The expansion of the capacitance measurement function of 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.
