Sensitive Method For Characterizing Liquid Helium Cooled Preamplifier Feedback Resistors
Larry G. Smeins, R. F. Arentz · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 1983
A technique is presented for measuring many of the non-ideal characteristics of the large value [1E8 to 1E13 ohm] resistors used in the feedback loops of liquid helium cooled infrared (IR) detector circuits. These resistors are sensitive to both temperature and voltage and have a far from ideal response to transient events. Since these resistors are used to determine the transfer characteristics of a photodetector circuit, their non-ideal behavior can have a large effect on a system's photometric accuracy and linearity. They can also seriously degrade a system's signal to noise ratio in the presence of ionizing radiation. The method described in this paper has the sensitivity and versatility to accurately measure the non-linear characteristics of these resistors in the actual circuit configuration in which they will be used. This method relys upon the use of a small, stable, ceramic chip capacitor that is installed at the input summing node of a preamp, and an extremely narrow band, tuneable, RMS voltmeter. The capacitor is used to differentiate a number of controlled and well known input voltage waveforms into correspondingly well known input currents. These input currents flow through the resistor thereby creating the output voltage waveforms. These output waveforms contain information about the load resistor's electrical characteristics and allow the calibration of the resistor's response to a wide variety of signals. This method is useful in performing a dynamic system level calibration of a detector circuit since it simulates the many varied types of signal currents generated in either a photovoltaic or photoconductive IR detector circuit.