Voltmeters are a crucial tool that are widely used in a variety of electrical and electronic technology applications for measuring voltage valves. The two main types of these voltmeters are analog and digital. The pointer on an analog voltmeter moves across the dial in accordance with the measurement and displays the corresponding reading. Analog voltmeters are being replaced by digital voltmeter as a result of technological advancements and the quick development of digital systems. So today’s article focuses on describing the digital voltmeter’s operation, as well as its types, advantages, and applications.

What is a Digital Voltmeter?

The digital voltmeter is used, like an analog voltmeter, to measure the potential differences between two particular points in an electric circuit. Discrete numerals are used to display the measured voltage, which can be either alternating or direct current. The numerical displays are so accurate that there won’t be any observational errors made by technicians or operators, and there won’t even be any pointer deflection.

The digital voltmeter, also known as a DVM, is a tool with many uses in a variety of fields. Lessening the size, cost, and power requirements of DVMs became possible with the development of IC technology. The power supply, changes in the input impedance levels, and temperature are just a few of the variables that have an impact on the digital voltmeter’s accuracy.

Less accurate DVMs hold input resistance in the range of 10M, whereas high-accuracy DVMs hold input resistance in the range of 1 G or more for the minimum voltage levels (which is 20V). The device must be routinely calibrated in contrast to voltage standards to ensure that the digital voltmeter precision is within the manufacturer’s tolerance ranges.

DVM Block Diagram and Working Principle

The components and operation of the digital voltmeter are described in this section. The block diagram of the digital voltmeter is displayed in the image below.

DVM Block Diagram

Input Signal – The device for which the voltage level needs to be measured receives this signal as input.

Pulse Generator – This voltage source produces a rectangular pulse as output using either AC, DC, or both methods. Digital circuitry found inside the generator can be used to control the frequency and pulse width levels of the generated rectangular pulse. While analog circuitry controls the levels of rising and falling time.

AND Gate – As is common knowledge, an AND gate’s output is only HIGH when both of its inputs are HIGH. When a train pulse and a rectangular pulse are fed into the AND gate, the output is such that the train pulse and rectangular pulse have the same duration.

Decimal Display Section – The total number of impulses and the time between each impulse are both counted in this section. After calibration, the count is shown on either an LED or LCD screen.

Now, the working of DVM is explained as follows:

1. The pulse generator is fed an unidentified voltage signal. The output in this case is a pulse whose width is equal to the input signal.
2. One input to the AND gate is the generated output from the pulse generator, and the other input is a series of pulses.
3. An AND gate’s output is a positive-triggered series of pulses with a width corresponding to the pulse generated by the pulse generator.
4. The AND gate’s output is now inverted by using the positive triggered pulse output as an input to an inverter.
5. The output of the inverter is now sent to the counter, which keeps track of all the triggers that occurred during that time. The input signal is exactly the same as this.
6. The counter device must now be calibrated in order to have a precise measurement, and it displays a direct voltage reading in volts.

According to the digital voltmeter’s operating principle, the analog input signal is converted into a series of pulses, the number of which is directly proportional to the input signal, just like an analog to digital converter. As a result, a DVM can be created even using A/D conversion methods.

Types of Digital Voltmeter

DVMs are categorized into different types according to the type of analog to digital conversion techniques. These types are clearly explained in the section below.

Ramp Type Digital Voltmeter

The measurement of time is essentially necessary for the operation of DVMs of the ramp type. The system is composed of a ramp generator, which generates a signal that simulates a ramp. The device is known as a ramp type DVM due to the ramp generator that is utilized in this circuit.

Ramp Type DVM

A linear ramp voltage signal is thought to transform from the input signal voltage level because the device’s operation depends on time measurement. An electronic time interval counter device is incorporated into the circuit in order to determine the time interval, and it displays the measurement in digitized form to show the output of the voltmeter.

1. The ranging and attenuation section of this device receives an unknown voltage signal to start measuring voltage; depending on the situation, it either amplifies or attenuates the signal.
2. A positive or negative ramp voltage signal from the ramp generator is also taken into account. Consider a negative ramp signal that will be compared to the unidentified signal.
3. A comparator that compares the ramp and unknown input signals receives the amplified or attenuated signal.
4. When the input voltage and ramp voltage match during the comparison, a pulse is generated, opening the gate. The ramp voltage tends to drop over time until it reaches ‘0’ voltage. To close the gate, the ground comparator now sends a pulse.
5. Gating time intervals refer to the interval between a gate’s opening and closing. The pulses from the clock generator pass through the gate during this time interval, and the counter will count and display those pulses.
6. The oscillator produces clock pulses that are allowed to pass through the gate and are directed to a counter, which keeps track of how many pulses have successfully passed through the gate.
7. The ramp generator uses the initiating pulse from the sample rate multivibrator to start the next ramp voltage signal at the rate that is specified when the measurement cycles are started.

Integrating Type Digital Voltmeter

Here, the instrument measures the precise input voltage value corresponding to the constant time measurement. A voltage-to-frequency converter is used in this circuit, where it uses a feedback control system.

Integrating Type Digital Voltmeter

The integrator receives an unknown input voltage Vin and tends to produce a rising output voltage V out, which is then applied to the level detector.

The detector sends a pulse to the pulse generator gate when the output voltage reaches a particular reference level.

When Vout reaches the fixed voltage level, the detector emits the output pulse. The output from the integrator is now compared with the fixed level voltage of the internal reference source.

The gate is opened by the level detector’s output, allowing oscillator pulses to pass through the pulse generator. The pulse generator functions like a Schmitt trigger, giving each pulse it receives a fixed width and amplitude. As a result, the integrator’s input now has the opposite polarity. As a result, the output voltage Vout returns to its initial value.

The pulse generator receives no output from the level detector because the output voltage is less than the level detector’s reference voltage, and the gate enters a disabled state as a result. Vin will return to its original value and begin to rise as the pulse passes through the generator. The cycle is repeated, and a sawtooth waveform is displayed.

Thus, a pulse is produced for each cycle of the sawtooth wave, and the total number of pulses produced for the given time period. By counting the pulses during the designated time period, this can be determined.

The oscillator that determines the duty ratio of the necessary pulses provides input to the time base selector. The main gate receives the output from the Start/Stop gate after the first pulse has passed through it, meaning that the output from the pulse generator also passes through the main gate.

The main gate is also disabled by the subsequent pulse from the time base selector, which disables the Start/Stop gate. The counter then shows the total number of pulses that have occurred during the designated amount of time, which is a measurement of the voltage that needs to be taken.

Successive Approximation Digital Voltmeter

The output from the DAC is compared to the unknown voltage in this type of DVM, and the instrument can measure 100 readings per second.

Successive Approximation DVM

1. This device has an input amplifier that allows you to select the required input voltage range and reduce noise levels. An S/H circuit is used to feed the comparator’s input from the amplifier section.
2. After each clock pulse, the control register (SAR) receives 8 bits from the counter circuit as input, which is then given to DAC, which transforms the received signal into an analog voltage.
3. Now, an analog signal is fed into the comparator’s second input, and when the comparator’s output is positive, the AND gate’s output changes to logic ‘1’.
4. The control register ultimately provides the digital output. Due to digitization, the input levels vary, and a sample and hold circuit is used to eliminate this error.
5. Consider the input as having 8 bits, for instance. The control register sets the MSB bit D7 to 1 on the very first clock pulse, producing the value 10000000 as the control register’s output.
6. The comparator’s output is negative when Vout exceeds Vin, which resets the D7 bit.
7. The comparator’s output is positive and sets the D7 bit when Vout > Vin. D7 to D0 bits are set in a similar way, and the process continues. The 8-bit input is therefore converted.

SAR Working

Dual Slope Integrating Digital Voltmeter

The block diagram that demonstrates how the dual-slope integrating DVM functions is shown below.

Dual Slope Integrating Type DVM

In this case, switch ‘S’ is used to provide an unknown voltage for a predetermined amount of time as input to the integrator circuit. By measuring the clock frequency in decimal counter devices, this time period can be determined.

The rate at which C charges during this time is proportional to the input voltage Vx. The switch “S” will be moved from Vx to Vref, which has the opposite polarity of Vx, at the conclusion of the time period T. With time, the capacitor’s charging tends to diminish and eventually manifests as a downward linear ramp voltage.

Then Vref is recorded for unknown time ‘t’ for the second time period (known voltage). The ‘t’ in this case is determined by counting the number of timing pulses from the clock until the capacitor’s voltage reaches its fundamental reference. This produces a negative slope that is constant and proportional to the voltage of the input signal.

Therefore, these are the DVMs that are used to measure voltage, resistance, and current values.

Advantages

Digital voltmeters have a number of advantages over analog DVM types. The few benefits of a digital voltmeter are listed below.

• Digital voltmeters allow for the elimination of human error, providing exact and precise readings.
• It is possible to take readings without taking into account factors like the weather, temperature, and others.
• No manual involvement is required to find out the readings.
• DVMs provide enhanced reliability, stability dependability.
• Digital voltmeters are so versatile and inexpensive too.
• The output from DVMs can be provided to memory devices for the purpose of storage.
• DVMs work with minimal power.

Disadvantages of Digital Voltmeter

DVMs have a number of benefits, but they also have some drawbacks, including the following:

• DVMs can run on batteries or an external power source, and the output depends on the power source used.
• DVMs are incorporated with digitizing circuits, which slows down operation.
• Device damage results when the voltage level goes over the specified threshold.
• Digital voltmeters might get heated up upon extended usage and this results in wrong readings.
• DVMs lack the ability to calculate fluctuating readings and may even display output with inaccurate readings.
• It is complicated for DVMs to observe transient voltage spikes.

Applications

The applications of DVM are:

1. The actual voltage levels of various components can be quickly determined by using a digital voltmeter.
2. The known voltage values from the DVM can be used to determine current levels.
3. A digital voltmeter can be used to determine whether the circuit has power or not.
4. Knowing whether a battery is charged or depleted can be determined by using a DVM.