A Preprocessing Unit for Wide-Range Measuring Channels

The methods of handling small signals from multidecade sensors are discussed. A simple, fast and accurate preprocessing amplification unit (PPU) with the capabilities of automatic gain switching is presented. The unit could be used in the analog channels of a computer based data acquisition and process control system. The use of such a unit with some of the analog sensors would increase the resolution of the measuring subsystem involved through the combined action of a shared analog-to-digital converter (ADC). The PPU consists of an operational amplifier with suitable performance, a resistance feedback network whose value could be changed by switching, and FET switches. The switching control is done by a reversible counter, decoder, and state control logic all using TTL chips. A digital code representing the gain constant is available together with the amplified signal. An experimental unit was built for use in a wide range, neutron flux, measuring channel. This channel is connected to the control computer´s analog subsystem which has a 10 bit ADC. Accuracies better than 0.1% of full scale on the most sensitive range (10-9 amp.) and switching speeds less than 1 ¿.sec. are obtainable. The accuracy limits are imposed by the quality of a single amplifier, and the speed limits by the amplifier slew rate and the delay in a few IC logic chips.