A Servo-Stabilized Analogue-to-Digital Converter for High Resolution Pulse-Height Analysis

A 4096 channel pulse-height analyzer has been constructed to exploit the high resolution attainable with semiconductor junction charged particle detectors in the 0 to 40 Mev range. The use of so large a number of channels can only be justified if the drift in the system can be held to less than one channel. It was felt that, using conventional techniques, it would be very difficult to stabilize the linear amplifier gain, the converter gain and the converter zero to a part in 4000. A servo-stabilized system was therefore adopted which assures long term system stability without imposing unreasonable restrictions on component stability. A stable pulse generator produces two reference pulses, one 8 times as large as the other, and presents them alternately to the preamplifier input along with the detector pulses. Reference and detector pulses are amplified and measured by the same analogue-to-digital converter. The small reference pulses should fall in channel 512 and the large pulses in channel 4096. The amounts by which the reference pulses miss their intended channels constitute the error inputs to two integrating digital servos. The large-pulse servo controls the converter clock frequency and the small-pulse servo controls the converter zero setting. The servos have sufficient lock-in range to compensate perfectly for gain changes of ± 3% and zero changes of ± 1.5%.