RE: [SI-LIST] : Connector pin pwr/gnd ratio

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From: Peterson, James F (FL51) (james.f.peterson@honeywell.com)
Date: Tue Nov 14 2000 - 03:57:30 PST


Thanks for the input on this,
BTW : the dc requirements have been accounted for with a minimum # of 2.5v
and 3.3v pins.

The consensus seems to be that it depends on which plane the return currents
are flowing.

That's a very good point. Since 2.5 is just for cores, I'm going to assume
that we don't need to beef up the 2.5v pin qty beyond the dc #.
Almost all of the signals are from 3.3v totem pole outputs (lvcmos, lvttl,
and pci), so I would assume that return currents want to flow on both power
and gnd planes. With that said, I believe my qty of 3.3v connector pins
should at least be close to the qty of gnd connector pins.

thanks again,
Jim

-----Original Message-----
From: Ingraham, Andrew [mailto:Andrew.Ingraham@compaq.com]
Sent: Monday, November 13, 2000 10:50 PM
To: 'Peterson, James F (FL51)'
Subject: RE: [SI-LIST] : Connector pin pwr/gnd ratio

Oops, I mis-read your original question!

If you have enough pwr/gnd pins to more than handle the DC power needs, so
that you have a lot of flexibility in allocating them between power and
ground, then I think the best choice is partly a matter of your board layup.

For example, if all your signals that go through the connector are adjacent
to ground planes only in both boards, then ideally you want only ground pins
and no power pins. Any power pins would tend to have some signal return
current as the signal passes through the connector, but then where does that
return current go when it reaches the board? It goes into the power plane,
which has no signal return current. It wants to get to the gnd plane where
the return current is. You would need to have very good bypassing between
power and gnd planes on both sides of the connector. But if you had only
gnd pins, in theory you would not need to bypass at all at the connectors,
at least not for the sake of the signal return currents.

On the other hand, if your signals ran between pwr and gnd planes on both
boards, so that 50% of each signal's return current is in either plane, then
ideally you want to continue that 50-50 relationship through the connector:
try to have each signal pin next to both a power pin and a ground pin. Then
each gets half the return current, which simply flows from the pins into
both planes, without having to go through bypass capacitors to complete the
circuit.

Regards,
Andy

> ----------
> From: Peterson, James F
> (FL51)[SMTP:james.f.peterson@honeywell.com]
> Reply To: Peterson, James F (FL51)
> Sent: Mon, 13 November 2000, 16:43
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : Connector pin pwr/gnd ratio
>
> The original question was meant to be:
>
> On my board (CMOS logic), I have 85 connector pins allotted for 3.3v, 2.5v
> and gnd.
> How would you distribute? and why?
>
> Would you do 25% for 3.3v, 25% for 2.5v, and 50% for gnd? (which is a
> component approach, but maybe not a board approach.)
>
> Would you minimize the pwr pins and maximize the gnd pins? if so, why?
> (don't we have return currents flowing in both.)
>
> all inputs and opinions are welcome and appreciated.
>
> Jim
>
>
> -----Original Message-----
> From: Charles Grasso [mailto:chasgra@msn.com]
> Sent: Monday, November 13, 2000 4:25 PM
> To: Peterson, James F (FL51); si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Connector pin pwr/gnd ratio
>
>
> The allocation of grounds with respect to any number of signals is mainly
> based
> on rise time (hence frequency content). That is to say the faster the rise
> time the
> more grounds you need - the asymptote being a solid "bar" connection found
> in many impedace controlled connectors.
>
> ----- Original Message -----
> From: "Peterson, James F (FL51)" <james.f.peterson@honeywell.com>
> To: <si-list@silab.eng.sun.com>
> Sent: Monday, November 13, 2000 6:57 AM
> Subject: [SI-LIST] : Connector pin pwr/gnd ratio
>
>
> > Hello,
> >
> > I'm looking for some information on what the pwr/gnd ratio should be for
> a
> > board's connector. (I know there is something in the archives, but I
> > couldn't find it.)
> >
> > Someone said a rule of thumb is a 3 to 1 ratio, but I'm not sure what
> that
> > is based on.
> >
> > So let's say for example I have 85 connector pins for 3.3v, 2.5v and
> gnd.
> > How would you distribute? and why?
> >
> > thanks
> > Jim
> > > Jim Peterson
> > > james.f.peterson@honeywell.com
> > > Honeywell Space Systems
> > > M/S 934-5
> > > 13350 U.S. Hwy 19 N.
> > > Clearwater FL, 33764-7290
> > > Office : 727-539-2719
> > > Fax : 727-539-2183
> >
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