[SI-LIST] Re: Hspice simulation in relates with capacitor

You will notice that for any capacitor value you place in your topology, you
will eventually get a zero crossing point for your differential signal, if
you wait long enough.  As Andrew said, it will take the 10000pF capacitor 25
times as long as the 400pF to get to that point.  
So, you can fix this one of two ways in simulation:

1.  Simulate for a very long time and disragard the first couple
microseconds or so of your sim (which takes a lot of unnecessary sim time).

2.  Specify an initial charge on your capcitor.  All you have to do to do
this is include IC=#v after the capcitor value when it is called out.  For
example:
C1 1 2 10000pF IC=375mV

--Pat

-----Original Message-----
From: zanglinyuan [mailto:zanglinyuan@xxxxxxxxxx]
Sent: Wednesday, December 18, 2002 8:42 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Hspice simulation in relates with capacitor


Hi.everyone
Now I'm making simulation with hspice.
Here are two cases and both AC coupling:
1.transmitter(88e1043)----||-------receiver(HDMP1636)
2.transmitter(HDMP1636)---||-------reciver(88e1043)

Stimulus :1.25Gbps,LVPECL
Termination as per vendor's recommendation.

For case1,when we use 10000pf capacitor,the eye diagram crossing point is
abt 200mv.(eye diagram amplitude is 400mv)
and we try 400pf capacitor again,the eye diagram opening and amplitude same
as 10000pf,but the crossing point is 0mv.
That means the differential signal's common mode voltage is changed.

For case2,the simulation result is almost no difference between 10000pf and
400pf. 

Question is:
What on earth does the common mode voltage change with coupling capacitor
for case1 ?
is it due to hspice simulator or there is some problem in 88e1043 model? and
some other reasons? 

Thanks 


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