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]