Digital voltmeter (DVM) and its Classification
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This document discusses digital voltmeters (DVMs). It explains that DVMs display voltage measurements as numerical readings rather than using an analog needle gauge. The document covers various types of DVMs including ramp type, dual slope integrating type, successive approximation type, and microprocessor-based versions. It provides block diagrams and explanations of the operating principles for different DVM designs. Advantages of DVMs like accuracy, ease of reading, and versatility are also summarized.
Digital voltmeter (DVM) and its Classification
- 2. Digital Voltmeter (DVM) Voltmeter is an electrical measuring instrument used to measure potential difference between two points. The voltage to be measured may be AC or DC. Two types of voltmeters are available for the purpose of voltage measurement i.e. analog and digital. Analog voltmeters generally contain a dial with a needle moving over it according to the measure and hence displaying the value of the same. Digital voltmeters display the value of AC or DC voltage being measured directly as discrete numerical instead of a pointer deflection on a continuous scale as in analog instruments.
- 3. Advantages: Read out of DVMs is easy as it eliminates observational errors in measurement committed by operators. Error on account of parallax and approximation is entirely eliminated. Reading can be taken very fast. Output can be fed to memory devices for storage and future computations. Versatile and accurate Compact and cheap Low power requirements Portability increased
- 4. Very high accuracy (5 to 0.05%) Has very input impedance (2MΩ) which ensures less loading effect on the input. The numeric display of digital meters provide zero parallax error
- 5. Block Diagram of DVM
- 6. Classification of DVM’s : • Ramp type digital voltmeter • Dual slope Integrating type voltmeter • Micro processor based ramp type digital voltmeters • Successive approximation type digital voltmeter • Continuous balance type digital voltmeter
- 7. Digital Voltmeter(DC) The voltage is first attenuated and then converted into a digital word which is displayed.
- 8. Features Polarity detection Auto ranging Auto zeroing Polarity detection: The information is given by the ADC. When the voltage is +ve, the characteristic curve of the converter is in first quadrant. The characteristic curve will be in the fourth quadrant, if the voltage is –ve
- 9. DVM (AC) The AC is processed first i.e., phase compensated attenuation with rectification and filtering is done. The dc voltage obtained is converted into a digital word, which is displayed. For high frequencies, a precision diode with small reverse recovery time is required.
- 10. RAMP TYPE DVM The main building block of ramp type dvm is a ramp generator. This is generating a waveform which is representing a ramp. The heart of the circuit is the ramp generator. Therefore it is called ramp type digital voltmeter(DVM). The input which should be measured is given at input voltage. This input fed to ranging and attenuator circuit which will amplify the signal if it is small or attenuates the signal if it is large.
- 11. This is given to an input comparator which will compare two signals and generate the output. One input to the input comparator is from the input voltage and another input from the ramp. This input voltage and ramp signal are compared and output is given. If the ramp signal is more than input voltage there will be no output but if the input voltage is greater than the ramp signal then a is generated which will open the gate. Now when the gate get opened, clock oscillator will send clock pulses which are counted by the counter and displays on the screen.
- 12. Operating Principle: The operating principle of ramp type digital voltmeter is to measure the time that a linear ramp voltage takes to change from the level of the input voltage to zero voltage (or vice versa). This time interval is measured with an electronic time interval counter and the count is displayed as a number of digits on electronic indicating tubes of the output readout of the voltmeter.
- 13. Timing Diagram:
- 14. Dual slope Integrating type DVM The dual-slope type of A to D conversion is a very popular method for digital voltmeter applications. When compared to other types of analog-to-digital conversion techniques, the dual-slope method is slow but is quite adequate for a digital voltmeter used for laboratory measurements. For data acquisition applications, where a number of measurements are required, faster techniques are recommended.
- 15. Introduction: Dual Slope Integrating Type DVM – In ramp techniques, superimposed noise can cause large errors. In the dual ramp technique, noise is averaged out by the positive and negative ramps using the process of integration.
- 16. Principle of dual slope DVM: The input voltage ’ei’ is integrated, with the slope of the integrator output proportional to the test input voltage. After a fixed time equal to t1, the input voltage is disconnected and the integrator input is connected to a negative voltage — er The integrator output will have a negative slope which is constant and proportional to the magnitude of the input voltage.
- 17. BLOCK DIAGRAM:
- 19. Successive approximation type DVM: Successive Approximation type ADC is the most widely used and popular ADC method. The conversion time is maintained constant in successive approximation type ADC, and is proportional to the number of bits in the digital output, unlike the counter and continuous type A/D converters.
- 20. The basic principle of this type of A/D converter is that the unknown analog input voltage is approximated against an n-bit digital value by trying one bit at a time, beginning with the MSB. The principle of successive approximation process for a 4-bit conversion is explained here. This type of ADC operates by successively dividing the voltage range by half, as explained in the following steps.
- 21. It consists of a successive approximation register (SAR), DAC and comparator. The output of SAR is given to n-bit DAC. The equivalent analog output voltage of DAC, VD is applied to the non-inverting input of the comparator. The second input to the comparator is the unknown analog input voltage VA. The output of the comparator is used to activate the successive approximation logic of SAR. When the start command is applied, the SAR sets the MSB to logic 1 and other bits are made logic 0, so that the trial code becomes 1000.
- 23. Advantages: 1 Conversion time is very small. 2 Conversion time is constant and independent of the amplitude of the analog input signal VA. Disadvantages: 1 Circuit is complex. 2 The conversion time is more compared to flash type ADC.
- 24. Digital Frequency Meter Digital frequency meter is a general purpose instrument that displays the frequency of a periodic electrical signal to an accuracy of three decimal places. It counts the number events occurring within the oscillations during a given interval of time.
- 25. A frequency meter has a small device which converts the sinusoidal voltage of the frequency into a train of unidirectional pulses. The frequency of input signal is the displayed count, averaged over a suitable counting interval out of 0.1, 1.0, or 10 seconds. These three intervals repeat themselves sequentially.
- 26. Block Diagram:
- 27. The range of modern digital frequency meter is between the range from104 to 109 hertz. The possibility of relative measurement error ranges between from 10- 9 to 10-11 hertz and a sensitivity of 10-2 volt. For testing radio equipment Measuring the temperature, pressure, and other physical values. Measuring vibration, strain Measuring transducers
- 28. Direct probes Isolation Probes High impedance or 10: 1 Probes Active Probes Current Probes Differential Probes