WB> We've seen specifications on differential impedance, but no
WB> one has been able to define what that number represents. We
WB> use the rule of thumb that the differential impedance is 2
WB> times the odd mode impedance. Does that make sense? Can
WB> someone give me a real definition of differential impedance
WB> and it's relation to odd mode impedance?
Exactly correct. Here's why:
Differential impedance is defined conventionally (see e.g. Gray &
Meyer, Analog Integrated Circuits) as
Vdifferential
Zdifferential = ---------------
I1
where I1 is the current in ONE terminal of the diff pair.
Differential voltage is defined as
Vdifferential = V1 - V2.
Odd-mode impedance is defined as
Vodd
Zodd = -------
Iodd
Here you have to be careful. Let's let V1, V2 be the voltages on the
two lines and I1, I2 the currents.
V1 = Vodd, V2 = -Vodd
I1 = Iodd, I2 = -Iodd
So, Vdifferential = V1 - V2 = 2*Vodd.
Thus we have
Zdifferential = (V1 - V2)/I1 = 2*Vodd/Iodd = 2*Zodd.
You can also take a look at
http://www.ansoft.com/~bracken/differential/differen.html
for more info.
-- J. Eric Bracken, Ph.D. Tel: 1.412.261.3200 x135 R&D Manager, Signal Integrity Products Fax: 1.412.471.9427 Ansoft Corp., Four Station Square, Suite 660 bracken@ansoft.com Pittsburgh, PA USA 15219-1119 http://www.ansoft.com