[SI-LIST] Re: return currents

Larry,

Please see my comments in lines. Thanks.

Larry Smith wrote:

> Zhiping - Voltage is always with respect to (WRT) some other node.  All
> voltages on printed circuit boards, electronic packages or chips are
> differential.  Voltage is the potential of one node WRT another node.
> We sometimes talk about the voltage of a node WRT the center of the
> earth or WRT spice node zero (they are the same thing) but those
> voltages are irrelevant for signal integrity in our products.  The only
> thing that counts is the difference in voltage between two nodes, for
> example a signal WRT local ground.  For traces in a PCB, the difference
> voltage is always vertical, the voltage of a trace WRT the local
> reference plane.  TEM mode analysis is good for all the stuff we do.
>

This is very true!

>
> The PCB stackup in question has ground planes that surround both the
> analog and digital traces.  The only thing that is important is the
> differential voltage between the trace and reference (ground) planes.
> That difference voltage will propagate down the trace (transmission
> line).  Eventually, the voltage/current waveform will arrive at a via
> that takes it to the surface of the PCB.  There should always be a
> nearby reference (ground) via.  A chip will sense the differential
> voltage between the signal and local ground.  This will be the same
> voltage that was driven into the trace at the other end of the line,
> if we have done a good job of maintaining a controlled impedance
> environment (50 ohms) for the whole length of the trace. The voltage
> that went in on one end is the same as the voltage that comes out on
> the other end a time delay later, each WRT to their local grounds.
> (For the moment, let's ignore the lossy nature of transmission lines.)
>
> Now suppose we had a way to measure the voltage between two ground
> points spaced several inches apart horizontally (I am not sure how to
> do it, but let's say we could).  Suppose there were several volts
> difference between the two local ground points.  Would that make any
> difference to the waveform propagating on the 50 Ohm transmission
> line?  I don't think so.  What ever voltage went into the transmission
> line WRT it's local ground will come out of the transmission line WRT
> local ground, even if the local grounds are several volts apart.  The
> driving chip drives WRT local ground and the receiving chip receives
> a signal WRT local ground.  If there happens to be a bunch noisy
> digital lines on the opposite side of a ground plane, ...so what?

I agree with your digital transmission line analysis, but how about the
analog circuit? If the voltage difference betwen two points on the ground
plane is not zero and those 2 points happen to be the reference points for
a differential amplifier inputs.  For my understanding, the voltage between
those 2 ground points will effect the output of the amplifier. Is that true or
I missed something here? Thanks.

>
> BTW, I agree with you that low frequency can be a killer.  But signals
> behave well at high frequency.  The thickness of a ground plane is
> all you need to isolate one signal from another.
>
> regards,
> Larry Smith
> Sun Microsystems
>
> > Date: Wed, 26 Sep 2001 12:27:38 -0700
> > From: Zhiping Yang <zhiping@xxxxxxxxx>
> > X-Accept-Language: en
> > MIME-Version: 1.0
> > To: ldsmith@xxxxxxxxxxxxxxxxxx
> > CC: si-list@xxxxxxxxxxxxx, james.f.peterson@xxxxxxxxxxxxx
> > Subject: Re: [SI-LIST] Re: return currents
> >
> > Larry,
> >
> > What you said about the current distribution at high frequency(>100Mhz)
> > is true, but my questions is whether the current distribution is important
> > or the voltage variation on the power plane is more important than current
> > distribution?
> >
> > Let's say, there are 2 points on the power plane and the most (>93%) current
> > flows
> > on the surface near digital circuit. The current flow near analog circuit is
> > very small (<7%), but it has big loop (large inductance) and it produces 
> > same
> > voltage drop between those 2 points as large current on the surface of 
> > digital
> > circuit.
> > IF the anolg circuit is sensitive to the voltage noise, then it is a 
> > problem.
> >
> > Jim, another thing you need to be aware is that low frequency may be a 
> > killer
> > for your anolog circuit. In your current stack up, it is diffcult for to
> > control
> > the lower frequency current return path on layer 4.
> >
> > Thanks.
> >
> > Zhiping
> >
> >
> > --
> >    Zhiping Yang, Ph. D.
> >    Hardware Engineer
> >    Cisco Systems
> >    270 West Tasman Drive
> >    Mail Stop:SJCG/2/2
> >    San Jose, CA 95134             |          |
> >    email: zhiping@xxxxxxxxx      :|:        :|:
> >    Tel  : 408 525 5690          :|||:      :|||:
> >    Fax  : 408 526 5504       .:|||||||:..:|||||||:.
> > *****************************************************
> >
> > Larry Smith wrote:
> >
> > > Jim - I don't believe that the high frequency return currents on your
> > > digital traces will have much effect on your analog traces even though
> > > they share the same ground plane (layer 4).
> > >
> > > The skin depth at 100 MHz is about 0.26 mil compared with the 0.7 mil
> > > thickness of half oz copper.  The skin depth is essentially the depth
> > > that the magnetic field penetrates into the copper.  At 100 MHz, very
> > > little magnetic field (approximately 1/[e^(.7/.26)] = 7% ) will
> > > penetrate through the copper plane.  Even less of it will reach an
> > > analog trace.  At higher frequencies, the penetration will be even less.
> > >
> > > How sensitive are your analog signals?  For digital signals, I would
> > > not worry about 7% magnetic field penetration.
> > >
> > > regards,
> > > Larry Smith
> > > Sun Microsystems.
> > >
> > > > Delivered-To: si-list@xxxxxxxxxxxxxx
> > > > From: "Peterson, James F (FL51)" <james.f.peterson@xxxxxxxxxxxxx>
> > > > To: si-list@xxxxxxxxxxxxx
> > > > Subject: [SI-LIST] return currents
> > > > Date: Wed, 26 Sep 2001 07:32:10 -0400
> > > > MIME-Version: 1.0
> > > > Content-Transfer-Encoding: 8bit
> > > > X-archive-position: 946
> > > > X-listar-version: Listar v1.0.0
> > > > X-original-sender: james.f.peterson@xxxxxxxxxxxxx
> > > > X-list: si-list
> > > >
> > > >
> > > > hello,
> > > >
> > > > Stackup :
> > > >
> > > > 1 - gnd
> > > > 2 - digital sig
> > > > 3 - digital sig
> > > > 4 - gnd
> > > > 5 - analog sig
> > > > 6 - analog sig
> > > > 7 - gnd
> > > > 8,9,10,11 .....
> > > >
> > > > notice that the digital signals from layer 3 and the analog signals from
> > > > layer 5 will probably have return currents on layer 4.
> > > >
> > > > question :
> > > > will the digital return currents cause noise in the analog section, 
> > > > since
> > > > they both share layer 4 for return currents? (My first guess is yes, but
> > > > someone mentioned that the skin depth  for the return currents is small 
> > > > so
> > > > they can share layer 4 without effecting each other.)
> > > >
> > > > thanks for your input.
> > > > Jim

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