ADL5306:  60 dB-range (100 nA-100 µA) Low-Cost Logarithmic Converter

The ADL5306 is a low cost micro-miniature logarithmic converter optimized for the determination of 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 5.5 V is adequate; dual supplies may optionally be used. The low quiescent current (typically 5 mA) permits use in battery-operating applications

The input current I_PD of 100 nA to 100 µA applied to the INPT pin is the collector current of an optimally-scaled NPN transistor, which converts this current to a voltage (its V_BE) with a precise logarithmic relationship. A second such converter is used to handle the reference current, I_REF, applied to pin IREF. These input nodes are biased slightly above ground (0.5 V). 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 I_REF. The output of the logarithmic front-end is available at pin VLOG.