Like the question, I often see two capacitors, one large and one small, in parallel with the IC power pins. Am I doubting that both capacitors work?
The large capacitor limits a smaller upper cut-off frequency f1, and the upper cut-off frequency f2 determined by the small capacitor is higher.
So even if there is no small capacitor, when the power supply has interference that exceeds f1, this interference will be filtered out, and the interference above f2 will naturally be filtered out.
Caesar 发表于 November 10, 2020
Due to the wiring and PCB wiring, the capacitor is actually a parallel circuit of inductance and capacitance (and the resistance of the capacitor itself, sometimes not negligible), which introduces the concept of resonance frequency: ω=1/(LC)1/2 ,
The capacitor is capacitive below the resonant frequency, and inductive above the resonant frequency.
Therefore, generally large capacitors filter low frequency waves, and small capacitors filter high frequency waves.
Davi 发表于 November 10, 2020
Small capacitors filter high frequencies, but large capacitors are too late to respond.
Wisdom 发表于 November 10, 2020
Generally, the IC power supply terminal needs to be connected with two capacitors of 10～33 uF and 104 uF, which are used for smoothing and filtering. This is especially important when there are high-power components such as relays on the board!
Bear 发表于 November 10, 2020
I think I know the answer. Decoupling capacitors often filter out high-frequency signals.
And every capacitor, in the high frequency state, there will be distributed inductance,
As a result, in addition to the upper cut-off frequency, there is also a lower cut-off frequency.
Therefore, a large capacitor and a small capacitor are connected in parallel.
