60 dB-range (100 nA-100 µA) Low-Cost Logarithmic Converter
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The input current IPD 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 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 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.
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By Eric J. Newman and Matthew Pilotte, Analog Devices, Inc. (Analog Dialogue, Vol 37, December 2003)