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 large dynamic range in versatile and easily used form. A single-supply voltage of between 3V and 12V is adequate; dual supplies may optionally be used. The low quiescent current (typically 5mA) permits use in battery-operated applications.

The input current, IPD, of 10nA to 1mA applied to the INPT pin is collector current of an optimally scaled NPN transistor, which converts this current to a voltage (VBE) with 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 (0.5V). This is generally acceptable for 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.

The basic logarithmic slope at this output is nominally 200 mV/decade (10 mV/dB). Thus, a 100 dB range corresponds to an output change of 1 V. When this voltage (or the buffer output) is applied to an ADC that permits an external reference voltage to be employed, the AD8305 voltage reference output of 2.5V at Pin VREF can be used to improve the scaling accuracy. Suitable ADCs include the AD7810 (serial 10-bit), AD7823 (serial 8-bit), and AD7813 (parallel, 8-bit or 10-bit). Other values of the logarithmic slope can be provided using a simple external resistor network.