Digital Multimeter

Analog multimeters were manufactured in the early twentieth century. The most common forms measured by this multimeter were d.c. and a.c. voltage, resistance, and d.c. current. These meters were known as volt-ohm-milliammeters or VOMs. After commercial manufacturing of digital voltmeters in 1955, digital measurement and display technologies developed in parallel with digital integrated circuit technology until finally, in late 1960s, portable digital versions of the VOM appeared. This form of meter has since been known as the digital multimeter.

Construction and Working

It shows the basic flow of information from the analog input signal through the various analog signal conversion circuits which convert the measured quantity to a d.c. voltage equivalent. Then the ADC translates this d.c. signal to digital form, and the display system shows the resultant value with appropriate annunciation, such as measurement units of the original input signals. Thus, a digital multimeter is made up of following three basic elements:

(i) signal conditioning
(ii) Analog-to-digital conversion (ADC)
(iii) Numeric digital display

A.C. or d.c. voltage, A.C. or d.c. current, and resistance are the most common parameters to be measured by digital multimeter. Now digital multimeters are also being used to measure additional functions such as frequency, temperature, etc. To measure these parameters, first these inputs must be switched and conditioned to present an equivalent d.c. voltage of the appropriate range to the analog-to-digital converter (ADC) or a sealed input to the counting circuits. The basic functioning of elements such as signal conditioning, ADC and digital displays have already been explained in detail in Section 3.3 while describing digital voltmeter (DVM).

For d.c. voltage measurement by digital multimeter, a wide range of d.c. voltage inputs is scaled to the limited range of the ADC. A resistive divider and switching are generally used for this function.

For a.c. voltage measurement by digital multimeter, the signal is scaled and then converted to a d.c. equivalent value before sending it to the ADC. The methods of this conversion are already explained in Section 3.2 while describing analog electronic voltmeter (EVM).

For d.c. and a.c. current measurements by digital multimeter, it is necessary to convert the current at the input to a voltage for use by the ADC. This is done through a series of switched resistors, called shunts. A small-valued resistor through a series of switched resistors, called shunts. A small-valued resistor in the input is placed in series with the current to be measured. Ohm's law (V = IR) defines the small voltage proportional to the input current which is then measured. AC current measurement uses the same method except that the voltage across the shunt is routed through in a.c.-d.c. voltage converter before going to the ADC.

For resistance measurement, it is necessary to create a voltage proportional to the resistance because the ADC measures only d.c. volts. The digital multimeter input circuit must provide a d.c. current flowing through the resistor and then measure the resulting voltage. Many digital multimeters provide a calibrated constant current source for this purpose. Many others use reference resistors in series with the resistor being measured, and then measure the unknown and reference voltage drops. When combined with the known reference resistor values, this voltage-ratio technique also yields the desired resistance value.

Advantages of Digital Multimeter (digital multimeter)

Following are the advantages of digital multimeters (digital multimeter):

(i) digital multimeters offer high measurement accuracy.
(ii) These instruments have a high input impedance.
(iii) They are smaller in size.
(iv) These meters eliminate observational, parallax and approximation errors.
(v) The measured output of these instruments can be directly fed to a computer for further analysis and use.