Re: gEDA-user: Question for DJ about his alarm clock

[John Doty <jpd@xxxxxxxxx>]

On Aug 15, 2008, at 4:58 PM, Stuart Brorson wrote:

>>> I noticed that on p. 3, the ST part (U300) has PVDD and PVSS
>>> connections through 4.7 ohm resistors to PWR and GND,  
>>> respectively.  I
>>> can see using a small resistor + shunt cap to GND in series with the
>>> power connection to attenuate power supply noise.  However, why  
>>> is the
>>> PVSS connection to GND also going through a resistor?
>>
>> There's noise on both power and ground lines.  The P* pins are for  
>> the
>> PLL's power supply, which need as clean a power source as you can
>> provide.
>
> Did you use a ground plane?  If so, then I'd expect that the plane is
> quiet, and it would be better to connect directly.  Or am I wrong?

You're right. I'm always amused by the "noise on ground" notion. It's  
the sound of one hand clapping. Noise relative to what? I'm not  
saying you can't have troublesome noise currents flowing on a ground  
connection, but that has to be analyzed from a transmission line, not  
a network theory, point of view.

>
> Of course, if you used tracks for ground on the same plane as other
> signals, then you're right.  Putting a cap between PVDD and PVSS, and
> a resistor to PWR and GND makes sense.
>
> But anyway, thanks for the answer.  It makes sense.
>
> Stuart
>
>
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John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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