Electronic – Placement of signal, power and ground lines within a ribbon cable


Just a couple of assumptions before I start as this is a theoretical question

  • All signals are <1MHz digital signals
  • All signals are single ended, no differential pairs
  • The 3 power lines are +5V, -5V and 12V
  • The ribbon cable is unshielded
  • The ribbon cable is inside a system with various other wiring looms carrying voltages/signals

What is the best placement for signal, power and ground lines within a ribbon cable with regards to EMI.

I have drawn up the two possibilities I could think of which are:

  1. Interleaving ground lines between each signal/power line
  2. Using the ground lines to create a barrier between the power and signal lines

enter image description here

Is one of these a better practice than the other or is there an even better solution which I didn't think of?

Best Answer

Interspersed signals with grounds or rails is best for EMI and crosstalk purposes, so your option 1 is better here.

However, in your example you have seven ground wires and only one wire for each power rail. Assuming your signals are mostly logic level, i.e. 5V rail, it would be preferable to reduce the ground lines to four and add three more 5V lines. This will help balance the return current paths for both signal levels.


simulate this circuit – Schematic created using CircuitLab

Note, with your current setup, when the signal is high the return current is through the shared grounds, when it is low, the return current is through the single wire. Add more signals and the single wire must share all those signals and noise will be seven times more significant on the rail wire.

If the other rails are driving significant currents you should also increase their counts appropriately to try and balance the currents over the available wires.

In addition, as analogsystemsrf mentioned, if the cable is of any appreciable length, it is prudent to slow down the edges of logic signals passed along the cable. Your signals may be low frequencies, but those digital edges are not.