Electrical schematic diagram of DC voltmeter input at non-inverting terminal

The non-inverting terminal inputs A, and the electrical schematic diagram of a DC voltmeter composed of a milliamp meter is shown in the figure.     In the figure, the operational amplifier uses LM324.     In order to reduce the impact of the meter parameters on the measurement accuracy, the meter is placed in the feedback loop of the operational amplifier, and the range can be switched by simply changing a resistor . After the resistance is determined, the ammeter can be calibrated directly with the voltage value.     It should be noted that the circuit in the figure is suitable for measuring related circuits that share a common ground with the operational amplifier circuit. In addition, when the measured voltage is high, an attenuator should be set at the input end of the op amp.