Block diagram of Digital multimeter:
The Key process that occurs within a Digital multimeter for any measurement that takes place is that of voltage measurement. If you measure voltage then you can easily measure other electrical parameters with the help of mathematical formulas. To understand how digital multimeter works, first of all, we have to understand this process.
As we know, Digital multimeters gave output in numeric form due to ADC registers present inside these multimeters. One that is most widely used in digital multimeters, DMMs is known as the successive approximation register or SAR. For better accuracy, these SAR ADCs may have resolution levels of 12 bits.
Generally, Digital multimeter have resolution levels of 16 bits with speeds of 100k samples per second. These levels of speed are more than adequate for most DMM applications, that’s why we are using these registers depending upon the requirement.
Operation of Digital multimeter :
The flow chart given below shows the operation flow of digital multimeter.
If you want to know how to read a Digital multimeter then immediately follow this one: Read
(i) Settling time: When the value to be measured is applied to the input of the circuit it would take a certain time to settle is known as settling time. This will overcome any input capacitance levels when high impedance tests are made.
(ii) ADC calibration time: In some DMMs a calibration is periodically performed must be accounted for especially where measurements are taken under automatic or computer control.
(iv) Auto-zero time: To ensure accuracy it is necessary to zero the meter when auto-range is selected, or range changes are made.
(v) Signal measurement time: This is the basic time required to make the measurement itself. For AC measurements, the frequency of operation must be taken into account because the minimum signal measurement time is based on the minimum frequency required of the measurement.
Digital Multimeter Accuracy:
- DMM Accuracy = ±(ppm of reading + ppm of range)
- DMM Accuracy = (% Reading) + (% Range)
- DMM Accuracy = (% Reading) + Offset
Factors affecting Accuracy of Multimeter are:
(i) Temperature: At large extent temperature can affect the accuracy of Digital multimeters. Today many multimeters have an inbuilt temperature feature which eliminates the need of any external device.You can express them as ±(ppm of reading + ppm of range)/°C.
(ii) Resolution: Resolution is directly proportional to accuracy. If you want accuracy you have to take care of resolution also.The resolution of Digital multimeter is expressed in terms of a number of digits displayed.Typically this will be a number consisting of an integer and a half i.e.3 ½ digits etc. By convention, a half digit can display either a zero or 1.
Note: Different multimeters from different manufacturers may work in different ways. It is always recommended to consult the manufacturer’s instructions to understand how a particular digital multimeter works.