Small electric current measurement method for e.g. biological membrane currents

摘要

A definite charge (Q) is injected from a changeover switch (1) into a 1 pF capacitor (2), producing a potential difference (U1) of alternating polarity, with up to 4000 polarity swaps per second, in a fixed timing pattern. The voltage (U2) at the input terminal (6) is limited to the threshold of inverse-parallel diodes (3,4). The gain of the input amplifier (5) is set for an output voltage (U3) not exceeding 10 V, which is compared (7,8) with positive and negative reference voltages defining the sensitivity. The signals (U4, U7) created by the input current (+i) and injected charge are applied to AND gates (9,10) which open and close a time gate.