Digital Multimeter Introduction 2) DIGITAL MULTIMETER Digital multimeters offer higher accuracy and...

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.