Digital Circuits

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.

Op-Amps and ICs

The op-amp is a differential amplifier on an integrated circuit (IC) chip, with terminal access to all points in both stages. Connections can be made to these terminals to have the op-amp perform a wide variety of functions. It can be an ordinary amplifier with one input, in which the outputs can be inverted or noninverted or both. The two outputs can be used independently or as a differential output. The op-amp can also be used as a source follower. With some amplifiers, if the alternate input is used, the amplifier is the same, except that the inverted and noninverted outputs reverse. The two stages in the op-amp can also be used to gate an input. The input applied to the first stage will be fed internally to the second stage, which will block the signal until it receives a gate voltage.

Pulse Generators

The pulse generator circuits are the principal circuits which determine the accuracy of the digital meter. The clock is a signal generator, or stable oscillator, that creates and supplies the steady stream of pulses to the variable gate. This stable signal source is also passed through a sequence of decade dividers that each reduce the pulse frequencies to Y10 of their value.

In this example, the original clock frequency of I megahertz is subdivided three times, first to I 00 kilohertz, then to I O kilohertz, and then to I kilohertz. The subdividing can continue down to I hertz so that a gate pulse of any width will be accessible. Also, the basic clock frequency and all frequencies subdivided by 10 are available for use individually for selective counting in different ranges.

Oscillators and Waveshapers

The clocks and the decade dividers, as well are not usually just one stage. With the clock, for example, in addition to the oscillator, there are generally amplifiers and wave shaping circuits used to get each wave in the pulse train properly shaped. When a sine wave oscillator is used, clipper and clamper circuits, or a Schmitt trigger, are used to reshape the sine waves into square wave pulses.

There are countless varieties of oscillator circuits. The main requirement for a circuit to oscillate and produce a stream of waves is that positive feedback, also called regenerative feedback, must take place. The feedback from the output must be in phase with the input for regeneration to occur. Several types of oscillators include: a two-stage oscillator, using the noninverted (in phase) output for feedback; a one-stage oscillator using RC circuits to shift the feedback phase 180 degrees to make it positive; and an LC tuned or resonant circuit to cause regenerative feedback. The tuned oscillator also has a crystal, which is not always used, but which has a natural vibrating frequency to control the oscillator.