Electrical & Electronic Measurements : Digital Voltmeters

By Yash Bansal|Updated : November 24th, 2021

                                                                          

Analog to Digital Conversion

A voltmeter is an electrical measuring instrument that is used to measure the potential difference between two points. The voltage to be measured may be AC or DC. Two types of voltmeters are available for the purpose of voltage measurement i.e. analogue and digital. Analogue voltmeters generally contain a dial with a needle moving over it according to the measure and hence displaying the value of the same.

  • These are generally known as DVM that use digital formatting of the input fed to its input leads. Here, the result of measurement is shown in form of discrete numbers for which they employ display devices for the decimal number system.
  • Digital voltmeter also attains an inherited greater speed of operation. Because output obtained from these instruments comes to be a digital form so it becomes easier to use them directly as an input to many other devices like memory devices so that the result may be used further in future, this is called storage of data
  • Because of high accuracy, high-speed operation and greater reliability they are frequently used in laboratories and industries for the purpose of experimentation and obtaining highly accurate results.
  • It can be accepted as a disadvantage of digital voltmeters that they always need some external power supply for its operation that make it less portable and also bulkier but with the advancements made in the field of integrated circuits, it has become possible to make such digital device very compact, more efficient, low cost and having even greater accuracy.
  • This advancement has led to the verge that now some digital devices are there having less cost than their competent analogue ones with the same extent of accuracy. 
  • A/D converts an analogue signal into the digital code which is proportional to the magnitude of the coming signal.

Vin ≈ k ×Digital output

Where k is step size or resolution.

Quantization error or Conversion error of a A/D

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Conversion time, Tc: The time requires to convert an analogue signal to the corresponding digital code.

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On the basis of the A/D conversion method used digital voltmeters can be classified as:

  • Ramp type digital voltmeter
  • Integrating type voltmeter
  • Potentiometric type digital voltmeters
  • Successive approximation type digital voltmeter
  • Continuous balance type digital voltmeter

Now-a-days digital voltmeters are also replaced by digital millimeters due to its multitasking feature i.e. it can be used for measuring current, voltage and resistance. But still there are some fields where separated digital voltmeters are being used.

Ramp-type Digital Voltmeter

Its Operating principle is based on the measurement of the time it takes for a linear ramp voltage to rise from 0 V to the level of the input voltage, or to decrease from the level of the input voltage to zero. This time interval is measured with an electronic time-interval counter, and the count is displayed as a number of digits on electronic indicating tubes.

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The working principle i.e., the Conversion from a voltage to a time interval is illustrated by the waveform 

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  • At the start of the measurement cycle, a ramp voltage is initiated; this 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, a coincidence circuit, or compactor, generates a pulse which opens a gate.

  • The ramp voltage continues to decrease with time until it finally reaches 0 V (or ground potential) and a second compactor generates an output pulse which closes 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 Multi-vibrator 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 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 0 state, removing the display momentarily from the indicator tubes.

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