PRODUCT DESCRIPTION

The AD8304 is a monolithic logarithmic detector optimized for the measurement of low frequency signal power in fiber optic systems. It uses an advanced translinear technique to provide an exceptionally large dynamic range in a versatile and easily used form. Its wide measurement range and accuracy are achieved using proprietary design techniques and precise laser trimming. In most applications only a single positive supply, VP, of 5 V will be required, but 3.0 V to 5.5 V can be used, and certain applications benefit from the added use of a negative supply, VN. When using low supply voltages, the log slope is readily altered to fit the available span. The low quiescent current and chip disable features facilitate use in battery-operated applications.

The input current, IPD, flows in the collector of an optimally scaled NPN transistor, connected in a feedback path around a low offset JFET amplifier. The current-summing input node operates at a constant voltage, independent of current, with a default value of 0.5 V; this may be adjusted over a wide range, including ground or below, using an optional negative supply. An adaptive biasing scheme is provided for reducing the dark current at very low light input levels. The voltage at Pin VPDB applies approximately 0.1 V across the diode for IPD = 100 pA, rising linearly with current to 2.0 V of net bias at IPD = 10 mA. The input pin INPT is flanked by the guard pins VSUM that track the voltage at the summing node to minimize leakage.

FEATURES

Optimized for Fiber Optic Photodiode Interfacing

Eight Full Decades of Range

Law Conformance 0.1 dB from 1 nA to 1 mA

Single-Supply Operation (3.0 V– 5.5 V)

Complete and Temperature Stable

Accurate Laser-Trimmed Scaling:

Logarithmic Slope of 10 mV/dB (at VLOG Pin)

Basic Logarithmic Intercept at 100 pA

Easy Adjustment of Slope and Intercept

Output Bandwidth of 10 MHz, 15 V/μs Slew Rate

1-, 2-, or 3-Pole Low-Pass Filtering at Output

Miniature 14-Lead Package (TSSOP)

Low Power: ~4.5 mA Quiescent Current (Enabled)

APPLICATIONS

High Accuracy Optical Power Measurement

Wide Range Baseband Log Compression

Versatile Detector for APC Loops