Figure 10-12 Layout Recommendation

When configuring the OPA855 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 10-13. 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 3. The added PCB trace inductance between the feedback network increases the denominator in Equation 3 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 10-13 can be improved by following some of the guidelines shown in Figure 10-14. The two key rules to follow are:

• Add an isolation resistor R_ISO as close as possible to the inverting input of the amplifier. Select the value of R_ISO to be between 10 Ω and 20 Ω. The resistor dampens the potential resonance caused by the trace inductance and the amplifiers internal capacitance.
• Close the loop between the feedback elements (R_F and C_F) and R_ISO as close to the APD pins as possible. This ensures a more balanced layout and reduces the inductive isolation between the APD and the feedback network.

Equation 3.

where

• Z_F is the total impedance of the feedback network.
• Z_IN is the total impedance of the input network.

Figure 10-13 Non-Ideal TIA Layout

Figure 10-14 Improved TIA Layout