Current measurement

An apparatus responding to the value of a small input current, comprising input terminals for said input current and output terminals, and an integrator, said integrator having an integrator input coupled to said input terminals, and an output, said integrator comprising an amplifier connected in a path between input and output of said integrator and a capacitor, whereby said integrator produces on said capacitor a voltage which is the continuing integral of an input current applied to said input terminals, said apparatus further comprising a differentiator having an input coupled to the integrator output and an output, to differentiate continuously the voltage at the integrator output whilst the voltage at said integrator output is varying, and an output coupled to said output terminals whereby an output is obtained at said output terminals which is directly proportional to said input current, said apparatus further including switching means associated with the input to said integrator, said switching means being of a nature to produce by its operation an unwanted input to said integrator and comprising means for developing a correction signal the effect of which is equal and opposite that of said unwanted signal, and, on operation of said switching means injecting said correction signal into said input.