# [SI-LIST] : Impedance of micro-strip over wide ground trace

Roland F. Portman ([email protected])
Thu, 26 Mar 1998 14:13:04 -0800

[The following is related to a question I sent to the si-list about a 2
layer flex circuit.]
[My thanks to all of you who replied.]

All the equations I have read assume an infinitely wide ground plane
when
calculating the impedance of a micro-strip. But in practice, how wide
does the ground
reference really need to be?

As the width of the ground trace increases beneath the fixed width of
the signal trace,
at what width does the ground trace begin to take on the characteristics
of a ground plane?

In other words, if a signal, 'w' mils wide, was centered over a 'W' mil
wide ground trace, at what width W would the resulting impedance be
within a reasonable tolerance to the same signal trace over an infinitly
wide ground plane?

----- w signal
H
--------------- W ground

Could it be related to the equation for current distrubution under a
trace?

i(D) = (Io/piH) * 1/(1+ (d/h)sqrd)

where

Io = total signal current (Amps)
H = height of trace above ground plane
D = perpendicular distance from the signal trace (inches)
i(D) = signal current density (Amps/inch)
pi = 3.1415...

[taken from "High Speed Digital Design" by Howard Johnson & Martin
Graham; eq 5.1, pg 190]

Roland

```--
Roland F. Portman
BiT Microsystems, Inc.
48499 Milmont Drive
Fremont, CA  94538

(510) 623-2341 Ext 129
(510) 623-2342 Fax

[email protected]
http://www.bitmicro.com
```