Test & Measurement Circuits

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           Edc 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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