[SI-LIST] Driver strength and trace capacitance

  • From: Ravinder.Ajmani@xxxxxxxxxxxxxx
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 30 Aug 2005 16:11:15 -0700

I was simulating a 2" long net driven by an ASIC and noticed significant 
ringing and non-monotonicity.  This is a bidirectional net and the driver 
at the other end is a 50 ohm driver, which does not require any 
termination.  From the IBIS model I deduced the driver impedance to be 
about 3 ohms, which I thought was quite low.  I was told that the ASIC 
vendor was asked to design the driver to drive a 100 MHz signal in to a 35 
pF load.  The trace capacitance is about 10 pF (including vias), and the 
receiver capacitance is less than 5 pF (package plus die).  To me this 
appears to be a case of over-design.  However, I have one question about 
the trace capacitance.  Can it be considered a lumped load or a 
distributed load.  The Lambda/20 length at 100 MHz is about 3" so I am 
inclined to treat the total trace capacitance as a lumped load.
Regards, Ravinder
Server PCB Development
Hitachi Global Storage Technologies

Email: Ravinder.Ajmani@xxxxxxxxxxxxxx

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