[SOLVED] impedance matched traces that branch out to multiple pins of an ASIC

[frznchckn]

I have several digital input pins to an ASIC that will have incoming signals >100MHz. Technically, it can be the same signal going to all pins, but I'm concerned about the traces branching off on the PCB and the affect the branching will cause on the incoming signal.

I will be driving this signal to the board using a 50 ohm impedance cable and was, thus, going to have impedance matched traces. Should I just drive these signals individually to the board or can I branch them to the various pins as close as possible to the pins?

Thanks.

 

[loosemoose]

Best would be to simulate the circuit. I would add terminations to each pins just in case. And the branches should be as short as possible in relationship to the whole trace length.

 

[frznchckn]

So branch off as close to the pins as possible and have a termination resistor for each pin to 50 ohm as well. So the pin would see VDD/2 at full rail with termination, yes?

 

[marce]

If you branch and use 50 ohms per pin your signal will see the parallel combination of theses resistors. I would daisy chain if close together and have a final pull up at the last pin.
or to terminate each branch:
If you had 5 braches with 50ohms, the signal sees 10ohms, so to match 50ohms with 5 branches you require 250ohms pull ups.

 

[frznchckn]

Marce,

Thanks, in speaking with another engineer that's what I decided to do. I should've closed this thread. However, why do you suggest a pull-up instead of a single termination resistor to ground?

Thanks.

 

[marce]

Woops sorry, read Vdd wrong, a pull down.
A good source of info for this sort of stuff is "High Speed Digital Design" by Howard Johnson and Martin Graham. It has examples of exactly what you wanted to do. So I checked in there first.