GB1414985A - Current measurement - Google Patents

Abstract

1414985 Measuring current TAYLOR SERVOMEX Ltd 22 Feb 1973 [22 Feb 1972] 8102/72 Heading G1U In apparatus for measuring a small D.C. i, this is first integrated and the slope of the integrator output is detected to provide a signal representing the input current and a resetting switch connected to the integrator has means for compensating any unwanted signals introduced by operation of the switch. In all the detailed embodiments, the integrator is followed by a differentiator, but it is stated that the time taken for the integrator output to rise to a given value may be measured. The basic principle is shown in Fig. 1, where an operational amplifier Al has i as an input and operates as an integrator by means of capacitor Cl. Capacitor C2, amplifier A2 and resistor R1 form a differentiator whose output is -i.R1.C2/C1. The capacitors are periodically discharged by gauged switches S1, S2. In order to avoid the significant periods when the capacitors are being discharged, a modification Fig. 3 (not shown) provides for inversion of the input, feeding both inputs alternately to an integrator, inverting the integrator output, differentiating both outputs and feeding them alternately to the apparatus output. While each differentiator capacitor is not connected to the apparatus output it is discharged to earth. The resetting switches may comprise insulated gate F.E.T's which disadvantageously inject charge into the system due to their possessing finite capacitance. This effect is overcome in accordance with the requirements of the invention by injecting compensating pulses into the integrator of opposite polarity to the gate switching pulses Fig. 8 (not shown). Alternatively, devices such as photoresistive choppers or reed relays may be used. Fig. 10 (not shown) illustrates a detailed embodiment of an integrator wherein a high gain amplifier has inverting and non-inverting inputs (A,B) and an output (C). The output is also fed to a current generator (49) having an output (D). During the reset period, the inverting input is connected to earth and the generator output is connected to the non-inverting input (B). At the end of the reset period any change in output (D) results in current from a further integrator output (F) being injected into the inverting input (A) as a correction. A further embodiment of integrator Fig. 13 (not shown) has differential inputs (M,N) and two such units are used to measure the difference between two input currents Fig. 14 (not shown).