Patent ID: 7696742

Claim:
A circuit for measuring intensity of an electric current by integrating the electric current over integration cycles having a time period ti, comprising; a first operational amplifier (OPAMP) having first and second inputs and an output; means for measuring a peak value of a sawtooth voltage at the output of the first operational amplifier at the end of the integration cycle; a voltage adder having an addend input, a subtrahend input and an output; a first low-pass filter with an upper cut-off frequency, which has a value of the order of magnitude (0,1×2π×ti) −1 , for connecting the output of the first OPAMP to the addend input of the voltage adder; a second OPAMP having first and second inputs and an output; a second low-pass filter having an input and an output, the input of the second low-pass filter connected to the output of the second OPAMP; a first switch having an input and an output, the input of the switch connected to the output of the output of the second low-pass filter; means for connecting the output of the first switch to the subtrahend input of the voltage adder and to a capacitor to provide a correction voltage from the output of the second OPAMP and a capacitor, whose one terminal is connected to mass and provided to store the correction voltage being stored in the capacitor; means for connecting output of the voltage adder to the first input of the second OPAMP; and the second OPAMP, the second low-pass filter, and the switch constituting a correction circuit, wherein the time constant of the correction circuit is approximately three times shorter than a time interval in which the switch remains closed.