[SI-LIST] Termination resistance value for GTL+ bus

  • From: Samuel Dadel <samuel_dadel@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Mon, 9 Sep 2002 12:11:15 -0700 (PDT)

Hi! All,  I am new to transmission lines: so I would greatly  appreciate of 
someone could give me some leads on the following.  I am employing a dual-ended 
parallel termination scheme for a GTL+  bus. The driver has the capacity to 
sink 50mA. The Vol is .5 and  the Voh is 1.5, and the Vref is 1.0 [Nominal]. 
When employing  a 50ohm termination scheme I see a voltage swing of 1.15 Volts  
at the driver, with Vol gone down to 0.3 Volts. The nominal values

  are 1.5 and 0.5. I get bit errors, that go away when Vref

  is lowered to 0.8 Volts.

  Could this be a case of unmatched termination?  In addition: these is this 
additional equation that I   cannot relate to the characteristic impedance of 
the trace.  Iol[max] = .050 Amps.  Iol = Vtt - Vol / R[eff].  R[eff] = 1.5 - 
0.5 / .050 = 20 ohms.  For a dual ended termination scheme Rtt = 20 * 2 = 40 
ohms.Thanks,Daniel



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