Picoammeter for Ion Chamber Spans Six Decades without Range Switching

Precise current measurements in the range 10-13A to 10-7A can be done without range switching using an active integrator built around a varactor-bridge Op-Amp. The information is supplied by the time required to cross the voltage gap between two thresholds and is carried out by a pulse-width analog signal. Conversion into current or gamma dose rate is done by a simple digital encoder. Suitable contrivances have been adopted in the circuit to minimize leakage and drift effects and to prevent the need of any recalibration of the electrometer.