RE: [IBIS-Users] [RAMP] for differential output LVDS

From: Tom Dagostino <tom_at_.....> Date: Thu May 26 2005 - 15:32:46 PDT · This archive was generated by hypermail 2.1.8 : Thu May 26 2005 - 15:31:56 PDT

For LVDS IBIS models we use two 50 Ohm resistors, one each from each output
of the LVDS driver.  The other end of each resistor is tied to Vcm of the
driver.  This gives a set of waveforms that match the expected operating
conditions the driver will see.  You can also tie those resistors to Vouthi
and Voutlo of the output to get waveforms that best approximate the IBIS
gnd/vcc conditions.  I would not use a termination to ground or vcc for LVDS
modeling.  In all three cases above V_fixture is the termination voltage,
Vcm, Vouthi, Voutlo.  The resistor is 50 Ohms and never use a C_fixture in a
model.  My experience is they cause simulators to fail.  If you feel the
need to put C_fixture in make it equal to 0.00 pF.

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org]On
Behalf Of CW - Doran David
Sent: Thursday, May 26, 2005 1:51 PM
To: ibis-users@eda.org
Subject: [IBIS-Users] [RAMP] for differential output LVDS

Hello experts:

I'm simulating a LVDS driver that has differential outputs
(one inverting and one non-inverting) and trying to get my
[RAMP] data for IBIS model.

Typically, IBIS spec says to place 50-ohm resistor to Vdd for down-ramp,
and 50-ohm resistor to Vss for the up-ramp.

However, the vendor spec for all timing & waveforms says to place
a 100-ohm termination between the two outputs.

So in this case, can I ignore the IBIS spec and not use the Vdd & Vss
resistor, but use a resistor between the two outputs instead ?

Also, same question for my AC test fixture for rising & falling waveforms:
is it OK to ignore the IBIS spec and not use the Vdd & Vss R_fixture
but use a 100-ohm termination between the two outputs ?

I'm confuse here since IBIS test fixture for rising & falling waveforms
requires listing a R_fixture, V_fixture, C_fixture... and none of these
are for the connection between outputs.  They are only for connecting
to V_fixture, which I don't have.  So I cannot even specify a V_fixture
in this case with termination between the two outputs.

Also, the IBIS spec mentions "true differential models" in Section 6b
with model types Input_diff, Output_diff, I/O_diff, and 3-state_diff...
but not sure this applies to my LVDS driver.

My spec simply states that this driver must have a 50-ohm transmission
line for each output, and then terminated by 100-ohm resistor across
the outputs at the far end.

any inputs appreciated!

If it helps, below are my model types used for each output (P & N) :

[Model]          TX1P
Model_type       Output
Polarity         Non-Inverting
Enable           Active-Low
Vmeas =  1.200V
Rref  =  50.000
....

[Model]          TX1N
Model_type       Output
Polarity         Inverting
Enable           Active-Low
Vmeas =  1.200V
Rref  =  50.000
Cref  =  0.000pF
....

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