[sdiy] filter cap needed?

[Scott Nordlund]

You're thinking about it the wrong way.  High frequency signals (hopefully) aren't emanating from the power supply to the rest of the circuit.  

Switching in a CMOS circuit will cause a transient spike in the current supplied to the chip.  Parasitic inductance or long leads in the power supply lines will limit how quickly the power supply can respond to this, leading to a local drop in supply voltage.  This enters nearby circuits via the power rails and can cause noise, glitches, etc..  A decoupling capacitor on the power leads of each chip will stabilize the power during these transient conditions and limit their affect on other parts of the circuit.

Or you could think about it this way: decoupling caps provide a low impedance path to ground for high frequency signals (noise).

----------------------------------------
> Date: Fri, 27 Mar 2009 09:09:28 -0700
> From: dixon at interchange.ubc.ca
> Subject: RE: [sdiy] filter cap needed?
> To: simon.oo.o at xs4all.nl; synth-diy at dropmix.xs4all.nl
> CC:
>
>>> However, something has been troubling me about these caps: If several of
>>> them are connected between a rail and ground at various locations,
>> wouldn't
>>> they end up fulfilling the same function as one cap, several times as
>> large
>>> (given that they are capacitances in parallel)?
>>>
>> If they are at different locations, there must be a conductor between
>> them which has resistance and inductance.
>> So, for high frequencies, no they wouldn't .
>
> What do you mean by "high"? Assuming the trace represents a resistor R = 1
> ohm (which is probably conservative), and the cap is C = 0.1uF, then this
> makes a low-pass filter with a cutoff frequency of about 1.6 MHz. How does
> this help?
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy

_________________________________________________________________
Express your personality in color! Preview and select themes for Hotmail®.
http://www.windowslive-hotmail.com/LearnMore/personalize.aspx?ocid=TXT_MSGTX_WL_HM_express_032009#colortheme