DIGITAL VOLTMETER

All types of digital meters are basically modified forms of the digital voltmeter (DVM), irrespective of the quantity that they are designed to measure. Digital meters designed to measure quantities other than voltage are in fact digital voltmeters that contain appropriate electrical circuits to convert current or resistance measurement signals into voltage signals. Digital multimeters are also essentially digital voltmeters that contain several conversion circuits, thus allowing the measurement of voltage, current and resistance within one instrument.

Digital meters have been developed to satisfy a need for higher measurement accuracies and a faster speed of response to voltage changes than can be achieved with analog instruments. They are technically superior to analog meters in almost every respect. However, they have a greater cost due to the higher manufacturing costs compared with analog meters. The binary nature of the output reading from a digital instrument can be readily applied to a display that is in the form of discrete numerals. Where human operators are required to measure and record signal voltage levels, this form of output makes an important contribution to measurement reliability and accuracy, since the problem of analog meter parallax error is eliminated and the possibility of gross error through misreading the meter output is greatly reduced. The availability in many instruments of a direct output in digital form is also very useful in the rapidly expanding range of computer control applications. Quoted inaccuracy figures are between 0.005% (measuring d.c. voltages) and 2%. Additional advantages of digital meters are their very high input impedance (10Mcompared with 1–20 for analog meters), the ability to measure signals of frequency up to 1MHz, and the common inclusion of features such as automatic ranging, which prevents overload and reverse polarity connection, etc.

The major part of a digital voltmeter is the circuitry that converts the analog voltage being measured into a digital quantity. As the instrument only measures d.c.
quantities in its basic mode, another necessary component within it are one that performs a.c.–d.c. conversion and thereby gives it the capacity to measure a.c. signals. After conversion, the voltage value is displayed by means of indicating tubes or a set of solid-state light-emitting diodes. Four-, five- or even six-figure output displays are commonly used, and although the instrument itself may not be inherently more accurate than some analog types, this form of the display enables measurements to be recorded with much greater accuracy than that obtainable by reading an analog meter scale.
Digital voltmeters differ mainly in the technique used to affect the analog-to-digital conversion between the measured analog voltage and the output digital reading. As a general rule, the more expensive and complicated conversion methods achieve a faster conversion speed. Some common types of DVM are discussed below.

Voltage-to-time conversion digital voltmeter:

 This is the simplest form of DVM and is a ramp type of instrument. When an unknown voltage signal is applied to the input terminals of the instrument, a negative-slope ramp waveform is generated internally and compared with the input signal. When the two are equal, a pulse is generated that opens a gate, and at a later point in time, a second pulse closes the gate when the negative ramp voltage reaches zero. The length of time between the gate opening and closing is monitored by an electronic counter, which produces a digital display according to the level of the input voltage signal. Its main drawbacks are non-linearities in the shape of the ramp waveform used and lack of noise rejection, and these problems lead to a typical inaccuracy of š0.05%. It is relatively cheap, however.

Potentiometric digital voltmeter:

 This uses a servo principle, in which the error between the unknown input voltage level and a reference voltage is applied to a servo-driven potentiometer that adjusts the reference voltage until it balances the unknown voltage. The output reading is produced by a mechanical drum-type digital display driven by the potentiometer. This is also a relatively cheap form of DVM that gives excellent performance for its price.

 

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