An integrated-circuit based wide range electrometer implemented with automatically ranged linear display

In the measurement of dc current for the range of 10-6-10-‘2 A, a stable electrometer is required. The electrometer circuit is usually implemented by simply probing the voltage drop on a load resistor using a sensitive voltage amplifier.’ In this scheme, different values of the load resistor are needed to cover multi decade range without saturating the voltage amplifier. This results in different circuit impedances and input biasing to the voltage amplifier. It may consequently cause the offset variation when the load resistor is changed. Another scheme for the galvanometric current measurement is obtained by a current to voltage converter using the nearly ideal character of an operational amplifier.2 In this case, the input circuit is not changed during the range selection so that the problem of offset variation can be greatly reduced. Although the principle of operation is simple, the commercially available electrometer uses complicated circuitry and high quality contact switches for wide range measurement. It is rather expensive and inconvenient for general purpose current monitoring where the close attachment of the electrometer to the current source is usually needed. In this report, we will describe a novel circuit based on a modified currentvoltage (Z-v) converter which makes possible the usage of complementary metal-oxide semiconductor (CMOS) bilateral switches for range selection. The resultant electrometer is very compact in size and can be easily built.