Digital Voltmeters: Basic Features, Operation & Applications

The digital voltmeter (DVM) displays measurement of ac or dc voltages as discrete numbers instead of a pointer deflection on a continuous scale as in analog instruments. A digital voltmeter basically consists of an analog-to-digital converter, a set of seven-segment numerical displays and the necessary BCD-to-seven-segment drivers. A digital voltmeter is a versatile and accurate instrument that is used in many laboratory measurement applications. Because of the advancement and perfection of IC modules, their size, power consumption and cost has considerably reduced and hence DVMs are extensively used in measurement applications.

A block diagram of a simple digital voltmeter is shown below:

With reference to the above figure, the unknown signal is fed to the pulse generator which generates a pulse whose width is directly proportional to the input unknown voltage. The output of the pulse generator is applied to one leg of an AND gate. The input to the other leg of the AND gate is a train of pulses. The output of the AND gate is, thus a positive trigger train of duration t second and the inverter converts it into a negative trigger train. The counter counts the number of triggers in t seconds which is proportional to the voltage under measurement. Hence, the counter can be calibrated to indicate in volts directly.

What I have just described above is essentially an analog to digital converter (ADC) which converts an analog signal into a train of pulses, the number of which is proportional to the input voltage. Thus a digital voltmeter can be constructed by using any of the analog to digital conversion techniques. We can therefore classify digital voltmeters on the basis of ADCs use, common DVM types include:

• Ramp-type DVM
• Dual-slope Integrating type DVM
• Integrating-type DVM (voltage to frequency conversion)

The input range of the DVM may vary from ±1.00000 V to ±1000 V and its limiting accuracy is as high as ±0.005% of the reading. Its resolution may be 1 part in 10⁶, giving 1 µV reading of the 1 V input range. It has high input resistance of the order of 10 MΩ and input capacitance of the order of 40 pF.

Applications of Digital Voltmeters (DVMs)  

Digital voltmeters are frequently used in data processing systems or data logging systems where a number of analog input signals are scanned sequentially by an electronic system and then each signal is converted to an equivalent digital value by the analog to digital converter in the DVM. In this way, a number of input signals can be automatically scanned or processed and their values either logged or printed.

Related articles:

• Electrical vs. Electronic Instruments
• Digital Multimeter: Principle of Operation and How it is used
• Digital vs. Analog Instruments
• Null vs. Deflection Type Instruments
• Performance Characteristics of Digital Measurement Instruments