Digital vs. Analog Instruments

Electronic instruments mainly voltmeters used either transistor or vacuum tubes. The later one is called The Vacuum Tube Voltmeter (VTVM) and the former one is called the Transistorized Voltmeter (TVM). In almost every field of electronics, VTVMs have been replaced by TVMs because of their numerous advantages. In TVM, due to the absence of a heating element, warm-up time is not required. The TVM is portable due to the light weight of the transistor. VTVMs cannot measure current due to the very high resistance, whereas due to the low resistance of the TVM, it can measure the current directly from the circuit. VTVMs also cannot measure high-frequency signals. The only disadvantage of TVM over VTVM is that the TVM has very low input impedance. But by using Field Effect Transistor (FET) in the input stage of the voltmeter it overcomes this low-impedance problem, because FET offers input impedance almost equal to a vacuum tube.

Advantages of Digital Instruments over Analog ones

Detection of low level signals

Analog instruments use permanent magnet moving coil instrument (PMMC) movement for indication. This movement cannot be constructed with a full scale sensitivity of less than 50 mA. Any measurement using a PMMC movement must draw a current of 50 mA from the measured quantity for its operation for full scale deflection if conventional voltmeters are used. This would produce great loading effects especially in electronic and communication circuits. Electronic voltmeter avoids these loading errors by supplying the power required for measurement by using external circuits like amplifiers. The amplifier not only supply power for the operation but make it possible for low level signals which produce a current less than 50 mA for full scale deflection, to be detected which otherwise cannot be detected in the absence of amplifiers.

High Input Impedance

The conventional PMMC voltmeter is a rugged and an accurate instrument but its main problem is that it lacks both high sensitivity and high resistance. The electronic voltmeter can have high input resistances with the input resistances remaining constant over all ranges instead of being different at different ranges, hence less loading effects.

High Frequency Range

The response of electronic voltmeters can be made practically independent of frequency within extremely wide limits.

Some electronic voltmeters permit the measurement of voltage from direct current to frequency of the order of MHz The high frequency range may also be attributed to low input capacitance of most electronic devices. The capacitance may be of the order of a few pF.

Accuracy

Since there are few moving parts (or even no moving parts) in the digital instruments, they are generally more accurate than the analog instruments. The human error involved in the reading digital instruments is very small, which adds to the accuracy of these instruments. However, the overall accuracy of any digital instrument will largely depend on accuracies of the larger number of individual electronic components used for building the instruments.

Better Resolution

The resolution (smallest reading perceivable) of analog instruments is limited by space on the scale marking and also by the ability of the human operator to read such small deviations in scale markings whereas in a digital instrument the measured value is displayed directly on LED or LCD panel whose resolution is solely determined by resolution of the analog to digital converter (ADC). Use of 12 bit (or higher) ADC can make a digital instrument to read as small as 0.001 V in 0-5 V range.

Storage capability

Digital instruments readings can be stored for future reference. Since the value displayed is obtained through an ADC, the digital data can easily be stored in a microprocessor or PC memory. Such storage facility can only be made available in analog instruments by the use of chart recorders where the pointers has an ink source that keeps on marking the values on a roll of moving paper.

Disadvantages of Digital Instruments

• Analog instruments have higher overload capacity than digital instruments. The sensitive electronic components used in digital instruments are prone to damage in case of even momentary overloads.
• Effects on noise are more predominant on digital instruments than analog instruments. Analog instruments due to inertia of its moving parts normally remain insensitive to fast varying noise, while digital instruments continue to show erratic variations in presence of noise.
• Digital instruments and their internal electronic components are very much sensitive to external atmosphere conditions. In case of high humidity and corrosive atmosphere the internal parts may get damaged and indicate the faulty values.
• Digital instruments can sometimes loose its reliability and tend to indicate erratic values due to faulty electronic circuit components or damaged display.

Also read: Performance Characteristics of Digital Measurement Instruments