Op amps as electrometers
R S McDougall · Physics in Medicine and Biology · 1972
Photomultipliers, ionization chambers, solid state detectors and a host of other scientific instruments have extremely high effective source impedances and low output currents (e.g. 1 pA). The elements of a system to measure such low levels of current include a transducer, connecting cable (long or short) and an input device. In recent years op amp designers have introduced a number of devices capable of functioning as input devices and signal conditioners. A good example is the parametric or varactor bridge amplifiers with bias currents less than 10 -14 A.