Expansion of the Capacitance Measurement Function of a Digital Multimeter
1. Online measurement of capacitance
According to the properties of differential and integral circuits, capacitance measurement can be converted into voltage measurement.
The core part of the circuit, CX/V, adopts a simple active RC inverse differential and integral 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 clutter outside of the fixed frequency, it is then subjected to AC/DC to obtain a DC output voltage V proportional to CX. When the AC signal Vr excites the CX/V circuit, the output voltage of the inverter integrator
That is, the measured capacitance CX is proportional to the output voltage C0, thus achieving CX → V conversion. In order to correspond the basic level of the capacitor to the 2V level of the digital multimeter, the oscillation frequency of the Venturi oscillator is selected as 400Hz, the effective voltage value is 1V, R1 is taken as 20k Ω, and C1 is taken as 0.1 μ F. R2 changes from 200 Ω -2k Ω -20k Ω -200k Ω -2M Ω, corresponding to a measurement capacitance range of 20 μ F-2 μ F-200nF-20nF-2nF.
2. Measure small capacitance
The range for measuring capacitance with a typical three and a half digit multimeter is 2000pF~20 μ F. It appears powerless to measure small capacitors below 1pF. According to the capacitive impedance method and the use of high-frequency signals, the measurement of small capacitors can be achieved. The measurement circuit diagram is shown in Figure 2. CX is the measured capacitance, and Rf is the feedback resistance at the reverse phase 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 sine signal frequency f is inversely proportional to the measured capacitance CX. To measure smaller capacitors, high-frequency signals are used for measurement.
