What frequency does a Decoupling CAP work on?

[twonsr]

Dear all :

I connect a decopuling capacitor ( 0.1uF ) from the VDD pin to ground.
But the input resistance of VDD is about 5k.

but the power supply has a internal resistance about 10 ohm.

In this case , the lowpass filter will work at 159kHz.

f= 1 / (2pai * 5k||10 * 0.1u )

but even a power supply has a HUM for only 60Hz , this decoupling capacitor can't filter that.

what is the purpose of the capacitor actually ?

 

[Borber]

You described real situation which is present in all circuits. Actually two filterings are neccessary. First you must filter out hum and noise of power supply. This is usually low frequency signal. You must use large capacitors or even LC filters. Secondly it is neccessary to prevent AC currents from Vdd pin of a device to spread over supply rail to other parts of circuits and cause interferrence or instability of circuit (oscillations). This filtering is done with smaller caps or set of them or by LC filter.

 

[lastdance]

you can imagine that when u try to pull large current into your chip, in transient, if u do not have the cap at Vdd, the supply current has to source all the way back from the power source (assuming no other cap is anyway near on Vdd), u cause havoc. the inductance on the power bus is going to glitch ur supply badly.
a decoupling cap should be installed as a general practice. if u want to filter the supply ac noise, u need a special filter to do that at the front stage, mentioned by the last poster.