Practical Vcc trace implementation for pull-up resistor

Hello everyone,

In practical design cases, when there is a pull-up resistor at the output connected to Vcc, can we have this Vcc connected directly to other Vcc trace or, it must be an independent Vcc or, some kind of filter must be applied?

I have a transimpedance amplifier, whose output is in CML and requires pull-up resistors. Since the amplifier itself needs Vcc as well as the pull-up resistor, and in my understanding, if both Vcc are on the same trace, when the output is drawing current it may interfere the Vcc supplying to the amplifier. How should I design my Vcc trace in this case? Vcc is 3.3V and the differential output swing is up to 1V.


Thanks a lot!

Mario Young

Due to the differential design, neither the amplifier supply nor the pull-up resistor node should face large current changes in a first order. Depending on the amplifier gain, separate bypassing may be advantageous anyway.

Hi FvM,

Thanks for your reply. If the output is taken as single-ended, because there is nothing following it but a connector for measure purpose, would this make any difference? By the way, the signal if about 40GHz. Thank you!

Mario Young

Yes, I think so. You can calculate the trace and bypass capacitor inductances and estimate the AC magnitude at the supply voltage node, that is generated by the load resistor AC current. Check if it's still acceptable.