A New Approach to Direct Current Integration

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Abstract
A new all-solid state direct-current integrator integrates currents as low as 10-11 ampere with an accuracy of 1%. The operating principle is similar to that of more conventional current integrators in that the voltage across a storage capacitor in the input circuit is monitored by an electrometer type operational amplifier, the output of which triggers a discriminator at a given voltage level. The discriminator generates a reset pulse which removes a fixed charge from the integrating capacitor. This is where the similarity ends. The diode pump circuit, which has long been recognized as an ideal circuit for removing precise quantities of charge from the integrating capacitor, has been replaced by complementary silicon planar transistors that operate in the inverted-mode and serve as current switches. These transistors normally have both their base-emitter and base-collector junctions reverse biased and look like open switches with leakage currents of about 10-13 ampere. Reset pulses switch these transistors from the reverse-biased state to the active state for a precise time during which a constant current flows to or from the integrating capacitor. Therefore, each reset pulse removes from the integrating capacitor a precise charge that is the product of the current and the period of the active state. By the employment of sufficient gain in the operational amplifier to maintain small voltage excursions at the input, the total effective leakage may be kept as low as 10-14 ampere.

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