Metering and Instrument Circuits

Precision Rectifier for Digital Voltmeter(DVM)

The voltmeter is an electrical measuring instrument used to measure the potential difference between two points. The voltage to be measured may be AC or DC. Two types of voltmeters are available for the purpose of voltage measurement i.e. analog and digital.

This simple circuit, based on a single opamp in non-inverting mode, is a precision rectifier extension for digital voltmeters. The circuit can be connected to a high-impedance voltage divider without the need for an additional buffer stage that increases the cost and, more importantly, the power consumption. Another advantage of the circuit is that the accuracy is not affected by the offset voltage of the opamp. The output of the rectifier is differential to allow ready connection to the IN-LO and IN-HI inputs of the digital voltmeter (DVM) ICs like the familiar 7106 and similar types.

Circuit IC-1 is a LinCMOS operational amplifier operating in the high-bias mode. The TLC271 used here achieves a good high-frequency response at low current consumption of about 1 mA. For all practical purposes, the gain of the operational amplifier is 2R1 / R2, where R1 = R3 and R2 = R4. With the values shown, the gain is nearly equal to 1.1107, which is the r.m.s. (root-mean-square) shape factor for sinusoidal waveforms. Capacitors C1 and C2 are optional. They improve the response and stability of the rectifier at high frequencies.

Any DC component at the input as well as the offset voltage of IC-1 appears as the common-mode voltage across C3 and C4 and is therefore rejected. The frequency response of the rectifier is determined by the time constant R2-C3 or R4-C4). With the component values specified, the 1%-accuracy bandwidth extends from 25 Hz to about 20 kHz. The circuit is powered by the 9 V battery used for the DVM module. The ground of the rectifier is connected to the COM terminal of the module, which is at a potential of about 28 V below the positive supply. The DVM should be set to a full-scale input voltage of 200 mV.


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