Digital Multimeter working principle

What is Digital Multimeter?

A digital multimeter or DMM is a test instrument used for measuring resistance, voltage, current measurements and other electrical parameters as needed and displays the results of mathematical calculations on LCD or LED readouts. It is a kind of multimeter that works digitally. Digital multimeters are widely accepted worldwide because they have better accuracy levels and range from a typical 3 ½ to 4 ½ handheld DMM to a very specialized system DMM.

Features of Digital Multimeter

 

The digital multimeter is the most advanced measuring instrument that uses modern integrated circuits to create electrical measurements. Some of the features that make it famous in the eyes of professional technicians are:

1.  It is light in weight.
2. Capable of giving more accurate readings.
3.  It measures a lot of physical quantities like voltage, current, resistance, frequency etc.
4. It is less costly.
5.  It measures various electrical parameters at high frequencies with the help of special probes.

Block diagram of Digital multimeter

The process that takes place in a digital multimeter for any measurement is voltage measurement. If you measure voltage, you can easily measure other electrical parameters with the help of mathematical formulas. To understand how a digital multimeter works, we must first understand this process. As we know, digital multimeters give output in the form of numbers due to the ADC registries present inside this multimeter. One of the most widely used digital multimeters, DMMs, is continuously known as approximate registers or SARs. For better accuracy, this SARC may have a resolution of 12 bits. Typically, a digital multimeter has a resolution level of 16 bits with a sample speed of 100k per second. This speed level is more than sufficient for most DMM applications, so we use these registers depending on the requirements. As shown in the diagram, the first stage of the process is a sample and the hold is used to sample the voltage at the input of the digital multimeter and then to stabilize it. In the first stage, the output becomes an input of the operational amplifier and another input of the amp is the digital output response via DAC. The work obtained becomes the input of SAR, which produces the digital form result with a good resolution level. With a constant input voltage, the registration starts at half of its full-scale value. It sets the most significant bits, MSB to "1" and everything else to "0". Take a simple example of a 4-bit SAR to see how it works. Its output will start at 1000. If the voltage is less than half of the maximum capacity, the comparative production will be low and this will force the article to the level of 0100. If the voltage is above, the register will go to 0110 and much more.

Operation of Digital multimeter

The flow chart below shows the operation flow of a digital multimeter. As shown above, sample acquisition is done with the help of a sample and hold circuit. Inside the piece and hold circuit, a capacitor receives a charge corresponding to the input analog voltage known as the acquisition process. When the capacitor is released from the acquisition circuit, the voltage is considered to be the sample. After that, the noise usually comes, which adversely affects the accuracy of the digital multimeter. To overcome this, we buffered and averaged the pieces to achieve high accuracy and resolution.

Knowing this, you can easily use a digital multimeter to measure electronic parameters such as AC and DC voltage, current, resistance, capacitance, etc.

Working Principle of Digital Multimeter

As shown in the block diagram, the input signal in a simple digital multimeter, i.e., AC or DC voltage, current, resistance, temperature, or any other parameter, is converted to DC volt within the ADC range. The digital converter analog then converts the pre-scaled DC voltage to its equivalent digital number, displayed in the display unit. Sometimes, a digital controller block is applied by a microcontroller or manages data flow between a microprocessor device. This block will integrate all internal functions as well as transfer data to external devices such as a printer or a personal computer. In the case of some handheld multimeters, all or some of these blocks can be applied to a VLSI circuit when the A / D converter and the display driver are on the same IC.

Digital Multimeter as Voltmeter, Ammeter and Digital Ohmmeter

We can include many kinds of meters such as ohmmeters, ammeters, and voltmeters for measuring electrical parameters in digital multimeters. Its block diagram is shown below in the figure. Let's take a look at its functions and specifications.

(i) Digital voltmeter (DVM):

Digital voltmeters are basic instruments used to measure voltage using analog to digital converters. The basic principle behind a digital multimeter is to convert analog to digital because without it we would not be able to convert analog output to digital. There are several ADCs available in the market but we usually use its flash type ADC due to its simplicity and fast speed. Let's take a look at its initial activity.

(A) Flash AD Converter: It has a comparator, encoder and digital display. A register divider network drives the comparators. The encoder converts its inputs to the relevant outputs driven by the digital display. 

As shown above, the three resistors of value R drive comparatively C1, C2, C3. The input voltage is equal to vi = 1V, + V = 4V and the comparator i.e. C1, C2, C3 voltage is equal to 1V, 2V and 3V respectively. If C1 = +1 and C2 = C3 = 0 are the outputs, we feed 001 as the input encoder which further converts it to 0001.

This binary output displays seven sections for reading 1V in it. With this method, we read voltages of 1V, 2V, 3V lengths and we add more comparators for more accurate readings according to our requirements.

(ii) Digital Ammeter (DAM):

A digital maltimeter uses shunt resistors to produce a calibrated voltage proportional to the current flow. As shown in the diagram, to read the current we first need to convert it to measure current as a voltage using a known resistance RK. The voltage is so calibrated to read the input current.

(iii) Digital ohm meter (DOM):

A digital ohmmeter is used to measure the electrical resistance which prevents the flow of current. As shown in the figure, a resistance network would develop a voltage across an unknown resistance consisting of a known resistance RK and unknown resistance. Voltage is given:

The meter can be calibrated in ohm condition after calibrating the voltage.

DMM Parts and functions

A digital multimeter is divided into three parts:

(i) Display: The LCD screen at the top of the multimeter basically displays four or more digits and also displays negative values if required. A few multimeters today illuminated the display for better viewing in low light conditions.

(ii) Selection dial: This allows the user to set the multimeter to read the milliamps (mA) of various electrical parameters such as current, voltage, resistance, capacitance etc. You can easily rotate the dial anywhere to measure specific parameters.

(iii) Ports: Two ports are available in front of each multimeter except for a few ports for current measurement in MA or A. We have plugged two probes into these ports which are of different colors i.e. one is red and the other is black. The different ports on a multimeter are:

(A) COM: It stands for general and is considered to be almost connected to the ground or -ve connection of a circuit. We usually insert a black probe into the COM port.

(B) MAV Ω: This port allows measurement of current (up to 200 mA), voltage and resistance or considered as a + ve connection of a circuit. We usually insert the red probe into the MAV6 port.