> First, my experience regarding layout is minimal at best since I seem to 

> exhaust all the wrong ways first.  I once took a class in multilayer 
layout 
> and was told that the capacitance between the power/ground planes 
> themselves was sufficient for decoupling high frequencies and that 
adding 
> pacitors could cause tuned circuits and troublesome resonances.  I'm 
> simply asking, any views on that theory?


That may be true at very low frequencies like 74LSxx parts, but as the 
edge rate goes up the capacitance needs to be closer to the pin. Most 
manufacturers today say to put decoupling capacitors as close to the power 
pins as possible and they try to make a power and ground such that the cap 
can be attached to both pins.

>>common in today's circuits.  Also, 0.01 uF caps are less expensive and 
take
>>up less space (0805 vs. 1206).

Huh? Why would you use a 1206 0.1uf for anything unless you needed 50WVDC 
or more rating? 

Everything I have read on bypassing for digital says to use 0603 or 
smaller parts since they have lower inductance, and at the moment 0603 is 
the most cost effective. On a current design I have 0603 0.1uf, 0.01uf and 
smaller, 0805 1uf and 1210 10uf ceramic caps, besides the larger 
tantalums.

The previous designers, who no longer work for us, used 1210 0.1uf for all 
the bypassing and they put them on the other side of the board far away 
from the power pins with really small traces so they'd be comletely 
useless. It really makes life suck now that we need to get RFI compliance 
testing done, especially when they routed digital across ground plane cuts 
and routed ADC power on traces that snake all over the board, 
and.......I'm going home for a beer.

Robert D. LaMoreaux
MTS Systems Corp. 
Powertrain Technology Division
4622 Runway Blvd.
Ann Arbor, MI 48108
734-822-9696
Fax 734-973-1103
Main Desk 734-973-1111



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