[SI-LIST] Re: Drivers on a 50Ohm line
- From: steve weir <weirsp@xxxxxxxxxx>
- To: nitin.bhagwath@xxxxxx, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 28 Jul 2004 13:07:13 -0700
The current depends on whether the termination is parallel or series, and
if parallel Vterm. A series termination presents 2*Zline for 2*Tflight,
while a parallel termination presents Zline indefinitely to Vterm. For the
case you cite, it is unlikely that Vterm would be 0 or 2.5V. More likely
it would be Vdd/2.
Steve
At 11:22 AM 7/28/2004 -0700, Bhagwath, Nitin wrote:
>Hi All,
>
>I have a simple question, for which I'm yet to find a complete answer.
>
>A standard PCB trace has an impedance of 50Ohms. Does this imply that,
>say, for a 2.5V signal, a driver must be able to drive 2.5/50 =3D 50mA =
>of
>current on this signal to allow for a full swing of the signal?
>
>If this is the case, then how can a driver, such as a clock oscillator,
>which has a drive strength in the order of 20-30mA drive a buffer
>through a 50ohm line without the voltage being attenuated? This would
>get accentuated at 3.3V, or 5V where the current requirement would
>increase.
>
>Any thoughts appreciated
>
>Thanks
>
>-Nitin
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