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Question asked by staringlizard on Mar 1, 2015
Latest reply on Mar 3, 2015 by staringlizard



Question about TMDS routing/trace.


So as I understand it, the resistance for a TMDS pair should be equivalent to 100 Ohms (50 Ohms for each line). This is used as termination for the signal, e.g. as balance to avoid reflections. In other words, when designing the PCB, the trace for one TDMS line should be equivalent to 50 Ohm (this is done by adjusting the trace width etc.).


Instead of routing a trace for a TMDS line so that it will ultimately end up at exactly 50 Ohm I would like to keep the trace as short as possible between transmitter and connector and insert a ~50 Ohms resistor in serial instead. Is this possible/recommended and if it is, what is good to have in mind when doing it this way ?


Also, I have stumbled across a very different design online (suggested by Analog Devices) when it comes to the automobile industry:


Here it looks like the trace resistance for the TDMS lines has been ignored altogether where they have 100 Ohms pull-up and then 100 Ohms in serial for each line ? I quote:

"Using the 100Ω pull-up resistors in parallel with two 50Ω resistors in series with each other, gives an overall impedance of 50Ω."

As explained above, I would expect the TDMS lines themselves to carry 50 Ohms and make the statement above somewhat wrong.