Digital Multimeter working principle

What is Digital Multimeter?

A digital multimeter or DMM is a test instrument used for measuring resistance, voltage, current 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 levels of accuracy 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. Giving able to give more accurate readings
3. It measures a lot of physical quantities like voltage, current, resistance, frequency etc.
4. It measures various electrical parameters at high frequencies with the help of special probes.
5.  It is less costly.

Block diagram of Digital multimeter

Voltage measurement is the process that takes place in a digital multimeter for any 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 in digital multimeters, DMMs are 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 level of speed is more than sufficient for most DMM applications, so we are using these registers depending on the requirements. As shown in the diagram, the first stage of the process is to sample the voltage at the input of the digital multimeter and then hold it to a constant and hold. 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 output obtained becomes the input of SAR which produces the result in digital form with a good resolution level. With a constant input voltage, the register starts from half of its full-scale value. It basically 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 maximum capacity of the voltage is less than half, the comparative output will be less and it will push the article to the level of 0100 If the voltage is above, the registrar will move to 0110, and soon.

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 sample and hold circuit. Inside the sample and hold circuit a capacitor is present which receives a charge corresponding to the input analog voltage known as the acquisition process. The voltage is considered as a sample when the capacitor is released from the acquisition circuit. After that, the noise usually comes which adversely affects the accuracy of the digital multimeter. To overcome this, we buffered and averaged the samples 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 range of ADC. The digital converter analog then converts the pre-scaled DC voltage to its equivalent digital number which will be displayed in the display unit. Sometimes, a digital controller block is applied by a microcontroller or manages the flow of data 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

In digital multimeters, we can include many types of meters such as ohmmeters, ammeters, voltmeters for measuring electrical parameters. Its block diagram is shown below in the figure. Let's take a look at its functions and specifications. Digital voltmeters are basic instruments used to measure voltage using the 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 the 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. Comparators are driven by a register divider network, the encoder converts its inputs into related displays driven by digital displays. 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 as encoder which converts it to more 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 ammeter 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 measure the current as voltage using a known resistance RK. Such an advanced voltage calibrator is used 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 consisting of a known resistance RK and an unknown resistance formed a voltage across the unknown resistance. Voltage is given: 

V = VB Ru / RK + Ru
where VB = Voltage of the built-in battery

The meter can be calibrated in ohm conditions after the voltage is calibrated.

What do symbols on Digital Multimeter mean?

Some common digital multimeter symbols and their details are given in the table below. These symbols are often found on multimeters and their schematics are designed to symbolize the elements of electrical parameters and reference values.

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, if necessary, displays negative values. A few multimeters today have illuminated the display for better viewing in low light conditions.

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

(iii) Ports: Except for a few four ports for measuring current in MA or A, two ports are found on the front of each multimeter. The different ports in a multimeter are:

(a) COM: It stands as normal and is almost as connected to the ground or as a one-open connection of a circuit. We usually insert a black probe into the COM port.

(b) mAVΩ: This port allows measurement of current (up to 200 mA), voltage and resistance or are considered as a + ve connection of a circuit. We usually think Amaechi Ω port, insert the red probe.

DMM leads:

In a digital multimeter box, we found different colored leads. Here we are going to explain these leads in detail. DMM tops are divided into four parts:

(i) Red lead

• Connected to voltage, resistance or ampere port.
• Considered as a +ve connection of a circuit

(ii) Black lead

• Connected to the common or ground port
• Considered as a -ve connection of a circuit

(iii) Probes: These are the handles used to hold the tip in the tested connection. There are different types of probes available, they are:

• Bananas on alligator clips: These are just great for connecting large wires or pins to a breadboard. Good for long term testing where you should not keep probes while operating a circuit.
• IC hook from banana: IC hooks work well on small ICs and IC legs.
• Banana to Tweezers: Tweezers are effective if you have to test SMD components.
• Banana to Test Probes: If you ever investigate a search, it is cheaper to replace them.

(iv) Tip:

These are present at the end of the probe and, basically, provide a connecting point.

Measurement time: Professional technicians have always preferred instruments that play an important role in the measurement and lead to better results with better accuracy. The timing depends largely on the following factors:

(i) Settling time: When the value to be measured is applied to the input of the circuit it will take a certain amount of time to dispose of it known as the disposal time. This will exceed any input capacitance level if the high disability is tested.

(ii) ADC calibration time: In some DMMs, a calibration is performed periodically, especially where measurements are taken automatically or under computer control.

(iii) Switch time: Switch time is the time required for the device to be disposed of after the input has been switched on. This includes the time of disposal after a change in the type of measurement, e.g., resistance to voltage, etc.

(iv) Auto-zero time: The meter needs to be set to zero if auto-range is selected or the range is changed to ensure accuracy.

(v) Signal measurement time: This is the time it takes to create 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 for the measurement.

Digital Multimeter Accuracy:

Digital multimeters are an ideal choice for every professional technologist due to their better accuracy. This is the amount by which the reading displayed may differ from the actual input. Digital multimeters generally define accuracy as a percentage of reading and one percent of full-scale quality. Accuracy varies from manufacturer to manufacturer depending on the specification of the instrument. There are several ways in which a multimeter can express accuracy:

• DMM Accuracy = ±(ppm of reading + ppm of range)
• DMM Accuracy = (% Reading) + (% Range)
• DMM Accuracy = (% Reading) + Offset

Note: Here ppm refers to every million parts.

(i) Temperature: Large temperatures can affect the accuracy of a digital multimeter. Many multimeters now have a built-in temperature feature that eliminates the need for an external device. You can express them as ± (read ppm + range ppm) / ° s.

(ii) Resolution: The resolution is directly proportional to the accuracy. If you want accuracy, you also need to take care of the resolution. The resolution of the digital multimeter is expressed in terms of the numbers displayed. Normally it is an integer and a number consisting of half i.e.3 ½ numbers etc. According to the convention, half a number can represent zero or 1.

DMM Safety Precaution:

We need to follow some safety precautions before operating multimeters. Here we are going to explain to you some safety information about DMM.

• If the DMM test leads are damaged, never use the meter.
• Always make sure that the test lead and dial are in the correct position for the desired measurement.
• Do not touch the probes to any voltage source when the test lead is plugged into 10A or 300 mA input jack.
• When power is applied it can never provide resistance in any circuit.
• Always place your finger behind the finger guards in your test probe when making measurements.
• To avoid damage or injury, never use meters in circuits that exceed 4800 watts.
• Replace the battery as soon as possible to avoid false texts that could cause electric shock or personal injury.
• Use caution when working with voltages above 60 volts DC or 30V AC RMS. This type of voltage poses a shock hazard.