Extension of the Capacitance Measurement Function of Digital Multimeters
Based on the characteristics of differential and integral circuits, capacitance measurement can be converted into voltage measurement.
The core CX/V conversion circuit adopts a simple active inverting RC differential and integral configuration. A Wien bridge oscillator generates an
AC reference signal Vr at a fixed frequency, which excites the CX/V conversion circuit and produces an AC output voltage V0(V1) proportional to the capacitance CX. After filtering out interference signals outside the fixed frequency via a second‑order band‑pass filter, the signal passes through an AC/DC converter to obtain a DC output voltage V proportional to CX.
When the AC signal Vr drives the CX/V circuit, the output voltage of the inverting integrator is proportional to the measured capacitance CX. This realizes the conversion from capacitance CX to voltage.
To match the basic capacitance range with the 2 V voltage range of a digital multimeter, the Wien bridge oscillator is set to 400 Hz with an RMS voltage of 1 V; and C1=0.1 μF. The resistance R2 is switchable among and , corresponding to capacitance measuring ranges of 20 μF, 2 μF, 200 nF, 20 nF and 2 nF respectively.
2 Small Capacitance Measurement
Ordinary 3½‑digit digital multimeters have a capacitance measuring range from 2000 pF to 20 μF and cannot measure tiny capacitances below 1 pF. By applying the capacitive reactance method with high‑frequency excitation, ultra‑small capacitances can be measured. The measurement circuit is shown in Figure 2, where CX is the measured capacitance and Rf is the feedback resistor at the inverting input.
When a sinusoidal signal Vi of frequency f is applied, capacitive reactance appears across CX. The gain of the operational amplifier is determined accordingly. With fixed gain and feedback resistance, the signal frequency f is inversely proportional to the measured capacitance CX. Therefore, high‑frequency signals are adopted for measuring extremely small capacitances.
The functional block diagram of the measuring principle is shown in Figure 2(b). During measurement: a high‑frequency sinusoidal signal generated by the oscillator is applied to CX; the capacitance is converted into capacitive reactance XC; a C/ACV converter transforms XC into an AC voltage, which is amplified and isolated by a transformer before being sent to a phase‑sensitive demodulator.
The second input of the demodulator is a square wave obtained by shaping the high‑frequency sine wave through a waveform converter; the two input signals share the same frequency and phase. The demodulated signal passes through a low‑pass filter to produce a DC voltage proportional to CX, which is then displayed directly on a DC voltmeter.
The waveform converter consists of an inverting zero‑crossing comparator, converting the standard 1 MHz sine wave from the Wien oscillator into a stable inverted square wave. Since the demodulator output is a pulsating DC containing high‑frequency harmonics, a π‑type filter is used to suppress ripple components and deliver a smooth, steady DC voltage to the meter.
To align the capacitance ranges with the multimeter's 2 V voltage range, the high‑frequency signal is set to 1 MHz (excessively high frequencies will introduce parasitic parameters) with an RMS voltage of 1 V. The product of circuit gain and feedback resistance is configured so that the multimeter's 200 mV DC range corresponds to 0.2 pF, and the 200 V range corresponds to 200 pF. The overall measuring range is 10−4 pF to 102 pF, with a resolution of 10−4 pF.
