Introduction to digital voltammeters
These are generally known as DVM that use digital formatting of the input fed to its input leads. Here, result of measurement is shown in form of discrete numbers for which they employ display devices for decimal number system. As in case of digital read out displays there is nothing like a scale embedded with some marking upon as in these cases some observational error are committed very commonly like parallax and approximation error which in not the case of digital voltmeters which results in improves accuracy of the measurement system. Digital voltmeter also attains an inherited greater speed of operation. Because output obtained from these instruments comes to be digital form so it becomes easier to use them directly as an input to many other devices like memory devices so that the result may be use further in future, this is called storage of data.
Because of high accuracy, high speed operation and greater reliability they are frequently used in laboratories and industries for the purpose of experimentation and obtaining highly accurate results. It can be accepted as an disadvantage of digital voltmeters that they always need some external power supply for its operation that make it less portable and also bulkier but with the advancements made in the field of integrated circuits it has becomes possible to make such digital device very compact, more efficient, low cost and having even greater accuracy. This advancements has led to the verge that now some digital devices are there having less cost than their competent analogue ones with the same extent of accuracy.
What are the different types of digital voltmeters?
Upon the basis of circuits employed within the devices it makes a number of categories of digital voltmeters which is nothing but a result of popularity of these devices. Generally used DVMs can be listed as under:
1. Ramp type of digital voltmeters
2. Voltmeters of integrating type
3. Digital voltmeters of potentiometer type
4. Successive approximation type digital voltmeters
5. And continuous balance type digital voltmeter
As all the digital voltmeters can be operated upon dc power and not upon ac one. So the first job regarding these instruments becomes, first, to converts the ac voltage into equivalent dc component or if it is concerned about the signal it requires use of A/D converters that converts analogue signal to corresponding digital form so as to feed it to further digital section of circuitry use in concerned device. For the sake of better understanding we can take an example of voltage signal in which its value is made to get changed to equivalent intervals of time that is further used to activate clock oscillator. This activation actually stops and restarts the oscillator consequently depending upon the pattern of time intervals. Now oscillator delivers its output to a digital counter circuit that also records and converts thus obtained output into equivalent voltage value.
Ramp type digital voltmeter
The working principle of ramp type digital voltmeter is that:
It takes measurement of the time that is taken by instrument for its input voltage to change its state from input level to zero level and also the reverse is also true. An digital counter is employed in this system that measured time intervals and gives this output to the display unit that merely displays the output inform of digits with the help of indicating tubes thus making this section to work as output device.
A voltage, referred as ramp voltage, is initiated at the beginning of measurements. Generally used ramp is negative going type but we also use one of positive going type. A continuous comparison is made between ramp voltage and that the voltage under measurement. As soon as, it is found that ramp voltage becomes equal to the input voltage a pulse in formed by coincidence generally called comparator circuit. At the same time value of ramp voltage is also made to fall down till the time when it reaches zero level. When value of ramp voltage has fallen to zero or ground another circuit called ground comparator comes into play and result in to close the gate. Some time is taken by the circuit between opening and closing of the gate can be given by T, now, clock pulse generator circuit generates some pulses during time T that are given to gate as a result of them counting an displaying can be performed. Thus, Input voltage that was initially fed to the instrument is read in digital form.
It employs multi-vibrator circuits that are basically used to determine rate according to which cycles of measurement are started this multi-vibrator is called sample rate multi-vibrator. It is because the initial pulses that are responsible for activation or generation of next ramp voltages are generated by sample rate multi-vibrator as with as multi-vibrator also sends a pulse to counter circuit that makes it to come again to 0 states to clean up the digital display.
Integrating type digital voltmeter
This class of voltmeter is used to measure the average value of applied input voltage in a wide range. In opposite of integrating type voltmeter in case of ramp type voltmeter sampling of input voltage in done at the ending of the whole process. As the name suggests an integrating circuit is used in this case that performs the function of converting voltage to its equivalent frequency i.e. it works as voltage to frequency converter. It behaves as a feedback system and tends to decide the rate at which pulses are formed in correspondence to quantity of voltage feed at input section. Voltage to frequency conversion merely means to generate a train of pulses such that it frequency is dependent upon the magnitude of instantaneous voltage applied in this way an parent of variations in voltage is copies into an equivalent pattern of variations in frequency. After formation of a pulse train it is counted that how man pulse the present in fixed interval of time with help of a suitable electronic circuit. As is discussed that thus obtained pulse train has frequency whose value at any instance depends upon the corresponding value of input voltage thus there exists a relation between pulse train and input voltage that is used further to recover the original value of input voltage.
The most important part of this whole system is integrator section which is nothing but an operational amplifier that produces an output voltage whose value can be given by a relation depending upon input voltage Ei. The relation is given by:
Where, Eo is output voltage obtained at integrator while Ei is input voltage. When Ei is made to remain constant Eo tends to change in its magnitude according to time such that Eo is always 180 degree out of phase i.e. having opposite polarity. By the graphical analysis it is seen that if input attains a shape of straight line horizontal to X-axis then its corresponding output comes to have a shape of straight line having declination with X-axis.
As soon as a constant input voltage is applied to integrator circuit a corresponding output is obtained whose value increase with time representing a straight line with a slope depending upon magnitude of input voltage. This rising voltage is applied to a level detector section that further sends a pulse to pulse generator circuit when a particular level of voltage is encountered by it. Functioning of level detector can be understood by considering it as voltage comparator. Integrator and level detector works together in such a way that output voltage from integrator is kept comparing with a fixed reference level, as soon as it reaches that level it generates the massaging pulse. Consequence of this pulse is that it triggers pulse generator gate that make the pulse generated at clock oscillator of fixed frequency to leave from pulse generator. This pulse generator is nothing but a Schmitt trigger.
Now this pulse, having polarity opposite to that of input with even greater magnitude which is further applied to integrator. This results into the total input to integrator with reversed polarity. And consequence of this opposite polarity inputs makes the output of integrator having downward slope. And as soon as output from integrator drops down a level detector circuit switches off the pulse generator gate and now no pulse is generated from clock oscillator. Once after passing of the pulse from pulse generator the input is reset to previous value and also makes output of integrator to increase again and the whole procedure is repeated again and again. This repetition results in formation of a saw tooth waveform whose frequency is function of magnitude of input voltage. Thus frequency of thus produces signal makes it possible to find out the value of input voltage applied.
Potentiometer Type of digital voltmeter
It used the method of voltage comparison for its operation. In the procedure of voltage measurement with this instrument includes a fixed voltage level whose value is calibrated with help of a potentiometer and input voltage is made to compare with this fixed value with the help of some suitable method. Upon application of input voltage potentiometer looses it balance point which is set again to null position by changing potentiometer sating accordingly. Here, as the measurement system is digital type so potentiometer adjustment is not done manually rather is obtained all automaticall.
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I want comparison between all types of DVM and Diagrams also.