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Hi,
This is fairly standard.
1) The routes should be as close as possible. ( Cross talk is not a issue. Common mode interference is desired. )
2) At higher frequencies the difference in length matters.
3) Impedance should be properly matched.
You must put the pair into a well (ofcoure, if your process lets this) If you dont want any body-effet. Connect the well to the reference. So you can short the drain and sources of the transistors.
Two traces which travel parallely from source to designation is referred as differential routing.
Differential impedance changes with coupling,
which changes with trace separation. Since it is always
important that the trace impedance remain constant over the
entire length, this means that the coupling must remain constant
over the entire length. And this leads to the separation
between the two traces (of the differential pair) must remain
constant over the entire length.
i am working on a board layout involving routing of differential pair signals. i have routed them with the spacing between them almost constant around 12-15 mils and when i see the final lengths there is a difference of around 25 mils. the frequency is around 200 Mhz and the the lengths are 500 mils and 525 mils. is this fine ?
i did some calculations,
signal propagation rate on FR4 board is 150ps/inch
so for a difference in length of 25 mils(0.025 inch), time lag =0.025*150 = 3.75 ps
frequency of signal = 200 MHz , T = 5ns = 5000 ps
hence a delay of 3.75 ps is not much of a delay factor ! does this sound reasonable
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