DC, RMS, And Peak-to-Peak High Impedance Voltmeter

Analog meter normally doesn’t have high impedance since they don’t have buffer circuit inside. With active buffering, the input impedance of this circuit depends on the impedance of the op-amp.

You can use other op-amp for higher impedance, but a low-cost 741 is enough for may application. The rectification inside the feedback loop path remove the effect of diode’s voltage drops, and now the RMS and peak-to-peak conversion is simply a matter of division process in relation to DC reading. [Circuit’s schematic diagram source: electronicsforu.com]