Dry goods big cattle share the experience of using filter capacitors

Simple inductive circuits work well when used in low-impedance circuits, with attenuation of more than 40dB, but they may have no effect at all in high-impedance circuits.

The circuit of a single capacitor works well in high-impedance circuits, but poorly in low-impedance circuits.

A filter composed of multiple components will have a good effect, but the premise is that it must be constructed correctly, so that the capacitor faces high impedance and the inductor faces low impedance.

Since the capacitor leads have parasitic inductance and resistance, the actual capacitor model is a series structure of capacitance, inductance (equivalent series inductance ESL), and resistance (equivalent series resistance ESR). It has a self-resonating frequency. Capacitors should work at their self-resonating frequency in order to function.

Table 1: Ceramic capacitors with a lead length of 1.6mm

Capacitance	Resonance frequency (MHZ)
1 mF	1.7
0.1 mF	4
0.01 mF	12.6
3300 pF	19.3
1100 pF	33
680 pF	42.5
330 pF	60

It can be seen that the resonance frequency of general capacitors is very low and cannot be used in the high frequency and microwave frequency bands. In order to increase the resonance frequency, surface mount capacitors, three-port capacitors and core-piercing capacitors can be used.

Three-terminal capacitor：

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Installation of three-terminal capacitors：

The ground terminal is as short as possible to reduce the lead inductance of the ground terminal.

Deficiencies of three-terminal capacitors：

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Heart-piercing capacitor：

Because its ground terminal can be in close contact with ground, the inductance of the lead wire is smaller, so the performance is better. feedthrough filters based on through-core capacitors are widely used in RF filtering.

The input and output terminals of the through-core capacitor are connected through the upper and lower ground plates (hence the name through-core capacitor), which is equivalent to the upper and lower capacitors connected in parallel to form a total capacitor. Since its grounding part is connected 360° to the ground in the circuit, the grounding inductance is very small.

In addition, temperature and voltage also have a great impact on the capacitance value.

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