A low-current logarithmic LED electrometer

A new circuit for measuring low dc currents using an LED-based logarithmic electrometer is presented. The electrometer is designed to measure currents from 1 pA to 0.1 mA. Temperature compensation is achieved using the ratio technique in the temperature range -20/spl deg/C-70/spl deg/C. Variations in the scale factor as a function of the input current and the device constant n as a function of temperature are also discussed.