Digital Electronic Voltmeter

A digital electronic voltmeter, also called digital voltmeter (digital electronic voltmeter), measures and displays d.c. or a.c. voltages as discrete numerals instead of a pointer deflection on a continuous scale as in analog electronic instruments. In 1965, the first digital voltmeter was marketed by M/s Non Linear Systems Inc. This meter scaled inputs and then digitized them, showing the results in a numeric digital display.

A pulse generator generates a pulse whose width is directly proportional to the input voltage. The output of the pulse generator is one of the inputs of an AND gate circuit. The other input signal to the AND gate is a train of pulses. The output of the AND gate is, thus, a positive negative trigger train. The counter, then, starts counting the number of triggers in T seconds which is proportional to the voltage to be measured. The counter can be calibrated in volt to read the measured input voltage.

Thus, it can be observed from the above description that the digital voltmeter described above, is basically, an analog-to digital converter (ADC) which converts an analog signal into a train of pulses, the number of which conditioning of the input voltage, digital electronic voltmeter can be used to measure many electrical and physical quantities such as a.c. voltages, d.c. and a.c. current, resistance, temperature, pressure, etc.

Digital voltmeters can be classified into the following broad categories on the basis of analog-to-digital converter (ADC) used:

(i) Ramp-type digital electronic voltmeter
(ii) Integrating-type digital electronic voltmeter
(iii) Continuous-balance or Servo-balance digital electronic voltmeter
(iv) Successive-approximation digital electronic voltmeter

Ramp-type digital electronic voltmeter

Working and Construction The operating principle of the ramp-type digital electronic voltmeter is based on measurement of the time taken by the digital electronic voltmeter for a linear ramp voltage to rise from 0 V to the level of the input voltage, or to decrease from level of the input voltage to zero. This time period is measured with an electronic time-interval counter, and the count is displayed as a number of digits on an electronic indicating tube or digital display. A block diagram and voltage-to-time conversion waveform of a ramp-type digital electronic voltmeter

At the start of the measuring cycle, a ramp voltage is initiated. The ramp voltage can be positive-going or negative-going. The negative-going ramp is continuously compared with the unknown input voltage. At the instant that the ramp voltage equals the unknown voltage to be measured, a coincidence circuit, or comparator, generates a pulse tothe gate. The ramp voltage continues to decrease with time until it finally reaches 0 V (or ground potential) and a second comparator generates an output pulse to close the gate. An oscillator generates clock pulses which are allowed to pass through the gate to a number of decade counting units (DCUs) which totalize the number of pulses passed through the gate. The decimal number, displayed by the indicator tubes associated with the DCUs, is a measure of the magnitude of the input voltage. The sample-rate multivibrator determines the rate at which the measurement cycles are initiated. The oscillation of this multivibrator can usually be adjusted by a front-panel control, marked rate, from a few cycles per second to as high as 1,000 or more. The sample-rate circuit provides an initiating pulse for the ramp generator to start its next ramp voltage. At the same time, A reset pulse is generated which returns all the DCUs to their zero state, removing the display momentarily from the indicator tube or display.

Advantages of Ramp-type digital electronic voltmeter Following are the advantages of ramp-type digital electronic voltmeter

(i) The ramp-type circuit is easy to design and its cost is low.
(ii) The output pulse can be transmitted over long distance.

Disadvantages of Ramp-type digital electronic voltmeter Following are the disadvantages of ramp-type digital electronic voltmeter

(i) The single ramp requires excellent characteristics regarding linearity of the ramp and the time measurement.
(ii) Large errors are possible when noise is superimposed on the input signal. Input filters are usually required for this type of converter.