For circuit operation, the variations between de and ac functions are not too dissimilar. For de signal voltages, the individual stages of the instrument are directly coupled. Direct coupling can also be used for ac voltage signals, but typically capacitors are used to pass only the ac signal voltages from one stage to another. The test indications for ac must be handled slightly differently because of the nature of ac waveforms and the different ways of describing their levels. One ac wave has average, effective (RMS), and peak values. The analog meter movement responds to the average ac value, but we all use the effective or RMS value to describe a waveform, such as 120-V ac line power. The ac meter is calibrated for the effective value, even though it reacts to the average value.
The purpose of the integrator and gate circuits is to pass on the number of pulses that match the test voltage being measured. Since a specific pulse frequency stream is being applied to the gate, the test voltage must somehow control how long the gate will be allowed to pass the pulses; the higher the test voltage, the longer the gate will be open, and the more pulses will be passed through- and the number of pulses will represent the digitized version of the analog test voltage.
Input Networks and Amplifiers
The input network and amplifier perform the same functions as they do for the electronic analog meter. The input network presents a high resistance (11 megohms) to the circuit under test to keep from loading it down; it also attenuates the input voltage with the range switch setting to keep the test signal at the input of the amplifier under 1 volt. Although identical input and amplifier circuits can be used for both digital and analog meters, the example we are using demonstrates the use of an amplifier that can take up to 1 volt of input, and the ranges vary from 2 volts to 2000 volts, in multiplier ranges of 2, 20, 200, and 2000 volts. Since digital measurements use ten digits (0- 9), the counters, and especially the pulse generators deal in multiples of ten for convenience. The follower and amplifier circuits are both op-amps connected to accomplish their functions.