Expansion of Capacitance Measuring Function of Digital Multimeter
Abstract: This paper introduces the techniques used to measure capacitance online and expand the measurement range of capacitance using the DC voltage range of ordinary digital multimeters. Key words:
General-purpose three-and-a-half-digit or four-and-a-half digit multimeters are equipped with capacitance measurement function, but the measurement range is narrow and the measurement accuracy is low, and generally there is no online measurement function. This article discusses how to extend these capabilities.
1 Online capacitance measurement
According to the nature of differential and integral circuits, the measurement of capacitance can be converted into voltage measurement.
The core part of the circuit, CX/V, uses a simple active RC inverting differential and integral circuit. The Wien Oscillator generates a fixed-frequency AC signal Vr, which excites the CX/V conversion circuit to obtain an AC voltage V0 (V1) proportional to CX, which is filtered by a second-order band-pass filter to filter out signals other than the fixed frequency. After the clutter, the DC output voltage V proportional to CX is obtained after AC/DC. When the AC signal Vr excites the CX/V circuit, the output voltage of the inverting integrator
That is, the measured capacitance CX is directly proportional to the output voltage C0, thus realizing the conversion of CX→V. In order to make the basic range of capacitance correspond to the 2V range of the digital multimeter, the oscillation frequency of the Wien oscillator is 400Hz, the effective value of the voltage is 1V, R1 is 20kΩ, and C1 is 0.1μF. R2 changes from 200Ω-2kΩ-20kΩ-200kΩ-2MΩ, and the corresponding measurement capacitance range is 20μF-2μF-200nF-20nF-2nF.
2 Measuring small capacitance
The general three-and-a-half-digit digital multimeter has a range of 2000pF to 20μF for measuring capacitance, and it is powerless to measure tiny capacitances below 1pF. According to the capacitive reactance method and using high-frequency signals, the measurement of tiny capacitance can be realized. The measurement circuit diagram is shown in Figure 2. CX is the measured capacitance, and Rf is the feedback resistance of the inverting terminal. When the sinusoidal signal Vi with frequency f is input, the impedance presented on CX and the gain of the operational amplifier are: when A and Rf are constant, the sinusoidal signal frequency f is inversely proportional to the measured capacitance CX. To measure small capacitances, high-frequency signal measurements are used.
The block diagram of the circuit principle to realize the measurement is shown in Fig. 2(b). The measurement process is: the high-frequency sinusoidal signal generated by the high-frequency signal generator is applied to the measured capacitor, and CX is converted into capacitive reactance Xc, and then Xc is converted into AC voltage signal through C/ACV conversion, which is amplified by the amplifier and output by the isolation transformer. Send it to the phase-sensitive demodulator for demodulation; the other input of the phase-sensitive demodulator is a square wave (that is, a demodulated signal) generated by a high-frequency sine wave through a waveform converter, and the two input signals have the same frequency and phase. The demodulated signal is filtered by a low-pass filter to obtain a DC voltage proportional to the value of the measured capacitor CX, which is sent to the DC voltmeter to directly display the measurement result. The waveform converter consists of a zero-crossing comparator with an inverting input, which converts a standard 1MHz high-frequency sine wave from a Wien oscillator into a standard inverting square wave. Since the output of the phase-sensitive demodulator is a pulsating DC voltage containing high-frequency harmonics, in order to obtain a stable and constant DC voltage output, a **π-type filter is used to filter out the harmonic components. Finally, the corresponding average voltage is sent to the DC voltmeter. In order to make the basic capacitance level correspond to the 2V level of the digital multimeter, the frequency of the high-frequency sinusoidal signal is selected as 1MHz (if the frequency is too high, the distribution parameters should be considered), the effective value of the voltage is 1V, and the product of the circuit amplification factor and the feedback resistance Rf is, so The digital multimeter’s DC voltage range of 200mV corresponds to the capacitance range of 0.2pF, and 200V corresponds to the capacitance range of 200pF. The measurement range is 10-4～102pF, the resolution is 10-4pF, and the measurement accuracy is
