How to properly de-coupling power supply with long cable.

[kudjung]

Hi All,

I need to feed the board power supply via a 3-meter cable(dc power supply is generating on the mother board). I just wonder how can we calculate a suitable de-coupling capacitor value in this situation? We always use a big enough cap and low esr type but is there any proper way to calculate the right value?


Thanks in advance.
KJ.

 

[crutschow]

Generally 0.1µF ceramic type is used in parallel with a larger electrolytic type capacitor on the circuit at the far end of he cable. The larger capacitor value is somewhat arbitrary but values such a 100µF or 270µF are common and typically do the job.

If the load is drawing varying current from the supply, such as an audio amp, than a larger value such as 470µF or 1000µF would be better. If you know the value of the varying current and frequency drawn by the load you can calculate the capacitance required to keep the ripple voltage below a desired level (such as 10mv for example).

Of course all ICs should also have a 0.1µF ceramic directly across their power pins for good decoupling.

 

[snail888]

it's also be careful of resonance between wire inductance and decoupling capacitor.