If return path as GND plane, then return current/signal will be ended to GND pin of driver IC. Right?

Similarly, if return path as POWER plane, then the return current/signal will be ended to VDD pin of driver IC. Right?

Consider my VDD pin of driver is 3.3V and return current of signal flows through across split planes (1.5V, 3.3V, 5V). If so what will be the voltage level of return signal in each power planes?

  1. When it flows through 1.5V plane, then return signal voltage level will be 1.5V.
  2. When it flows through 3.3V plane, then return signal voltage level will be 3.3V.
  3. When it flows through 5V plane, then return signal voltage level will be 5V.

My understanding is correct?

  • \$\begingroup\$ I don't follow what you mean in your 3 points. What is "return signal voltage level be at xxV" mean ? \$\endgroup\$ – efox29 Aug 14 '15 at 8:37

For high frequency signals the return current for that signal is on the adjacent plane. It tends to be directly under the signal path. The value of the plane does not matter. So long as the plane is at a fixed voltage.

If your plane is ground, then the return path would follow the signal path (back to your driver), and end there.

If your plane is a power plane, then your return path would follow the signal path and then find its way back to ground through the past of least impedance. If you have a decoupling capacitor nearby, it will pass through the capacitor. Otherwise it will find its own way and may start interfering with other signals until it makes it way back to ground.

If you have a split plane, the return path will follow up to the split, and then it will find a way to get across. If there is no immediate least impedance path available, then it will fringe across. Fringe lines are not straight, and will spread out, and again, potentially interfere with adjacent signals.

If you have to use split planes, throw a capacitor on each side of the signal, so that you can establish a return path, through the capacitor. However, with capacitors internal resistances and inductance, you may see some noise on adjacent signals, or ringing on your signal. However this is better than not having the caps there. Ideally, you would route your signals such that they don't cross split planes.

  • \$\begingroup\$ Stitching capacitor will help to complete the path for this high speed return current. If it is low frequency, then the capacitor doesn't work for return currents. Right. Then what is the solution for low speed return currents when it travels through the split plane ? \$\endgroup\$ – Mohammed Azlum Aug 14 '15 at 11:50
  • \$\begingroup\$ @MohammedAzlum To be honest, I do not know about low frequency signals. But when you say low speed, are you referring to the physical frequency or the rise time/fall time. \$\endgroup\$ – efox29 Aug 14 '15 at 15:21
  • \$\begingroup\$ Example UART, I2C etc etc \$\endgroup\$ – Mohammed Azlum Aug 18 '15 at 10:36

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