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8.2 Typical Applications

8.2.1 Transimpedance Amplifier

The high GBWP and low input voltage and current noise for the OPA810 make this device an excellent

wideband transimpedance amplifier for moderate to high transimpedance gains.

VBIAS

20 pF

CPCB

0.3 pF

OPA810

5 V

CF + CPCB

1.03 pF

Oscilloscope

With 50-

Inputs

100 k

Supply Decoupling not

shown

Figure 8-12. Wideband, High-Sensitivity, Transimpedance Amplifier

8.2.1.1 Design Requirements

Table 8-2 lists the design requirements for a high-bandwidth, high-gain transimpedance amplifier circuit.

Table 8-2. Design Requirements

PARAMETER

DESIGN REQUIREMENT

Target bandwidth

> 2 MHz

Transimpedance gain

100 kΩ

Photodiode capacitance

20 pF

8.2.1.2 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 OPA810. 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.

The key elements to the design are the expected diode capacitance (CD) with the reverse bias voltage

(VBIAS) applied, the desired transimpedance gain, RF, and the GBWP for the OPA810 (70 MHz). Figure

8-12 shows a transimpedance circuit with the parameters as described in Table 8-2. With these three

variables set (and including the parasitic input capacitance for the OPA810 and the printed circuit board

(PCB) added to CD), the feedback capacitor value (CF) can be set to control the frequency response.

The Transimpedance Considerations for High-Speed Amplifiers application report discusses using high-speed

amplifiers for transimpedance applications. Set the feedback pole according to Equation 9 to achieve a

maximally-flat second-order Butterworth frequency response:

GBWP

R C

(9)

The input capacitance of the amplifier is the sum of the common-mode and differential capacitance (2.0 +

0.5) pF. The parasitic capacitance from the photodiode package and the PCB is approximately 0.3 pF. Using

Equation 5 gives a total input capacitance of CD = 22.8 pF. From Equation 9, set the feedback pole at 1.55 MHz.

Setting the pole at 1.55 MHz requires a total feedback capacitance of 1.03 pF.

Equation 10 shows the approximate –3-dB bandwidth of the transimpedance amplifier circuit:

OPA810

SBOS799E – AUGUST 2019 – REVISED AUGUST 2024

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