. A link to the calculator is provided at What You Need To Know About Transimpedance Amplifiers –

Part 1. Determine the total input capacitance (CTOT) to help with the component selection. CTOT is referred to as

CIN in the calculator. CTOT is the sum of CD, CDIFF, and CCM, which is 7.7 pF. As listed in Table 9-2, using the

value of CTOT, the targeted closed-loop bandwidth (f–3 dB) of 28 MHz, and transimpedance gain of 10 kΩ requires

an amplifier with approximately 380 MHz GBWP and a feedback capacitance (CF) of 0.8 pF. With OPA817's 390

MHz GBWP, it will be a suitable amplifier for the design requirements.

Table 9-2. Results of Inputting Design Parameters in the TIA Calculator

Calculator II

Closed-loop TIA Bandwidth (f-3dB)

MHz

Feedback Resistance (RF)

kΩ

Input Capacitance (CIN)

7.7

Op amp Gain Bandwidth Product (GBP)

379.3

MHz

Feedback Capacitance (CF)

0.793

OPA817

SBOS847 – JULY 2022

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