Subject: Re: Bypass capacitor position
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Date: Fri, 25 Oct 2002 23:37:06 GMT
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I've seen some good and reasonable advice. However I've read above people
getting suceptibility and compatibility being argued against each other.
Requirements for one doesn't necessarily work for the other.
What I mean here is Richard is , what do you want the bypass to do, stop the
IC being a source of interefence, or stop the IC from being a victim of
interference.? I suspect the latter.
For example, nice idea - have gnd close as possible to IC and cap, however,
if good grounding techniques are not used around the board / system, ie star
earths, seperate analogue, power and digital earths with a single common
point etc, then a noise spike from say a power fet could cause a negative
GND spike which goes quite nicely through that cap and negativley spikes the
vcc of the IC.
A complex area, I doub't you'll get any definitive answers, just rules of
thumb and experience answers and the "i tried this and it worked" type
answers. There are definite guidlines and I'd receommend a visit to the
library or technical bookshop for a book or 2 on EMC design.
Good luck, experiment and find out what works and doesnt, here's a cheap
tester for susceptibility.
Wire up a relay through one of its contacts to a battery so that it
oscillates on/off. have a 2 - 4 inch wire off one of the coil wires. Turn
this on at varying distances from your circuit ( feet to inches) when it's
working and see if it is a victim of induced noise current and voltage. A
nice simple victim circuit to build to experiment with would be say a 74
series based D flip flop driving an led and have varying amounts of lead
length on the clk input or the power/gnd and observe the results. Then place
a few bypass caps around and observe the results.
"Richard Haendel" wrote in message
> Everyone knows that bypass capacitors should be as close as possible to an
> ic's power and ground leads. That's easy, of course, in the case of some
> PICS (for example) when power and ground are directly opposite to each
> and your using an ic socket that has room for a small capacitor.
> But my question is: can the capacitor be as far away as, say, .2" or .4"
> still be effective (at about 8Mhz)? If the cap can't be equi-distance from
> both leads, should it be closer to positive or ground? Or does it really
> Note that I'm talking about pc boards with through hole devices, not
> breadboads and not smts.
> Richard Haendel