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Maxim > Design Support > Technical Documents > Reference Designs > Amplifier and Comparator Circuits > APP 4122
Maxim > Design Support > Technical Documents > Reference Designs > Optoelectronics > APP 4122
Keywords: MAX4007, Current Monitor, Current monitoring, current regulator, current mirror, photodiode
current
REFERENCE DESIGN 4122 INCLUDES: Tested Circuit Schematic Description Test Data
Simple Regulator Provides Constant Voltage for
High-Side Current Monitors
By: Arpit Mehta, Strategic Applications Engineer, Amplifiers & Sensors
Dec 21, 2007
Abstract: In a typical photodiode current-monitoring application, the voltage drop between the current
monitor and the avalanche photodiode (APD) varies with the temperature and current flowing through it,
thereby changing the overall gain. This application note describes a regulator circuit that solves this
problem by maintaining a constant voltage drop across the current monitor and APD.
Overview
The voltage drop between a current monitor and avalanche photodiode (APD) varies with the
temperature and current flowing through the photodiode current-monitoring application. Therefore, in
applications such as fiber and instrumentation transmission systems, the ability to detect the average
photodiode current is essential for efficient system management.
Typical Application Circuit
The MAX4007/MAX4008 are precision, high-side, high-voltage current monitors specifically designed for
monitoring photodiode currents. These devices provide a connection point (REF) for the reference
current and a monitor output that provides a signal proportional to the reference current. A suitable APD
(avalanche photodiode) or PIN (positive-intrinsic-negative) photodiode is connected at the REF pin of the
IC as shown in Figure 1. The REF pin provides the source current to the cathode of the photodiode.
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