I would like to implement a LED Matrix circuit like this one, where the LED matrix enclosed within the blue dots will be on a seperate PCB than the rest of the circuit.

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Since this will be a multiboard design, I am confused whether ground lines should be passed between the matrix board and the main board to protect against EMI and signal integrity as suggested very well by the answers here and here.

If the boards don't share ground at all, then the return paths will flow directly between the shift registers and the mcu in the ground plane of the main board (not the matrix board). Which of course, will not stop the system from operating but will create huge current loops.

Questions:

1) Is a very high speed multiplexing of the leds in the array considered "high frequency" in terms of its currents so that the return currents choose the path of least impedance underneath the rows and columns?

2) Should a ground trace per signal (in this case per row, or per column) be added in the connectors bewteen matrix board and main board?

Thanks

EDIT: The design will be adjusted so that instead of a shift register, two multiplexer's will be used to switch between rows and columns and select a certain LED. Probably the ADG704 because of low Ton=20ns. This hopefully clarifies what range of frequencies I expect the system to operate at.

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    "...If the boards don't share ground at all..." - the GND on U1 and the GND on MCU1 are common though surely? If not, it won't work. – Oliver Aug 10 at 14:04
  • @Oliver mcu1 and u1 have common ground yes, they are in the same board. But no ground signal is passed to the matrix board. – George Aug 10 at 15:27
  • Does this need to pass regulatory? What do you mean by high speed? – laptop2d Aug 10 at 18:04
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    By "Regulatory" he means the legal regulatory requirements for EMC (electro-magnetic compatibility), etc. Is there any particular reason you are multiplexing "as fast as possible"? Usually > 120 Hz would eliminate any visual flicker, would it not? – Transistor Aug 10 at 18:44
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    And the reason for "as fast as possible"? (You may have missed the edit to my previous comment.) – Transistor Aug 10 at 18:54
up vote 2 down vote accepted

1) Is a very high speed multiplexing of the leds in the array considered "high frequency" in terms of its currents so that the return currents choose the path of least impedance underneath the rows and columns?

First off the Uno can switch at roughly 30Mhz (with a function call, if you eliminate function calls it can go faster), This is approaching high frequency territory as capacitance and inductance are going to limit the rise time of the signal. It depends on how it is wired, and the better pathway back for ground you have (by minimizing inductance), the better rise times and signaling you'll get. I would expect that this setup with jumper wire would be similar to breadboard inductance's and more like 10Mhz max. Going higher than 30-50Mhz the need for transmission lines and a solid ground plane underneath (or differential signaling) becomes necessary.

Actually looking at the 74HC595 the max speed it can support is 5Mhz, so that will be the limiter of your design, I don't think circuit parasitics will affect you.

2) Should a ground trace per signal (in this case per row, or per column) be added in the connectors between matrix board and main board?

Probably not necessary at 5Mhz. The thing to watch for is going to be the ground return path back to the source of the 74HC595, make sure the ground is solid (less than 100mΩ) on the wiring. Make sure the shift register has a good filter cap.

  • Thanks for the answer. I will probably swap the shift register with a faster multiplexer with rise time arouns 20ns and also use an STM32F4 so I will definitely exceed 30 MHz. For your second point, yes, I'll try to have as uninterrupted a ground plane as possible. WHy did you stress the importance of the wiring being less than 100mΩ under the shift reg.? – George Aug 10 at 18:45
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    Because with large switching currents you could get common mode noise. If the cable was say 2Ω and 5mAx8=40mA then with several LED's on it could create a slight brightness difference for all at. – laptop2d Aug 10 at 20:04

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