Expansion of Capacitance Measurement Capability in Digital Multimeters
1. Online measurement of capacitance
According to the properties of differential integral circuits, the measurement of capacitance can be converted into voltage measurement.
The core part of the circuit CX/V adopts a simple active RC inverse differentiation and integration circuit. The Wen 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. After being filtered by a second-order bandpass filter to remove impurities outside the fixed frequency, the AC/DC output voltage V proportional to CX is obtained. When the AC signal Vr excites the CX/V circuit, the output voltage of the inverting integrator is
That is, the measured capacitance CX is proportional to the output voltage C0, thus achieving CX → V conversion. In order to match the capacitance basic level with the 2V level of the digital multimeter, the oscillation frequency of the Wen oscillator is selected as 400Hz, the effective voltage value is 1V, R1 is set to 20k Ω, and C1 is set to 0.1 μ F. R2 varies from 200 Ω -2k Ω -20k Ω -200k Ω -2M Ω, corresponding to a measured capacitance range of 20 μ F-2 μ F-200nF-20nF-2nF.
2. Measure small capacitors
The range of a typical three and a half digit multimeter for measuring capacitance is 2000pF to 20 μ F, and it is powerless for measuring small capacitors below 1pF. According to the capacitance impedance method and using high-frequency signals, it is possible to measure small capacitors. The measurement circuit diagram is shown in Figure 2. CX is the measured capacitance, and Rf is the feedback resistor at the inverting end. When the input frequency of the sine signal Vi is f, the impedance presented on CX and the gain of the operational amplifier are: when A and Rf are constant, the frequency of the sine signal f is inversely proportional to the measured capacitance CX. To measure smaller capacitors, use high-frequency signals for measurement.
The block diagram of the circuit principle for measuring is shown in Figure 2 (b). The measurement process is as follows: the high-frequency sine signal generated by the high-frequency signal generator is applied to the measured capacitor, CX is converted into capacitance impedance Xc, and then Xc is converted into an AC voltage signal through C/ACV conversion, amplified by an amplifier, and the output of an isolation transformer is sent to a phase sensitive demodulator for demodulation; The other input of the phase sensitive demodulator is a square wave (i.e. demodulation signal) generated by a high-frequency sine wave through a waveform converter, and the two input signals are of the same frequency and phase. The demodulated signal is filtered by a low-pass filter to obtain a DC voltage proportional to the measured capacitance CX value, which is then sent to a DC voltmeter for direct display of the measurement result. The waveform converter consists of a zero crossing comparator with inverting input, which converts the standard 1MHz high-frequency sine wave from the Wen oscillator into a standard inverted square wave. Due to the fact that the output of the phase sensitive demodulator is a pulsating DC voltage containing high-frequency harmonics, a π - type filter is used to filter out harmonic components in order to obtain a stable and constant DC voltage output. Finally, send the corresponding average voltage value to the DC voltmeter. In order to correspond the basic capacitance range with the 2V range of the digital multimeter, the frequency of the high-frequency sine signal is selected as 1MHz (distribution parameters should be considered if the frequency is too high), the effective value of the voltage is 1V, and the product of the circuit amplification factor and feedback resistance Rf is. Therefore, the DC voltage range of the digital multimeter is 200mV, corresponding to a capacitance range of 0.2pF, and 200V corresponds to a capacitance range of 200pF. The measurement range is 10-4-102pF, and the resolution is 10-4pF. The measurement accuracy is
