Patent ID: 6998850

Claim:
A circuit for measuring picoampere current levels, comprising: an operational-amplifier that has differential inputs and differential outputs; a switching structure that switchably couples an input line to one of said differential inputs in response to signals from a timing circuit; a first integrating capacitor coupled to one of said differential inputs and to one of said differential outputs; a second integrating capacitor coupled to the other of said differential inputs and to the other of said differential outputs; a charge injection compensation structure that alternately injects charge into said input line and removes charge from said input line in response to signals from said timing circuit; wherein said charge injection compensation structure comprises: a first charge injection device that has a first width; and a second charge injection device that has a second width that is different than said first width, wherein said second charge injection device is driven by a waveform from said timing circuit that is an inverse of a waveform that drives said first charge injection device.