Digital MultimeterIntroduction

2) DIGITAL MULTIMETER Digital multimeters offer higher accuracy and input impedance, unambiguous readings at greater viewing distances, smaller size, and a digital electrical output (for interfacing with external equipment) in addition to visual readout. The main part of most of the digital multimeter (DMMs) is the analog to digital converter (A/D) which converts an analog input signal to a digital output. While specifications may vary, virtually such multimeters are developed around the same block .

DMM

Specifications

• Voltage ranges are from + 200 mV to + 1000 V.• Accuracy is about + 0.03%• Resolution is about 10 µV.• The resistance range is from 200 Ω to 20 MΩ.• The current range is from 200 µA to 2000 mA.

Advantages

• High Accuracy• High Input Impedance.• Short measurement time• Programmability• Smaller size

Operation

• It consists of several A to D converters, decade counters and displays.

• It is basically a dc voltmeter. In order to measure unknown current, current to voltage converter is used.

• An unknown current to be measured is applied to one of the input terminals of Op-Amp and as the input current of Op-Amp is negligible therefore this current will pass through known Resistance Rf. The voltage developed across the known Rf is applied to A to D converter and digital display is obtained.

• To measure an unknown resistance; a constant current source is used.

• To measure ac voltage a rectifier and filter is used.

Advantages

• High Speed• Programmable• Automatic range selection• High Accuracy• Better Resolution• Output can be recorded.

Specifications

• Input range is from +1V to +1000V.• For a 3 digit DVM accuracy is about +0.1%• Resolution is 1 part in 1000000.• Input resistance is 10 MΩ.