Interfacing with APDs is similar to interfacing with PIN PDs, but APDs have additional noise factors due to

the internal gain. APDs have increased shot noise and the addition of a multiplication excess noise factor.

Decreasing capacitance, increasing diode shunt resistance, and decreasing reverse voltage bias applied to the

APD deceases noise at the expense of response time. The MPPCs total noise is comparable to APDs, but

with differing noise sources. This optical sensor includes digital-like noise factors such as dark count rate, after

pulsing, and optical crosstalk due to the parallel gain cells. For PMTs, dark count rate is lower. In general, PMT

total noise is comparable to PDs with an internal gain comparable to APDs. However, PMTs have the lowest

quantum efficiency of the optical sensor space.

9.2.2.1 Design Requirements

The objective is to design a low-noise, wideband optical front-end system using a diverse selection of optical

sensors: PD, APD, PMT, and MPPC with the OPA855 as the TIA. Table 9-1 lists the approximate design

requirements for each type of photodetector.

Table 9-1. Design Parameters

SENSOR

INTRINSIC

GAIN

(A/W)

REVERSE

BIAS

(V)

INPUT

CAPACITANCE

(pF)

TARGET

BANDWIDTH

(MHZ)

TRANSIMPEDANCE

GAIN RF

(kΩ)

TOTAL OPTICAL

GAIN

(kV/W)

PD (PIN)

1

30

3

15

100

100

APD

100

150

1

200

10

1000

PMT

1 × 106

1250

50

100

1

1 × 106

MPPC (SiPM)

5 × 105

50

100

10

1

5 × 105

9.2.2.2 Detailed Design Procedure

The OPA855 is decompensated and requires a high-frequency gain of 7 V/V or greater to be stable. Using

the OPA855 in lower gains results in increased peaking and potential instability. Decompensated amplifiers

are advantageous in TIA applications due the inherent characteristics of a TIA design. The zero and pole pair

introduced by the input and feedback capacitances along with the feedback resistor increases the noise gain

until the flattens out at a high gain with a magnitude shown in Equation 2.

1+CTOTCF

(2)

where

• CTOT is the total input capacitance of the amplifier (includes photodetector capacitance and the common-

mode and differential input capacitance of the amplifier)

• CF is the feedback capacitance of the amplifier

A decompensated amplifier allows for benefits such as increased open loop gain, increased bandwidth,

increased slew rate, and lower input-referred noise for the same quiescent current relative to the unity gain

stable counterpart.

Similar to the concept described in Section 9.2.1.2, the rise time and the internal capacitance of the

photodetector determines the closed-loop bandwidth. Both the closed-loop bandwidth and the transimpedance

gain (RF) determine the necessary gain bandwidth (GBWP) of the amplifier. Table 9-1 shows the standard

photodiode characteristics based on type of photodetector. Target values such as the system bandwidth and

gain are calculated using these concepts with the chosen photodiode characteristics. Detailed explanations and

equations can be found in the application reports discussed in Section 9.2.1.2.

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OPA855

SBOS622D – JULY 2018 – REVISED MAY 2025

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