AC voltmeter

An AC voltmeter consisting of an operational amplifier, a diode rectifier bridge and a DC milliammeter is shown in the figure. In the figure, the operational amplifier uses LM324.     The measured AC voltage Ul is added to the non-inverting terminal of the operational amplifier, so it has a high input impedance. And because negative feedback can reduce the nonlinear influence in the feedback loop, the diode bridge and meter are placed at the feedback of the operational amplifier. loop to reduce the impact of the nonlinearity of the diode itself.     The current I all flows through the bridge, and its value is only related to Ui/R, and has nothing to do with the bridge and meter parameters ( nonlinear parameters such as the dead zone of the diode ). The current in the meter header is proportional to the average value of the full-pass rectification of the measured voltage Ui. The upper limit frequency of the measured voltage is determined by the frequency band and rise rate of the operational amplifier circuit.