[SI-LIST] Re: differential impedance

Stephen:

Look at my earlier posting regarding differential and common mode impedance 
formulas.

Common mode impedance is, to a reasonable approximation, 1/4 the 
differential mode impedance Zdiff =< 100 ohms, Zcm >= 25 ohms.  So small 
amounts of common mode voltage result in relatively larger CM currents than 
the equivalent differential mode voltages.  With regard to whether you care 
about common mode transmission, in all serial designs and specifications 
now in use, the common mode is open circuited terminated, so not only can 
the currents become sizeable, there could be significant CM reflections as 
well.

If however, the edges are not well controlled (within a fraction of the 
risetime) as well as the propagation delay of the traces (including 
parasitics) then you will have conversions of differential mode to common 
mode energy.  Use of the signal decomposition theorem 0.5*[A(t)+B(t)] and 
0.5[A(t)-B(t)], where A(t) and B(t) are the incident driver waveforms, 
shows that when A(t) = -B(T), the common mode is identically zero, but only 
for that condition.

If A(t) is not equal to -B(t), then exactly what effect this has on your 
interconnect is dependent on the transmission line length and the specific 
receiver and driver characteristics.  If you control the driver edges to be 
in time phase, you control the driven amount of the common mode 
energy.  Most PCB designs are fairly well balanced if they are symmetrical 
w/r to some imaginary center line. So these

At 11:07 AM 1/30/2006 +0000, Stephen Greenhalgh wrote:
>There are many pcb trace geometries that give 100R differential impedance. 
>However, the even mode impedance will vary, depending on the amount of 
>coupling between the traces. Consider a situation where two signal boards 
>are connected together by being plugged into a backplane. Is there any 
>advantage in using the same pcb trace geometry on the backplane as on the 
>signal boards, or does it not matter (as long as both have 100R 
>differential impedance)?
>My reasoning is that it would make no difference in a perfect world. 
>However, in practice there will be common mode components in the signal 
>(for example, because of crosstalk and drivers not being perfectly 
>symmetrical). Therefore, changes in even mode impedance may be significant 
>in the real world, but how significant? Is this just a second-order effect?
>
>Why might a designer consider different geometries? Signal boards may 
>require very thin traces with relatively high coupling (for example, to 
>route signals between the balls of a BGA). By comparison, a backplane may 
>be better designed with wider traces (as these are more easily 
>manufactured and less subject to impedance variation with manufacturing 
>tolerances).
>
>Any insights will be appreciated.
>
>Stephen Greenhalgh
>
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