an aspiring electronics engineer here. I do some electronics design for work and I'd like to know the following:

If I have an assembly of a main board, to which an FFC cable connects into a second board (A), to which another FFC cable connects to a last board. Board/FFC assembly Some signals flow back and forth between these different boards (e.g. I2C), so I'd like to do some low-pass filtering for EMI. From my work I've learned you can put a filter in series close to the connector where the signal 'leaves' one board. Since my signal leaves my Main board, but also board A, does this mean I should put something like a T-filter in the signals in both the Main board and in board A? Or would the two filters -separated by some FFC length and PCB- still function as one filter anyway?

I've also understood you should put a T-filter on signals that are bidirectional like the I2C SDA line, which can be pulled high and low by both the MCU and the chips on board A or B. Like so: Bidirectional

And you should put a half T-filter on signals that are unidirectional (can only be dictated by the MCU) like the I2C SCL line: Directional

How do I design for EMC here?

  • \$\begingroup\$ I stopped reading at FFC - didn't know what it meant. \$\endgroup\$ – Andy aka Jun 18 at 7:28
  • \$\begingroup\$ An FFC is a Flat Flexible Cable: nl.farnell.com/productimages/standard/en_GB/2520212-40.jpg \$\endgroup\$ – steenou Jun 18 at 8:01
  • \$\begingroup\$ What EMI issues are you hoping to fix? \$\endgroup\$ – Andy aka Jun 18 at 8:20
  • \$\begingroup\$ Radiated emissions due to ringing of high frequency signals. The signals would be offloaded unto an FFC from the Main board, go back onto board A, then offloaded again from board A, to arrive on board B. Some signals would go the other way around: from board B to board A to Main board, or from board A to Main board. (I just use I2C as an example in my drawing) \$\endgroup\$ – steenou Jun 18 at 9:08
  • \$\begingroup\$ Why are those high frequency signals ringing? I ask because, the best way to cure EMI radiated is at source so, what causes them to ring? \$\endgroup\$ – Andy aka Jun 18 at 9:28

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