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From: landry@enginr.dec.com
Newsgroups: comp.sys.atari.st
Subject: Re: 1Mbit DRAM Decoupling
Message-ID: <3958@shlump.nac.dec.com>
Date: 8 Aug 89 17:14:46 GMT
Sender: news@shlump.nac.dec.com
Organization: Digital Equipment Corporation
Lines: 41

In article <1534@ns.network.com>, logajan@ns.network.com (John Logajan) writes...
"willing@ICSL.UCLA.EDU (Scott Willingham) writes:
"> Does anyone out there happen to know the optimum decoupling
"> capacitor value for 1Mbit DRAMS?  If so, do you also know of a
"> good supplier for them?
" 
"I've constructed RAM boards using .01uf per memory chip, but I usually use
"0.1 uf.  It is usually the number of outputs on a chip that determine the
"power glitch during switching.  A single bit ram needs less bypassing than
"a ram with 8 data lines.  But 0.1uf should be fine for your purposes.
"Keep the leads short!  And keep the caps near the chips.  Everybody sells
"0.1uf caps!!!!!!!
" 

	This isn't quite true!  The decoupling needs of dynamic RAMs
	have little to do with the output drivers.  By far the 
	biggest current changes occur when RAS turns on and off, not
	when the outputs are enabled.  

	0.1 uf caps are fine for decoupling 64K RAMs but 0.33 uf caps
	(multilayer ceramic) should be used for 256K and larger devices.  
	AVX has a technical report that shows there to be quite an
	improvement in power noise by going to 0.33.  At first we
	thought it was AVX marketing trying to sell us their new small
	0.33 caps so we repeated the tests on our own memory boards.
	Turns out they were absolutely right!

	Here are some numbers out of their report (for 256K RAMs):

	     cap value(uf)	total voltage drop (mv) 

		0.068			130
		0.1			115
		0.22			95
		0.33			85
		0.47			80

	(Diminishing returns above 0.33)


	chris