Expansion of the Digital Multimeter's Capacitance Measuring Capability
1 Online capacitance measurement
The measurement of capacitance can be transformed into a measurement of voltage depending on the characteristics of differential and integral circuits.
Using a straightforward active RC inverting differential and integral circuit, the circuit's central component, CX/V, operates. The CX/V conversion circuit is excited by a fixed-frequency AC signal from the Wien oscillator to produce an AC voltage V0 (V1) proportional to CX. This voltage is then filtered by a second-order band-pass filter to remove signals other than the fixed frequency. After the clutter, AC/DC is used to provide a DC output voltage V proportional to CX. The CX/V circuit is excited by the AC signal Vr, and the inverting integrator's output voltage changes as a result.
In other words, the conversion of CX to V is realized since the measured capacitance CX is directly proportional to the output voltage C0. The Wien oscillator's 400Hz oscillation frequency, 1V of effective voltage, 20k of resistance R1, and 0.1F of capacitance C1 must all be present for the basic range of capacitance to match the 2V range of the digital multimeter. The measuring capacitance range for R2 changes from 200-2k-200k-200k-2M to 20F-2F-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 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 the output of the isolation transformer is sent to the phase sensitive Demodulator 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 a capacitance range of 0.2pF, and 200V corresponds to a capacitance range of 200pF. The measurement range is 10-4 to 102pF, and the resolution is 10-4pF.
