[SI-LIST] Re: SSO SSTL Vs LVTTL
- From: Canes Venatici <starsilic@xxxxxxxxx>
- To: "Peterson, James F \(EHCOE\)" <james.f.peterson@xxxxxxxxxxxxx>
- Date: Mon, 30 Apr 2007 06:13:10 -0700 (PDT)
Jim Peterson,
Thanks for your reply. The only concern is the impedance of the driver may be
non-linear,
depending upon the state (cut-off/linear/saturation) of the transistors, so I
was wondering
can we approximate the driver impedance.
Also I've one more doubt regarding the simulations using T.Lines. I used 4inch
lossless transmission line.
The equivalent i/p inductance comes around 33nH and capacitance comes to be
13pF, by simple formulas
of T.lines. With this if I do simulations, I get very high power cell
requirement for each signal cell, or the quite node
noise is very high.
I could see in some docs related to SSO simulations, they were just putting
series resistor at the output of the driver, followed by
termination resistor pulled to Vtt for SSTL interfaces, instead of T.lines in
their SSO set-ups.
Could anyone clarify on how well it can represent SSO noise simulations?
Regards
Canes
----- Original Message ----
From: "Peterson, James F (EHCOE)" <james.f.peterson@xxxxxxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Cc: starsilic@xxxxxxxxx
Sent: Monday, April 30, 2007 5:07:14 PM
Subject: RE: [SI-LIST] SSO SSTL Vs LVTTL
Hello Canes -
Since you're measuring noise at a quiet node, I assume you are
discussing SSO noise. I'm going to step out a little here and make a
bold statement : if you are comparing the SSO characteristics of two
different technologies, say 2.5V LVTTL and 2.5V SSTL, and their rise
times are the same, and their source impedances are the same, then their
SSO noise will also be the same. So, based on that statement, changes in
these above mentioned items will cause the SSO to change. If you go to a
stronger driver or a larger voltage swing (3.3V LVTTL) - and the rise
time stays the same - then you've increased SSO noise. If you go with a
weaker driver, or you put a series resistor at the output (like SSTL)
then the SSO noise will decrease. Signal swing, rise/fall time, and
source-Z are the things that influence SSO noise.
As a side note, you mention "12ma driver". IC manufacturers use this as
a DC drive number not an AC number. That's why we should use the source
impedance of the driver as a better (but not perfect) description of a
driver's capability.
Regards,
Jim Peterson
Honeywell
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Canes Venatici
Sent: Monday, April 30, 2007 1:18 AM
To: si-list@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] SSO SSTL Vs LVTTL
Hi,
I did quite node simulations in LVTTL and SSTL class I and II pads. I
used transmission line models for all the
simulations with noise measured at the Far End.
I tried to operate the LVTTL pads at 3.3V with 12mA drive strength. I
could see
the noise is more than with SSTL2-I/II interface.
Since there were no terminations for LVTTL, I suspect it can give more
noise compared to
SSTL. Comments are appreciated.
Even between SSTL2-I and II the power:signal is nearly same (the quite
node noise is similar),
with class-II is slightly more than class-I and in lower power:signal
ratios, the noise is less in
class-II compared to class-I.
I feel SSTL2-class-II have two terminations, which makes the interface
less noisier compared
to SSTL2-Class-I. Comments are appreciated.
Regards
Canes
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