Digital Multimeter

Digital Multimeter Points : Digital Multimeter Digital multimeter (DVM) is basically a digital voltmeter and may be used or the measurement of voltage, current (d.c. or a.c) and resistance. All quantities other than d.c voltage is first converted into an equivalent d.c voltage by some device.

The block diagram of a basic digital multimeter is given in the fig for measurement of ac voltage the input voltage is converted into a d.c. voltage by means of rectifier. A compensate attenuator is employed. Many manufacturers provide the same attenuator for both d.c. measurements.

For measurement of resistance of constant current, depending on the range, supplied from a battery or a constant current source is passed through the resistance under measurement and the voltage developed across it is measured. The resistance value is displayed in ohms.

For measurement of current, the unknown current is passed through a precision resistor in many commercial digital multimeter and the voltage developed across the precision resistor is measured. The current value is displayed in m A.

For measurement of current a current-to-voltage converts may also be used, as illustrated block diagram in fig.

The current under measurement is applied to the summing junction at the input of the operational amplifier. The current in the feedback resistor I_R is equal to the input current I in because of very high input impedance of the operational amplifier. The current IR causes a voltage drop across one of Ii_n,. . Different resistors are employed for different ranges.