When configuring the OPA858 as a transimpedance amplifier, take care 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. Figure 9-10 shows that the added distance between the two device results in

increased inductance between the APD and op amp feedback network. The added inductance is detrimental to

a decompensated amplifiers stability because this inductance isolates the APD capacitance from the noise-gain

transfer function. The noise gain is given by Equation 4. The added PCB trace inductance between the feedback

network increases the denominator in Equation 4 thereby reducing the noise gain and the phase margin. In

cases where a leaded APD in a TO-can package is used, further minimize inductance by cutting the leads of the

TO-can package as short as possible.

Noise Gain= 1+ ZFZIN

(4)

where

• ZF is the total impedance of the feedback network.

• ZIN is the total impedance of the input network.

The layout shown in Figure 9-10 is improved by following some of the guidelines in Figure 9-11. The two key

rules to follow are:

1. Add an isolation resistor RISO as close as possible to the inverting input of the amplifier. Select the value

of RISO to be between 10 Ω and 20 Ω. The resistor dampens the potential resonance caused by the trace

inductance and the amplifiers internal capacitance.

2. Close the loop between the feedback elements (RF and CF) and RISO as close to the APD pins as possible.

This closure provides a more balanced layout and reduces the inductive isolation between the APD and the

feedback network.

Vbias

CF

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

RF

Figure 9-10. Nonideal TIA Layout

Vbias

Place RISO close to IN±

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

CF

RF

RISO

Figure 9-11. Improved TIA Layout

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OPA858

SBOS629B – APRIL 2018 – REVISED MAY 2025

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