Help to decide vtt caps
robertferanec , 08-24-2016, 06:18 PM
I am not saying this is bad or good, I would probably try to make it as a full plane rather than branches to keep the voltage in every point same or very similar. I am not sure how much, but branches may influence potential (voltage) distribution in different places of the branches and voltage may differ a little bit. Especially for VTT it could be visible due current peaks (you have there 24 parallel 47 OHM resistors). Also, in some designs they connect some VTT capacitors also between GND ,,, it may depend on what plane is reference to the tracks - if GND or VDD.
nachodizz990 , 08-25-2016, 01:49 AM
Now i understand why you referenced vtt caps to both vdd and gnd, this is because L3 and L8 run between both planes.
The common factor in this case is GND but a common sense approach should be to try to place 2 caps for each 3 terminated lines one cap for VSS and the oter to VDD
As you said (another funny discussion):
The main reason to decouple VTT is to provide a path for trace return current. If the traces have their return current on Vss, then it important to decouple to Vss. If traces have their return current on Vdd, then decouple to Vdd. And if return current is on a combination of both Vdd and Vss, decouple VTT to both ; -> http://www.sipiexchange.net/q?id=9168
@robertferanec , I modified the decoupling following your sugestion and also placed a VTT plane, now it looks much better
The common factor in this case is GND but a common sense approach should be to try to place 2 caps for each 3 terminated lines one cap for VSS and the oter to VDD
As you said (another funny discussion):
The main reason to decouple VTT is to provide a path for trace return current. If the traces have their return current on Vss, then it important to decouple to Vss. If traces have their return current on Vdd, then decouple to Vdd. And if return current is on a combination of both Vdd and Vss, decouple VTT to both ; -> http://www.sipiexchange.net/q?id=9168
@robertferanec , I modified the decoupling following your sugestion and also placed a VTT plane, now it looks much better
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