LINEAR Application Note 92 Specifications User Guide

Application Note 92 November 2002

Bias Voltage and Current Sense Circuits for Avalanche Photodiodes Feeding and Reading the APD Jim Williams, Linear Technology Corporation

INTRODUCTION cates optical signal strength. This information can be

combined with feedback techniques to maintain optical

Avalanche photodiodes (APDs) are widely utilized in laser

signal strength at an optimal level. The feedback loop’s

based fiberoptic systems to convert optical data into

operating characteristics can also determine if deleterious

electrical form. The APD is usually packaged with a signal

degradation of optical components has occurred, permit-

conditioning amplifier in a small module. An APD receiver

ting corrective measures to be taken. APD current is

module and attendant circuitry appears in Figure 1. The

typically between 100n A and 1m A, a dynamic range of

APD module (figure right) contains the APD and a trans-

10,000:1. This measurement, which should be taken with

impedance (e.g., current-to-voltage) amplifier. An optical

an accuracy inside 1%, normally must occur in the APD’s

port permits interfacing fiberoptic cable to the APD’s

“high side,” complicating circuit design. This restriction

photosensitive portion. The module’s compact construc-

applies because the APD’s anode is committed to the

tion facilitates a direct, low loss connection between the

receiver amplifier’s summing point.

APD and the amplifier, necessary because of the extremely high speed data rates involved. The APD module, an expensive and electrically delicate

device, must be protected from damage under all condi-

The receiver module needs support circuitry. The APD

tions. The support circuitry must never produce spurious

requires a relatively high voltage bias (figure left) to

outputs which could destroy the APD module. Particular

operate, typically 20V to 90V. This voltage is set by the bias

attention must be devoted to the bias supply’s dynamic

supply’s programming port. This programming voltage

response under programming and power-up/down condi-

may also include corrections for the APD’s temperature

tions. Finally, it is desirable to power the support circuitry

dependent response. Additionally, it is desirable to moni-

from a single 5V rail.

tor the APD’s average current (figure center), which indi-

APD RECEIVER MODULE

TYPICALY 20V TO 90V

APD BIAS

APD APDAPD

INPUT AMPLIFIER

5VIN BIAS POWER CURRENT

OUTPUT

SUPPLY MONITOR

AMPLIFIER

PROGRAMMING OUTPUT IS GROUND OPTICAL INPUT REFERRED APD BIAS CURRENT INPUT

MONITOR. APD CURRENT TYPICALLY 100n A TO 1m A AN92 F01

Figure 1. Avalanche Photodiode (APD) Module (Figure Right) Contains APD, Amplifier and Optical Port. Power Supply (Figure Left) Provides APD Bias Voltage. APD Current Monitor (Figure Center) Operates at High Common Mode Voltage, Complicating Signal Conditioning

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AN92-1