[SI-LIST] Re: Why Termination at Both End ?
- From: John Ellis <John.Ellis@xxxxxxxxxxxx>
- To: "Lakshmi Narayanan Sowrirajan, ERS-HCLTech" <lakshminarayanans@xxxxxxx>, Rohit MISHRA <rohit.mishra@xxxxxx>, Rajan Hansa <all.si.list@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 9 Aug 2011 06:01:56 -0700
Yes, assuming you have a good reference plane for return currents, increasing
spacing will decrease mutual inductance and capacitance, which reduces the
impedance variation. Lines that are source-only terminated should be kept as
short as possible to minimize the influence of transmission line effects.
John Ellis
Sr. Staff R&D Engineer
Mixed Signals &
I/O Libraries Group
Synopsys, Inc.
email: jellis@xxxxxxxxxxxx
Phone: 508.263.8194
-----Original Message-----
From: Lakshmi Narayanan Sowrirajan, ERS-HCLTech
[mailto:lakshminarayanans@xxxxxxx]
Sent: Tuesday, August 09, 2011 8:41 AM
To: John Ellis; Rohit MISHRA; Rajan Hansa; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Why Termination at Both End ?
Hi John Ellis,
I hope you can take care of this by proper spacing between the lines and does
not require a load end termination necessarily. If we include this will have
negative effect on signal rise time and timings.
Thanks & Regards
Lakshminarayanan. S
9840250902
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of John Ellis
Sent: Tuesday, August 09, 2011 5:47 PM
To: Rohit MISHRA; Rajan Hansa; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Why Termination at Both End ?
Hi Rajan,
Source only termination can work, however you must simulate to make sure that
the reflections are under control when neighboring lines are switching as well.
Under different switching conditions the effective impedance that a signal
sees can change dramatically. A trace may have a 50 Ohm impedance when its
neighbors are quiet, however when its neighbors are switching in the same
direction, the effective impedance can easily rise by 20% or more. Conversely,
when the neighbors switch in opposite directions, the effective impedance can
be reduced by 20% or more. With random bit streams running on all or the
lines, you're effective impedance on the line will be changing and swinging
throughout this range. Depending on the length and the frequency, you can
create some pretty ugly reflections. Source-only termination can work, but
don't attempt it without simulating under crosstalk conditions.
John Ellis
Sr. Staff R&D Engineer
Mixed Signals &
I/O Libraries Group
Synopsys, Inc.
email: jellis@xxxxxxxxxxxx
Phone: 508.263.8194
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Rohit MISHRA
Sent: Tuesday, August 09, 2011 6:26 AM
To: Rajan Hansa; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Why Termination at Both End ?
Rajan,
Here's my 2 cents :
I believe you can use a receiver without matching it's input impedance with
transmission line impedance But Only If the system satisfies these conditions :
1) Transmitter is connected to single receiver.
2) Transmission line is a controlled impedance line and has no
unintentional impedance discontinuities(like via, stub, package lead, etc)
In real system even though board is designed with controlled-impedance traces,
there is still the opportunity for a signal to see an unintentional impedance
discontinuity and in this case just source termination won't stop the ringing
that's why termination at both end.
Rohit
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Rajan Hansa
Sent: Tuesday, August 09, 2011 1:57 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Why Termination at Both End ?
Experts,
Can anyone explain that why in some designs we see source as well as load
terminations, I mean termination at both side of traces ? If ringing is a
issue then only source termination should be sufficient to control it and we
can use a receiver with very high input impedance i.e. no need to match
input impedance of receiver with transmission line characteristics
impedance.
Rajan
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