Impedance matching Analog - Digital

[bilal_oct]

The impedance of analog and single ended digital signals have to be same when the objective is to have no reflections ? I have 50 ohm impedance for analog traces but I am wondering about the impedance of single ended digital signals. Any idea ?

 

[KlausST]

Hi,

Are you asking us what impedance you have? How can we know?

You say you have 50 Ohms for analog traces....this is knly good, if your source impedance is 50 Ohms, too. And if your termination resistance is 50 Ohms, too.

And it makes only sense if the traces are long enough ... depending on signal frequency.
For an analog audio signal up to 20kHz, it makes no sense to worry about trace impedance if trace length is only 30cm...

So you need to give a lot more information than just "50 Ohms".

Klaus

 

[FvM]

I guess "single ended digital signals" refers to CMOS/TTL?

High speed point-to-point connections can use source side series termination to reduce reflections, the driver impedance has to be considered as part of the series termination. Trace impedances vary according to PCB technology (number of layers, minimal trace widtdh), I would rather expect 60 - 70 ohms than 50 ohms for typical single ended traces.

Some nets include branches, e.g. clock distribution nets. It's effectively impossible to avoid reflections in this case.

 

[marce]

Most times it doesn't matter what the impedance is within reason... Its only when you get to high speed interfaces, such as DDR memory that you have to start controlling trace impedance's. Most boards are designed with a trace width that will fit the design being done, anything from 0.1mm for tight boards to 0.3mm for less dense designs...

 

[qtrungnet]

you can reference the " Signal Integrity" book, it's may be help:
http://longfiles.com/gbt323eb7ork/S...omagnetics_and_Professional_Practice.pdf.html