Show to properly terminate a ribbon cable

Just checking my failing memory.

I have a 20 cm long flat ribbon cable that connects two circuit boards I have designed (the distance is for convenience as both boards are used for prototyping purposes - the final circuit will be on a single - but more complex - board). The signals are all in the 0-3.3V range and switching at up to 5MHz (there are power and ground lines as well).

When the cable is not connected I'm getting clean signals at the controller board, but when the cable is connected (with or without the other board) I'm getting huge amounts of spurious transitions and (what looks like) crosstalk.

I realise (now!) that I should be terminating the data lines but I'm just checking with the experts to see exactly what that termination should be. Is something like a 470Ω resistor to ground the thing to use or is it better to use a 100nF capacitor to ground? Or both?

Thanks

Susan

Typically the line termination value would be in the neighborhood of 100-130Ω, but that requires that your digital drivers can drive such a load. Also each signal line should have a return line in next to it. This provides a path for the return signal current and reduces crosstalk.

You do not want to put a capacitor. That would likely just make it worse.

It may be difficult now but ground signal ground is where you need to be.

Thank you to both of you. I'll try to resistors tonight and see how that goes. Yes, you are right - I should re-order the lines so there is a ground in between each and I'll also give that a go even though it will involve quite a lot more soldering but that's what prototyping is all about I guess.

Susan