what are the digital meters?

what are the digital meters?

what are the digital meters? Discuss their advantages and disadvantages. Nowadays, everyone knows the analog signals. They are used to move an electromagnetic meter to measure voltage, current, resistance, power, etc. Although bridges and multipliers use electrical components for these measurements, the instruments described do not use amplifiers to increase the sensitivity of the measurements. The Arsonval meter was at the heart of these instruments and, in general, could not be constructed with a total sensitivity of less than approximately 50 μA. Any measuring system that uses the Arsonval meter, without amplifiers, must have at least 50 μA of the circuit under test for a maximum deviation.

For current measurements less than 50 μA full-scale, an amplifier must be used. The resistance of a sensitive counter, for example a counter of 50 μA for a volt-ohm-millimeter, is of the order of a few hundred ohms and represents a low but finite power. For example, 50 μA in a 200 Ω counter represents ½ microwatt.

This represents the power required by a meter for a large-scale deviation and does not represent the power dissipated in the series resistance. The total power required by the counter example would, therefore, be greater than ½ μW and would depend on the voltage range. It does not look like much power, but many electronic circuits can not tolerate so much power that it drains. That is why electronic instruments are needed to measure very low currents and voltages.

Electronic instruments, mainly electronic voltmeters, used transistors or vacuum tubes. The last one is called a vacuum tube voltmeter (VTVM) and the first is called a transistor voltmeter (TVM). In almost all areas of electronics, VTVs have been replaced by TVMs due to their many advantages. In TVM, due to the absence of a heating element, no preheating time is required.

It is portable due to the light weight of the transistor. The VTVM can not measure the current due to the very high resistance, while, due to the low resistance of the TVM, they can measure the current directly from the circuit. VTVMs can not measure high frequency signals either. The only drawback of TVM compared to VTVM is that the latter has a very low input impedance. But the use of FET in the input stage of the voltmeter solves this problem of low impedance, because an FET provides an input impedance almost equal to a vacuum tube.

Advantages & disadvantages of Digital Instruments over Analog

Advantages of Digital Instruments over Analog instruments.

what are the digital meters? Discuss their advantages and disadvantages. Lets answer this question now.

Detection of Low Level Signals:

  • As indicated above, analog instruments use the PMMC movement for indication. This movement can not be built with a total sensitivity of less than 50 mA. Any measurement that uses a PMMC movement must draw a current of 50 mA from the measured quantity in order to operate in full deviation if conventional voltmeters are used. This would produce large load effects, especially in electronic and communication circuits. The electronic voltmeter avoids load errors by providing the necessary power for the measurement using external circuits, such as amplifiers. The amplifiers not only provide power for operation, but also allow the detection of low level signals, which produce a current below 50 mA for a maximum deviation, impossible to detect in the absence of amplifiers. Let’s examine the loading effect. A typical meter has a resistance of 200 W and its full scale operating current is 50 mA. This means that the power consumed is only (50 ¥ 10-6) 2 ¥ 200 = 0.5 mW. This is an extremely low power for power circuits, but not for many electronic and communication circuits. If this power is extracted from the measurand, the signal is strongly deformed if the power level is very low and, to compensate, this power is supplied from the outside by the use of amplifiers.

High Input Impedance

  • A conventional PMMC voltmeter is a robust and accurate instrument, but it has some disadvantages. The main problem is that it lacks high sensitivity and high input resistance. It has a sensitivity of 20 kW / V with a range of zero to 0.5 V and an input resistance of only 10 kW in its range of 0.5 V; this results in a maximum scale current of 50 mA that considerably charges the measurand. In electronic and communication circuits, even this low current value may not be tolerable because these circuits have very low operating currents. The electronic voltmeter (EVM), meanwhile, can have input resistance from 10 MW to 100 MW, the input resistance remains constant in all ranges instead of being different in different ranges, the EVM allows to reduce the effects of the load .what are the digital meters? Discuss their advantages and disadvantages.

Low Power Consumption

  • Electronic voltmeters use the amplification properties of vacuum tubes and transistors. Therefore, the energy required for the operation of the instrument can be provided by an auxiliary source. Therefore, while the circuit whose voltage is measured controls the sensor element of the voltmeter, the power absorbed by the circuit being measured is very small or even negligible. This can be interpreted as a very high input impedance voltage in the voltmeter circuit. This characteristic of the electronic voltmeter is indispensable to measure the voltage in many high impedance circuits, such as those found in communication equipment.

High Frequency Range

  • The most important feature of electronic voltmeters is that their response can be virtually independent of frequency within extremely wide limits. Some electronic voltmeters measure the DC voltage at a frequency of the order of hundreds of MHz. The high frequency range can also be attributed to the low input capacity of most electronic devices. The capacitance can be of the order of some peak F.

Better Resolution

  • (the smallest possible reading) of the analog instruments is limited by the space in the scale marks, as well as by the ability of the human operator to read such small deviations in the scale marks. While on a digital instrument, the measured value is displayed directly on an LED or LCD panel whose resolution is determined only by the resolution of the analog to digital converter (ADC). The use of a 12-bit ADC (or higher) can allow a digital instrument to read up to 0.001 V in a range of 0 to 5 V.

Storage Facility

  • Digital instruments have an optional additional benefit: their readings can be stored for future reference. Since the displayed value is obtained through an ADC, the digital data can be easily stored in a microprocessor or in a PC memory. Said storage facility may be available in analog instruments only through the use of graphic recorders where the pointer has an ink source that continues to mark values on a moving paper roll.

Accuracy

  • Since digital instruments have very few moving parts (if any), they are generally more accurate than analog instruments. Even the human error involved in reading these instruments is very small, which increases the accuracy of digital instruments. However, the overall accuracy of a digital instrument will depend to a large extent on the accuracy of the large number of individual electronic components used to build the instrument.
  • In addition, digital instruments are easier to use because they are easy to read, take up less space, are suitable for serial production, and are sometimes less expensive.

What are the disadvantages of Digital Instruments?

  • Noise effects are more frequent in digital instruments than in analog instruments. Analog instruments, due to the inertia of their moving parts, are normally insensitive to rapidly changing noises, while digital instruments continue to show irregular variations in the presence of noise.
  • Analog instruments have a higher overload capacity than digital instruments. Sensitive electronic components used in digital instruments are more likely to be damaged in the event of an overload, even momentary.
  • Digital instruments can sometimes lose their reliability and tend to indicate erratic values ​​due to defective components of the electronic circuit or a damaged screen.
  • Digital instruments and their internal electronic components are very sensitive to external atmospheric conditions. In case of high humidity and corrosive atmosphere, internal parts can be damaged and indicate erroneous values.

 

Performance Characteristics of Digital Meters

The performance characteristics of digital instruments are resolution, accuracy, linear errors, monotonicity, settling time and temperature sensitivity.

Resolution

This is the opposite of the number of discrete steps in the digital to analog converter (D / A) input. The resolution defines the lowest voltage increase that can be discerned. Obviously, the resolution depends on the number of bits, that is, the smallest output voltage increase is determined by the least significant bit (LSB). The resolution percentage is [1 / (2n – 1)] × 100, where N is the number of bits.

Accuracy

It is a measure of the difference between actual production and expected production. It is expressed as a percentage of the maximum output voltage. If the maximum output voltage (or the maximum deviation) is 5 V and the accuracy is ± 0.1%, the maximum error is (0.1 / 100) × 5.

= 0.005 volts or 5 mV. Ideally, the accuracy should be better than ± ½ of LSB. In an 8-bit converter, LSB is 1/255 or 0.39% of the full scale. The precision should be better than 0.2%.

Linear Error

Linearity means that equal increments of the digital input of the digital instruments should result in an equal increase in the analog output voltage. If the resistance values are very accurate and the other components are also ideal, there would be a perfectly linear relationship between the output and the input and the output-input graph would be a straight line. Due to the fact that the resistors used in the circuit have a certain tolerance, a perfectly linear relationship between input and output is not obtained. A special case of linear error is the displacement error which is the output voltage when the digital input is 0000.

Monotonicity

A digital to analog (D / A) converter is monotonic if it does not work in reverse when it is sequenced in the entire range of input bits.

Settling Time

When the digital input signal changes, it is desirable that the analog output signal immediately indicate the new output value. However, in practice, the D / A converter takes some time to establish itself in the new position of the output voltage. The stabilization time is defined as the time it takes for the D / A converter to stabilize within ± 0.5 LSB of its final value when a digital input signal change occurs. The finite time required to stabilize at the new value is due to the transients and oscillations of the output voltage.

Figure

Temperature Sensitivity

The reference voltage supplied to the resistors of a D / A converter is temperature sensitive. Therefore, the analog output voltage depends, at least to some extent, on the temperature. The temperature sensitivity of the compensation voltage and the bias current of OP-AMP also affect the output voltage. The temperature sensitivity range for a digital-to-analog converter is approximately ± 50 to ± 1.5 ppm / ° C.

Settling time of a digital instrument
Settling time of a digital instrument

Digital Voltmeters (DVMS)

The digital voltmeter (DVM) shows the measurement of AC or DC voltages as discrete numbers instead of a pointer deviation on a continuous scale as in analog instruments. It is a versatile and precise instrument that is used in many laboratory measurement applications. Due to the development and refinement of IC modules, their size, power consumption and cost of digital voltmeters have been significantly reduced and, as a result, DVMs are widely used for all measurement purposes.

The block diagram of a simple digital voltmeter is shown in Figure 10.7. The unknown signal is sent to the pulse generator, which generates a pulse whose width is directly proportional to the unknown input voltage.

Block diagram of DVM
Block diagram of DVM

The output of the pulse generator is applied to a leg of an AND gate. The input signal on the other leg of the AND gate is a train of pulses. The output of the AND gate is, therefore, a positive tripping sequence of t seconds in duration and the inverter converts it into a negative tripping train. The counter counts the number of shots in t seconds, proportional to the measured voltage. Therefore, the meter can be calibrated to indicate the voltage in volts directly.

Therefore, the DVMs described above are an analog-to-digital converter (ADC) that converts an analog signal into a pulse train whose number is proportional to the input voltage. Therefore, a digital voltmeter can be manufactured using any of the analog-to-digital conversion methods and can be represented by a block diagram shown in Figure 10.8. Therefore, DVMs can be classified based on the CAN used

Representation of DVM Using Blocks
Representation of DVM Using Blocks

The input range of the DVM can vary from ± 1.00000V to ± 1000.00V and its limit accuracy can reach ± 0.005% of the reading. Its resolution can be 1 part of 106, which gives a reading of 1 μV of the input range of 1 V. It has a high input resistance of the order of 10 MΩ and an input capacity of the order 40 peak f. what are the digital meters? Discuss their advantages and disadvantages.

What are the applications of digital volt meters?

DVMs are often used in “data processing systems” or “data recording systems”. In such systems, a series of analog input signals are scanned sequentially by an electronic system, and then each signal is converted to an equivalent digital value by the A / D converter in the DVM.

The numerical value is then passed to a pointer with information about the input line from which the signal was derived. All data is printed.

In this way, a large number of input signals can be scanned or processed automatically and their values printed or recorded.

Also read here for analog meters

What are the Analog Meters and their classification?

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