Test & Measurement Circuits

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High Resistance Voltmeter           Click here for the circuit diagram

The full-scale deflection of the universal high-input-resistance voltmeter circuit shown in the figure depends on the function switch position as follows:
(a) 5V dc on position 1
(b) 5V ac rms in position 2
(c) 5V peak ac in position 3
(d) 5V ac peak-to-peak in position 4
The circuit is basically a voltage-to-current converter. The design procedure is as follows:
Calculate RI according to the application from one of the following equations:
(a) dc voltmeter: RIA = full-scale EDC/IFS
(b) rms ac voltmeter (sine wave only): RIB = 0.9 full-scale ERMS/ IFS
(c) Peak reading voltmeter (sine wave only): RIC = 0.636 full-scale EPK/IFS
(d) Peak-to-peak ac voltmeter (sine wave only): RID = 0.318 full-scale EPK-TO-PK / IFS
The term IFS in the above equations refers to meter?s full-scale deflection current rating in amperes.
It must be noted that neither meter resistance nor diode voltage drops affects meter current.
A high-input-resistance op-amp, a bridge rectifier, a microammeter, and a few other discrete components are all that are required to realise this versatile circuit. This circuit can be used for measurement of dc, ac rms, ac peak, or ac peak-to-peak voltage by simply changing the value of the resistor connected between the inverting input terminal of the op-amp and ground. The voltage to be measured is connected to non-inverting input of the op-amp.

                        TABLE I
        Position 1 of Function Switch
dc input    Meter Current
               5.00V            44 ?A
               4.00V            34 ?A
               3.00V            24 ?A
               2.00V            14 ?A
               1.00V             4 ?A

                      TABLE II
          Position 2 of Function Switch
Erms input    Meter Current
                     5V                 46 ?A
                     4V                 36 ?A
                     3V                 26 ?A
                     2V                 18 ?A
                     1V                 10 ?A

                       TABLE III
       Position 3 of Function Switch
EPk input      Meter Current
           5V peak             46 ?A
           4V peak             36 ?A
           3V peak             26 ?A
           2V peak             16 ?A
           1V peak             6 ?A

                     TABLE IV
         Position 4 of Function Switch
EPk-To-Pk              Meter Current
      5V peak to peak         46 ?A
      4V peak to peak         36 ?A
      3V peak to peak         26 ?A
      2V peak to peak         16 ?A
      1V peak to peak         7 ?A



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