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How to set up a test for common-mode noise for LVDS

Mike Valvo
Intellectual 470 points
Other Parts Discussed in Thread: SN55LVDS31, SN55LVDS32

hello,

I'm in the process of evaluating LVDS (and MLVDS) communication between 2 or more boxes.  I'll be using the SN55LVDS31 drivers and SN55LVDS32 receivers.  I have several configurations including direct connection and AC coupling with capacitors or transformers.

Single-point grounding (SPG) will be implemented on the boxes and the boxes are isolated from each other. 

One question that I'm trying to answer is : whether I use AC coupling or not, will common-mode noise transmit from one box to another?  

I bread boarded a setup where the driver and receiver are powered from different power supplies and they are offset from each other.  I used AC coupling with capacitors and the transmission was good for any offset between the power supplies.  My colleague is telling me that this is not a test for common-mode noise.

I'm not knowledgeable about common-mode noise.  Any suggestions would be appreciated. 

  • 0
    Prodigy 100 points
    Use the RX GND as reference, and use sufficient bandwidth scope&probes to test RX box 's differential traces. Also, make sure your scope's protective earth terminal and TX box ground is also isolated(e.g. power your scope with a battery powered DC/AC converter). This way you will be able to properly measure common mode voltage on the differential terminals.
    For properly isolated systems usually there are no unbalanced return path for the common mode signal on the traces (e.g. unexpected parasitic paths) thus the common mode voltage won't be seen by the receiver.
    You can also use two traces on the differential traces and use math function. By examining the eye diagram on the math result you should be able to determine whether the signal quality and eye opening are good enough for the transmitting rate you are using.


    Good luck and hope it helps.
  • 0 in reply to Dan_1200
    Intellectual 470 points
    thanks Dan, those are some good suggestions for setting up the scope to measure common-mode noise.

    However, I'm more interested in how to set up a possible failure condition brought on by common-mode noise. Sorry if I wasn't clear in my original post. how would inject common-mode noise on box1 and see if communication still passes at box2? How does TI do common-mode testing of their receivers?

    thanks
  • 0 in reply to Mike Valvo
    Prodigy 100 points
    Keep in mind that LVDS drivers are inherently current sources unlike traditional CMOS signaling. None of the traces are truly referenced to GND at TX end.

    For a true common mode noise at the TX end, they are identical in every way on the two TX terminals. So there are practically no way they will be able to drive the termination resistor at the RX end (usually a 100ohm resistor). No current will flow due to common mode noise, nor there are signal degradation due to common mode noise. This matters very little for what type of coupling you use.

    So I think you don't have to worry about common mode induced failure for your application. It is exactly why LVDS is used, at the cost of doubling the connection.