Useful information | Experts share their experience in using filter capacitors

Latest update time：2019-05-08

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Simple inductor circuits work well when used in low impedance circuits, with over 40dB of attenuation, but may not be effective at all in high impedance circuits.

A circuit with a single capacitor works well in a high impedance circuit, but works poorly in a low impedance circuit.

Filters made up of multiple elements can work well, but only if they are constructed correctly, with the capacitors facing high impedance and the inductors facing low impedance.

Since the capacitor leads have parasitic inductance and resistance, the actual capacitor model is a structure in which capacitance, inductance (equivalent series inductance ESL), and resistance (equivalent series resistance ESR) are connected in series, and has a self-resonant frequency. The capacitor should operate below its self-resonant frequency to function.

Table 1: Ceramic capacitors with 1.6mm lead length

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

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

Three-terminal capacitor:

Installation of three-terminal capacitor:

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

Disadvantages of three-terminal capacitors:

Feedthrough capacitor:

Since its ground terminal can be in close contact with the ground and the lead inductance is smaller, the performance is better. Feedthrough filters based on feedthrough capacitors are widely used in RF filtering.

The input and output connection lines of the through-hole capacitor pass through the upper and lower ground plates (hence the name through-hole capacitor), which is equivalent to connecting the upper and lower capacitors 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 influence on capacitance value.