This technical column describes the basic facts about capacitors.
This lesson describes the method of measuring the electrostatic capacitance of ceramic capacitors.
1. Measuring instruments
The electrostatic capacitance of ceramic capacitors is generally measured using an LCR meter.
2. Measurement principle
The typical measurement system of LCR meters is the "automatic balancing bridge method," such as shown in the figure below. The measurement principle is as follows.
DUT is the abbreviation of "Device Under Test," and refers to the measured item. A high gain amplifier automatically adjusts the gain level so that the electric current flowing through the resistor R is equal to the current flowing through the DUT; that is to say, the low potential side (L-end side in the figure) of the DUT is always equal to the virtual ground level (electric potential = 0). The phase angles of the input voltage E1 and output voltage E2 at this time are also measured, and are as follows:
The DUT impedance Zx can be obtained from these phase angles and the feedback resistance R as follows:
Here, the real part is Rx, the imaginary part is Xx, and the DUT electrostatic capacitance Cx can be obtained from the acquired Xx using the equation Xx=ｊ/ωCx.
The electrostatic capacitance of ceramic capacitors must be measured using the correct measurement conditions noted in the specifications or other documents. Care should be taken as the conditions may differ depending on the nominal value of the electrostatic capacitance and other factors. These conditions are mainly the heat treatment before measurement, the measurement voltage, and the measurement frequency.
In addition, the residual impedance and admittance components of the measurement terminal, including the measurement cable, affect the actual measurement results, so the measurement terminal must be calibrated. The measurement terminal is calibrated by measuring the value up to the part where the tip of the terminal directly contacts the DUT, and then subtracting the impedance and other components from the measurement results. This means that the calibration value must be measured beforehand. Calibration generally consists of short calibration and open calibration. For a detailed explanation of these measurements, see the following document.
Person in charge: Murata Manufacturing Co., Ltd. Martin