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Digital Multimeter

 Electronic Multimeter

For the measurement of d.c. as well as a.c. voltage and current, and resistance, an electronic multimeter is commonly used. It is also known as Voltage-Ohm Meter (VOM) or multimeter The important salient features of VOM are as listed below.
  1. The basic circuit of VOM includes a balanced bridge d.c. amplifier.
  2. A RANGE switch is provided to limit the input signal's magnitude. By properly adjusting input attenuator input signal can be limited.
  3. It also includes a rectifier section which converts a.c. an input signal to the d.c. voltage. 
  4. It facilitates resistance measurement with the help of an internal battery and additional Circuitry
  5. The various parameters measurement is possible by selecting the required function using the FUNCTION switch.
  6. The measurement of various parameters is indicated with the help of Indicating Meter.
A multimeter measures a.c. and d.c. voltage, a.c. and d.c. currents and resistance.


Use of Multimeter for d.c. Voltage Measurement

For getting different ranges of voltages, different series resistances are connected in series which can
be put in the circuit with the range selector switch. We can get different ranges to measure the d.c. voltages by selecting the proper resistance in series with the basic meter.

Use of Multimeter as an Ammeter:

To get different current ranges, different shunts are connected across the meter with the help of a range selector switch. The working is the same s that of the PMMC ammeter.

Use of Multimeter for Measurement of a.c. Voltage

The rectifier used in the circuit rectifies a.c. voltage into d.c. voltage for measurement of a.c. the voltage before the current passes through the meter. The other diode is used for protection purposes.


Use of Multimeter for Resistance Measurement

Before any measurement is made, the instrument is short circuited and "zero adjust" control is varied until the meter reads zero resistance i.e. it shows full scale current. Now the circuit takes the form of a variation of the shunt type ohmmeter. Scale multiplications of 100 and 10,000 can also be used for measuring high resistances. Voltages are applied to the circuit with the help of battery.
Ohmmeter circuit R x 1 range
Advantages
  1. The input impedance is high.
  2. The frequency range is high.
  3. The circuit is simple.
  4. The cost is less.
  5. The construction is rugged.
  6. It is less suffered from electric noise.
Disadvantages
  1. The accuracy is less.
  2. The resolution is poor.
  3. It is difficult to interface the output with the external devices.
  4. Not compact in size.
  5. The reliability and repeatability are poor.

Digital Multimeter


The various measurements possible by DMM are resistance, a.c. voltage and current, d.c. voltage and current. The selection of the parameter is possible with the help of rotary switch connected to input probes of DMM.
(1) Resistance measurement : The rotary switch is in position '1' and resistance is connected to input probes. The constant current source drives a current through unknown resistance. This produces voltage across resistance which is directly proportional to the resistance. It is given to the buffer amplifier and then to analog to digital converter. The ADC converts it to equivalent digital signal and it is displayed with the help of digital display.
(2) A.C. Voltage measurement : The rotary switch is in position '2' and input a.c. voltage is applied to probes. If it is above the selected range, it is attenuated with the help of compensated attenuator. It is rectified to produce proportional d.c. voltage. Then it is given to ADC which displays it in volts.
(3) A.C. Current measurement : The rotary switch is in position '3' and unknown current is applied across input probes. It is converted to proportional voltage using current to voltage converter.


This I-V converter is op-amp circuit as shown in the Fig. The op-amp input current is zero hence lil flows through R1 and drop across R1 is Vo hence output voltage Vo  proportional to Iin
The resistances R1, R2 and R3 are used for the proper range selection. This voltage is rectified and then given to ADC which displays the current in amperes. 
(4) D.C Current measurement : The rotary switch is in position '4' and unknown d.c. current is applied across input probes. This is converted to proportional voltage With the help of current to voltage converter. This voltage is given to ADC without rectification. As this is proportional to d.c. current, ADC displays it in amperes on digital display. 
(5) D.C. Voltage measurement : The rotary switch is in position '5' and unknown voltage is applied across input probes. It is attenuated and directly given to ADC without rectification. The ADC displays it in volts.

Advantages
  1. The accuracy is very high.
  2. The input impedance is very high hence there is no loading effect.
  3. An unambiguous reading at greater viewing distances is obtained.
  4. The output available is electrical which can be used for interfacing with external equipment.
  5. Due to improvement in the integrated technology, the prices are going down.
  6. These are available in very small size.
  7. The requirement of power supply, electric noise and isolation problems are the two limitations.
Specifications of Digital Multimeter
i) D.C. voltage
  • There are five ranges available from ± 200 mV to ± 1000 V.
  • The resolution is 10 PV on the lowest range.
  • The accuracy is ± 0.03 % of the reading + two digits.
ii) A.C.voItage
  • There are five ranges from 200 mV to 750 V
  • The resolution is 10 PV on the lowest range.
  • The accuracy is frequency dependent but the best accuracy is 0.5 % + 10 digits
  • between 45 Hz and 1 kHz on all the ranges.
iii) D.C. current
  • There are five ranges from ± 200 uA to ± 2000 mA.
  • The resolution is ± 0.01 PA on the lowest range.
  • The accuracy is ± 0.3 % of reading + two digits.
iv) A.C. current
  • There are five ranges from 200 uA to 2000 mA.
  • The accuracy is frequency dependent but the best accuracy of ± 1 % + ten digits
  • between 45 Hz and 2 kHz on all the ranges.
v) Resistance
  • Six ranges are available from 200 Ohm to 20 M Ohm.
  • The accuracy is ± 0.1 % of reading + two digits + 0.02 Ohm on the lowest range.
VI) Input impedance
  • The input impedance is about 10 M Ohm on all the ranges.
vii) Normal mode noise reaiection
  • It is greater than 60 dB at 50 Hz while the common mode noise rejection is greater than 90 dB at 50 Hz and greater than 120 dB at d.c.
viii) Overload protection
  • The overload protection of 1000 V d.c. and 750 r.m.s. a.c. is provided.
ix) Diode test
  • The voltage drop across the diode can be measured for which 1 mA ± 10 % of constant current source is used.
x) Conductance
  • It can display conductance in siemens.
xi) Relative reference
  • When s REL' button is pressed, the displayed reading is stored as a reference and then subtracted from the subsequent readings to indicate only amount of deviation from the reference.
xii) Frequency
  • The frequency range is 200 Hz to 200 kHz autoselection.

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