Electronic amplifiers are used with analog meters to increase their ranges, to make them more sensitive, to improve their accuracy, and to minimize their effects on the circuit under test. But, any analog meter movement can be used effectively without amplifiers; they need only multiplier resistors and/or shunt resistors to work on different ranges.
Digital meters differ from analog meters because they need electronic amplifiers if they are to work at all. The electrical characteristics that we work with are analog in nature, so some electronic circuits are necessary for the test reading to go through an analog-to-digital conversion to get a digital readout. Several different techniques were tried in the past, including electromechanical servo control, oscillator frequency control, and the one method that works best, pulse gate control.
Servo systems are motor systems that can be used to position the component being driven by the motor. The servo motor does this by producing a feedback signal voltage of its own while being driven by the test voltage. When the motor reaches the position dictated by the value of the test voltage, the feedback voltage cancels, or nulls, the test voltage to stop the motor. Each different test voltage level will cause the motor to position the potentiometer at a different place to produce an equal but opposite null voltage. The shaft of the motor also drives a series of drum wheels, like a car’s odometer, with digital readouts to match the servo’s positions.
The digital servo was one of the earliest systems tried to produce a digital readout. It was expensive, a little clumsy, slow, required more power, and not accurate enough. It is not a purely digital device since the servo follows an analog positioning of the drum scale and the digits on the scale simulate digital positioning.
Analog-to-Digital Frequency Control
The first attempt to develop a digital meter that did not rely on electromechanical devices used an oscillator circuit to produce a string of pulses which could represent a string of digits. The frequency of oscillation of the circuit was designed to be dependent on the test signal voltage. The higher the test signal voltage, the higher would be the frequency produced, and the higher the frequency, the more pulses per second there would be. At the right frequency, each pulse could represent a digit, the higher the test voltage, the higher the frequency, the more the pulses, and the higher the pulse or digit count. For each test voltage level, the pulses can be counted during a fixed time interval. A gate circuit was used to go on and off at fixed intervals to let through a string of pulses to be counted, then to drive a digital readout.
The variable oscillator was a good start, but it had serious limitations. An oscillator circuit tends to function most efficiently at a given frequency, or a relatively narrow range of frequencies. Getting the oscillator to vary its frequency in a wide enough range, or sets of ranges requires numerous complex circuits that are difficult to get to function reliably and accurately.
Analog-to-Digital Pulse Gate Control
The pulse gate control method of analog-to-digital conversion solves the problem of an unstable oscillator that must have its frequency shifted because the gate control method uses a fixed frequency oscillator. Also, instead of having a fixed time interval gate, this method uses a variable interval gate, which is narrower for low test voltages and gets wider as the test voltages go higher. So, with a fixed frequency, the variable-width gate will let the number of sampled pulses go up as the test voltage goes up.
The design of this kind of digital meter circuit can establish a basic fixed oscillator frequency and a pulse count availability so that each pulse in a gate sampling will represent one digit, and the actual count of the number of pulses will be the actual digit value that is generated.
This type of analog-to-digital conversion does not merely produce a digit that simulates the analog value. It produces true digital values.