Hi Scott,
If you can find a board that doesn't look like a short when you TDR between
large ground and power shapes when (almost) adjacent pins of a device are used,
I'd like to see that, to understand the disconnect. When I've TDR'ed boards
with this configuration, my experience has been that the waveform looks like a
short until you stretch the time waaaaay out - then it looks like a charging
capacitor.
I can imagine boards and stackups in which this isn't true (small power and/or
ground shapes, or very large distances between them), but haven't experienced
them.
Jeff
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Scott McMorrow
Sent: Tuesday, November 01, 2016 3:36 PM
To: dmarc-noreply@xxxxxxxxxxxxx; paul.huang7531@xxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Transmission line terminated to Vdd only
Jeff
Their ain't no such thing as a dead short. The world is awash in inductance,
which manifests itself along with capacitance as transmission lines with finite
delay. How the planes react will depend on the edge rate.
Scott
Teraspeed Consulting - A Division of Samtec Scott McMorrow, Technical Director
SI/PI Office 401-284-1827 www.teraspeed.com | www.samtec.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Loyer, Jeff
Sent: Tuesday, November 1, 2016 6:21 PM
To: paul.huang7531@xxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Transmission line terminated to Vdd only
Some previous postings talking about this are available at
//www.freelists.org/post/si-list/reference-plane,10
Depending on your stackup, the inter-plane capacitance may provide the AC path
you're looking for. Think of TDR'ing between your ground and power planes.
For most stackups, they will be a dead short (if you make the o' scope trace
long enough, you'll see the classic cap charging signature).
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Paul Huang
Sent: Tuesday, November 01, 2016 3:05 PM
To: patolfo@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Transmission line terminated to Vdd only
Thank you Patolfo and Orin,
If I don't have a capacitor between Vdd and ground at the termination resistor
does it mean that my trace referenced to ground operates in unterminated mode
and the same trace referenced to vdd plane operates in matched impedance
terminated mode?
Then if I add a capacitor does it mean that I create an AC termination for
trace referenced to ground too? Does this mean that I lower the overall
termination and I may have reflections?
Thanks,
Paul
Sent: Tuesday, November 01, 2016 at 2:22 AM------------------------------------------------------------------
From: "Patolfo Nery Velya" <patolfo@xxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Transmission line terminated to Vdd only
The current will flow through the parasitics too, so ac will be couple
partly to ground plane, al cabo de la vida quisiera descansar en un
bosque de paz y sin preguntas y desde la espesura escuchar en sosiego
el preludio de un réquiem privadÃsimo Benedetti **Cansancio
On Mon, Oct 31, 2016 at 6:26 PM, Paul Huang <paul.huang7531@xxxxxxxx> wrote:
Hello Si-list,
I have a question: I have a CMOS driver that drives a strip-line
trace referenced at top to Vdd supply power plane and at bottom to
vss ground plane. The trace is terminated to vdd only. Since there
is no termination to ground, will it be any return current flowing
through the ground plane or only through the Vdd power plane?
Thanks,
Paul
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