Re: My continuing saga with power supply bypassing
- From: Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 27 Apr 2007 20:22:20 GMT
a7yvm109gf5d1@xxxxxxxxxxx wrote:
Hi gang,
As you may know, we have a consultant offering his advice on bypassing
a power supply for a power-hungry ASIC that needs quiet power. (few
100 uVs of noise) It boils down to the "many values of caps" strategy.
So he has sent me a list of values, along with the fact that the
values must come in certain physical sizes. He has offered no part
numbers except to say choose the cheapest caps on Digikey. So I'm
creating the parts in our CAD system, but some cap values don't exist
in 0201.
18000pF, or 18nF, or .018uF for example. Searching with all three
units only gets a hit for 18000pf in ceramic caps. Well guess what,
there's no 0201 case size. Neither at Mouser.
Take TDK P/N C0603X5R0J223K, in stock at Digikey, 15670 of them, just a few pennies a pop. 0603 in there seems to be the metric size (arrrgh!) which would be 0201 in "real life". 0.018uF would be a boutique part, I wouldn't use that. Purchasing wouldn't be happy. And they must be kept happy or they won't let you dig into the Christmas bonbons the distributors bring around in December ;-)
So my question to the group is, how the hell can this guy offer any
sensible advice and claim to have run simulations WITH PARTS THAT
DON'T EXIST???
The medical folks would politely say "that is sub-optimal". When I design stuff, like right now, there is always a 2nd PC running with the Digikey site on the browser. If it ain't in stock it ain't going into the circuit. And with stock I mean some sizeable quantity. It can still get you, as it did here last weekend. Saturday there were thousands of a particular ferrite choke. Monday the client took my list and whoopdidou, only 30 left and that wasn't enough. So it can happen.
I just need some backup from the folks in here. When I go see
management I'll have something more cogent to offer than the rage
that's building up...
I mean I can understand the rationale for specific cap values if you
know you're building a resonant circuit at a point, in my RF days I'd
spend a few hours killing tones by placing shim material here and
there, but every board and every piece of shim is unique, you can't
specify anything better than a rough place and a rough size. For
Cthulu's sake, even the shape of the solder blob affected stuff as low
as 3GHz, which is around the frequencies we're tooling around here. So
how in the name of the seven dark servants of Cthulu can anyone
backing this "multiple cap value" approach be so damn sure that *this*
560pF cap will kill *that* tone on every single board?????? Down to
-70dBm????
Again, I'm dumb here, so what am I missing? I don't get it.
Who's Chtulu? Anyhow, staggering caps is a good idea but with SMT you don't have to go to extremes anymore. Something like a 0.1uF, a 4700pF and maybe a 100pF should take care of the higher frequency scrubbing. Sprinkle a few 1uF across the planes (hoping it's planes...) plus here and there a nice low ESR 100uF should really keep things quiet.
Well one thing's for sure, my roses will bloom real good this spring.
Watch them aphids. They got ours almost every year.
--
Regards, Joerg
http://www.analogconsultants.com
.
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