[SI-LIST] Re: Driver strength and trace capacitance
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Ravinder.Ajmani@xxxxxxxxxxxxxx, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 30 Aug 2005 16:36:32 -0700
Ravinder, once again we find a topic that is handled very nicely in Eric's
book. A variation on this theme is useful when considering how bypass
capacitors affect signal edges. Dr. Johnson also deals with this in his
book "High Speed Signal Propagation" where he goes through the different
frequency regions of a transmission line behavior, RC, LC, etc. Both are
good reads and will answer your question, probably a lot more thoroughly
than you were considering when you framed it. I think if you consider that
it is 100Mbps and probabbly more like 300MHz knee frequency that 2" is
getting pretty close to significant wave effects.
Steve.
At 04:11 PM 8/30/2005 -0700, Ravinder.Ajmani@xxxxxxxxxxxxxx wrote:
>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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