how to properly choose decoupling caps

[Lissajous]

Supply Decoupling

Hi, just would like to ask how to properly choose decoupling caps for circuit supplies... some circuits have 2 (e.g. 10uF and 0.1uF) while others only 1 capacitor(0.01uF). i believe the choice must be something to do with "timing". followup to that is how do i choose between polarised and non-polarised?

thanks so much!

 

[IanP]

Re: Supply Decoupling

Choosing and Using Bypass Capacitors

**broken link removed**

Rgds,
IanP

 

[huojinsi]

Supply Decoupling

Ianp's answer is very strong.

 

[Lissajous]

Re: Supply Decoupling

alright thanks....

they say -- place capacitors as close to the DUT as possible... yes to flatten out noise and all..... however, if you wish to have microAmpere currents in this node, is a relay ever practical to switch out the decoupling capacitor?

case A: DC voltage supplied to a pin (needs a decoupling)
case B: you need to measure leakage in the uA range on this pin (need to disconnect the capacitor)...

thx again alright.

 

[dick_freebird]

You might keep the low value, high frequency caps hard-
attached (their leakage should be negligible) and only
relay out the bulk capacitors whose ESL/ESR probably
dwarfs relay contact R and L, but contribute the majority
of the leakage.

The whole purpose of decoupling caps is to "AC-short" the
switching or high frequency currents. The whole idea is to
make the current loop as electrically short (so also physically)
to minimize the induced voltage perturbation. If you think
in terms of the current loop, a lot of bypassing and grounding
questions become almost "instinct" (you still have some
detail-scrubbing to do,such as selecting the proper capacitor
type / attributes).