When configuring the OPA855-Q1 as a transimpedance amplifier additional care must be taken to minimize

the inductance between the avalanche photodiode (APD) and the amplifier. Always place the photodiode on

the same side of the PCB as the amplifier. Placing the amplifier and the APD on opposite sides of the PCB

increases the parasitic effects due to via inductance. APD packaging can be quite large which often requires the

APD to be placed further away from the amplifier than ideal. The added distance between the two device results

in increased inductance between the APD and op amp feedback network as shown in Figure 11-2. The added

inductance is detrimental to a decompensated amplifiers stability since it isolates the APD capacitance from the

noise gain transfer function. The noise gain is given by Equation 2. The added PCB trace inductance between

the feedback network increases the denominator in Equation 2 thereby reducing the noise gain and the phase

margin. In cases where a leaded APD in a TO can is used inductance should be further minimized by cutting the

leads of the TO can as short as possible.

The layout shown in Figure 11-2 can be improved by following some of the guidelines shown in Figure 11-3. The

two key rules to follow are:

inductance and the amplifiers internal capacitance.

This ensures a more balanced layout and reduces the inductive isolation between the APD and the feedback

network.

F

IN

Z

Noise Gain

1

Z

§

·

¨

¸

©

¹

(2)

where

Vbias

1

FB

8

PD

2

NC

7 VS+

3

IN±

6 OUT

4

IN+

5 VS±

Thermal

Pad

APD

Package

Trace inductance isolates

APD capacitance from the

amplifier noise gain

Figure 11-2. Non-Ideal TIA Layout

Vbias

Close the loop close

to APD pins

1

FB

8

PD

2

NC

7 VS+

3

IN±

6 OUT

4

IN+

5 VS±

Thermal

Pad

APD

Package

Figure 11-3. Improved TIA Layout

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OPA855-Q1

SBOSA57A – FEBRUARY 2021 – REVISED MARCH 2021

Copyright © 2021 Texas Instruments Incorporated

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