Re: My continuing saga with power supply bypassing

One of the things any real world engineer knows (but text book ones often do not) is there is a very big difference between what is in the data sheets and what you can actually get. This is ESPECIALLY true for caps.

Sounds like your boy doesn't get out into the real world enough. Esp since they sent you to Digikey and not Arrow, TTI, Avnet, or some such. I spend more time in selecting parts in seeing if they are actually available then finding the best part for the job.

Anyway .018uF does exist for 0201s but in Y5V dialectic and if you are having bypass issues i am sure you don't want Y5V. Does your boy understand dialectic types or has he been behind a text book and into Math-CAD and Spice Simulations to long to know any real world realities?

What are you tring to do? Is the bypass problem bypassing what is coming from the ASIC back to the power or the voltage going to the ASIC? If it is to the ASIC and you are having to do this then you are not fixing the problem, only masking a power supply design or PCB layout issue. The problem needs to be fixed there. Adding a pile of inductance from a pile of caps isn't going to really fix the issue. My guess is you have a PCB layout issue, not a cap issue.

Hawker


On 4/27/2007 12:12 PM, The digits of a7yvm109gf5d1@xxxxxxxxxxx's hands composed the following:
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.

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???

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.

Well one thing's for sure, my roses will bloom real good this spring.

.

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