GENERAL DESCRIPTION

The AD8305 is an inexpensive microminiature logarithmic converter optimized for determining optical power in fiber optic systems. It uses an advanced implementation of a classic translinear (junction based) technique to provide a large dynamic range in a versatile and easily used form. A single-supply voltage of between 3 V and 12 V is adequate; dual supplies may optionally be used. The low quiescent current (typically 5 mA) permits use in battery-operated applications.

The input current, IPD, of 10 nA to 1 mA applied to the INPT pin is the collector current of an optimally scaled NPN transistor, which converts this current to a voltage (VBE) with a precise logarithmic relationship. A second such converter is used to handle the reference current (IREF)

applied to pin IREF. These input nodes are biased slightly above ground (0.5 V). This is generally acceptablefor photodiode applications where the anode does not need to be grounded. Similarly, this bias voltage is easily accounted for in generating IREF. The output of the logarithmic front end is available at Pin VLOG.

FEATURES

Optimized for fiber optic photodiode interfacing

Measures current over 5 decades

Law conformance 0.1 dB from 10 nA to 1 mA

Single- or dual-supply operation (3 V to 12 V total)

Full log-ratio capabilities

Nominal slope of 10 mV/dB (200 mV/decade)

Nominal intercept of 1 nA (set by external resistor)

Optional adjustment of slope and intercept

Complete and temperature stable

Rapid response time for a given current level

Miniature 16-lead chip scale package

(LFCSP 3 mm × 3 mm)

Low power: ~5 mA quiescent current

APPLICATIONS

Optical power measurement

Wide range baseband logarithmic compression

Measurement of current and voltage ratios

Optical absorbance measurement