THERMALLY REGULATED LOW-NOISE, WIDE-BAND, I/V CONVERTER, USING PELTIER HEAT-PUMPS

An ultra-low-noise current-to-voltage converter capable of detecting currents of 1 pA or less in a bandwidth from dc to 20 kHz is described. This instrument exhibits the dc performances typical of a picoammeter together with the low noise level and the wide bandwidth of a modern current-to-voltage converter. Thanks to an excellent dc stability over time, continuous monitoring of phenomena involving currents in the picoampere range can be carried out, for hours or even days. Moreover, ultra-low-noise ac measurements of these currents in the kilohertz range can be performed. A new nonheating thermal controller, using the concept of letting the instrument operate 30-degrees-C below room temperature, has been designed and built. Two small Peltier heat pumps have been employed to cool the critical parts of the system, thus achieving a long-term stability of a few hundredths of degrees centigrade. With minor modifications in the input stage of this instrument, a high-transimpedance, ultra-low-noise prototype has been built, which shows an input equivalent current noise as low as 4 fa/square-root Hz at 100 mHz.