[SI-LIST] Re: Should I take into account PCB trace as a load ?
- From: "Lynne D. Green" <lgreen22@xxxxxxxxxxxxxx>
- To: yardala <yardala@xxxxxxxxx>
- Date: Sat, 23 Apr 2011 22:17:37 -0700
Hello, Yardala ,
1) The trace is a transmission line load. This
means it looks resistive to the driver when it is
switching state. The signal can reflect from one
or both ends.
2) The math: L and C depend on both Z0 and delay
(properly, Z0 and delay depend on L and C). So
one can never calculate L and C from only Z0. The
mathematical relationships are in any good
transmission line text, often with discussion of
interesting "limit" conditions and their solutions.
3) A field solver is used to find L and C. There
are also approximate solutions published in papers
and textbooks. Accurate solutions include the
influence of all other traces, as well as vias and
other "second order" effects that matter at fast
edge rates.
4) EDA software is worth buying for working on an
actual design.
Regards,
Lynne
Dr. Lynne Green
Chair, IBIS Model Review Committee
On 4/23/2011 9:54 PM, yardala wrote:
> Hey, Experts
> I have a basic question that PCB trace can be a load ?
>
> I want evaluate the driving capability (characteristics) of a buffer with
> load.
>
> As i know a trace ( or say a transmission line ) can be equivalent L, C,
>
> in it's characteristic impedance (Z0), when I simulate the buffer with an
> input device load
>
> should I take PCB trace into account ?
>
> I am confused how much L, C should be ?
>
> for example a trace Z0 = 50 Ohm, The L, C can be ( 7.4 nH/in, 2.9 pF/in )
>
> if I use 4 inches trace, the L , C are 29.6nH and 11.6 pF, but a 6 inches
> trace
>
> will 44.4 nH and 15.6 pF, I don't know how long that real applications,
> then which
>
> value should I use in evaluation ?
>
> or, I need not take PCB trace into account, and why ?
>
> Thanks!
>
> Yardala
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