Differential Trace Design Rules
Truth vs Fiction
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Truth vs Fiction
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Truth vs Fiction
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Truth vs Fiction Array (half of a 360 degree complete cycle), and if propagation time is 6” per nanocond in FR4, then one degree pha shift equates to 333 mils distance. If we t one degree as our threshold, (which might be too much!) then the corresponding offts would be:
Frequency (MHz) Offt (mils, or thousandth in.)
50 333
500 33
5 GHz 3
Conclusion: The equal length design rule is important IF the signal equal-and-opposite assumption is important. The signal equal-and-opposite assumption is important if we are worried about EMI or if we require discontinuities in the power system grounds between two circuits.
Clo together rule, part 1: It is generally understood that EMI is related to loop area.2 The loop area is defined as the area between the signal path and its return path. On differential traces, the signal is on one trace and the return is on the other trace. So the loop area is a function of how clo the traces are routed together.
If we are concerned about EMI, we must route the traces clo together. The more cloly we route them to each other, the smaller the loop area will be and the less EMI that will be generated.
Clo together rule, part 2: One of the primary advantages of differential signals is the signal-to-noi ratio improvement that is obtained. Since the signal is one polarity on one trace and the other polarity on the other trace, the resulting signal at the receiving device is twice what the single-ended signal would be. An additional advantage is that the receiving circuits are designed so that they are highly nsitive to the difference in signal level between the two traces, but highly innsitive to shifts in signal level that occur
on both traces. This is normally called common mode rejection at the receiving device.
Truth vs Fiction
In order to have good common mode noi rejection, it is important that any noi that is prent affects the signals on both traces equally. That is, if noi is coupled into one trace, an equal amount of noi must be coupled into the other trace. Then the common mode rejection capability of the receiving circuit will reject the noi. But if noi is coupled into one trace more strongly than into the other trace, the noi will appear as a differential mode signal to the receiver and be amplified.
The way to ensure that any noi is coupled equally into both traces is to route the two traces very clo together. Then they will both be in the same noi environment.
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Conclusion: The clo together rule is important IF we are worried about EMI or IF we are worried about common mode rejection of noi that has been coupled into our traces.
Continuous plane rule, part 2: Assuming signal integrity issues are important, we are probably worried about signal reflections at the end of traces. If we are worried about reflections, then we need impedance controlled traces. If we need impedance controlled traces, then it is almost axiomatic that we need continuous planes underneath tho traces.3 Otherwi, impedance control is very difficult to achieve and impedance discontinuities will develop.
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Constant spacing rule: When two traces are routed clo together, there is coupling between them. Ordinarily we call that crosstalk. But in the very special ca of differential signals, we don’t refer to it as crosstalk, and there is no negative implication to this coupling. The very special ca derives from the fact that the two differential signals (in an ordinary ca referred to as the aggressor and the victim signals) are perfectly correlated. One is the exact inver of the other.
There is a conquence of this coupling. The conquence is that the resulting impedance of the trace reduces from its single-ended value.4 The normal expression for the resulting impedance is Z = Zo – Z12
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where Z12 reprents the effects of the coupling.
The coupling is a function of the spacing. Traces spaced very far apart have little or no effective coupling, and their characteristic impedance is simply Zo. Traces placed very clo together have some degree of coupling, and therefore lower impedance. If we are concerned about using impedance controlled traces, then we are also concerned about having a constant impedance everywhere along the trace. Otherwi, the resulting impedance discontinuities may cau reflections.
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Conclusion: If we are using controlled impedance traces, then it is important that the paration between traces remain constant everywhere along their length. (Note that this conclusion says nothing about how cloly they should be spaced. That conclusion derives from EMI and common mode noi rejection considerations.)
Differential impedance rule: The so-called differential impedance rule simply says we must calculate a differential impedance for proper trace termination if differential signals are routed clo together. There is nothing magical about differential impedance. We would not have to u differential impedance calculations if traces were routed widely apart. But:dateofissue
