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SN65LVDT250: SN65LVDT250

Eduardo Mendes
Prodigy 80 points
Part Number: SN65LVDT250

Hello,

In our application we are using a 4x4 SN65LVDT250 cross-point switch to multiplex some clock signals.
We have the following configuration:
- Inputs:
IN4=opened
IN3=ground
IN2=Coming from a 240MHz oscillator
IN1=opened

-Outputs:
OUT4<=IN3
OUT3<=IN3
OUT2<=IN2
OUT1<=IN2

When we observe OUT3 using a scope, we observe a signal of amplitude ~10mV synchronous to the 240MHz oscillator connected to IN2.
We didn't find information about the cross-talk between inputs in the  SN65LVDT250  data-sheet.
QUESTION1) Is it normal to observe such a noise?
QUESTION2) Is there any application note or document with a crosstalk characterization for the device?

Thank you very much!
With my best regards,
Eduardo Mendes


  • 0
    TI__Intellectual 2020 points
    Hi Eduardo,

    Did you observe this on our EVM or on your own board? If it is on your own board, could you show me the layout of the board? Cross-talk may be caused by improper layout.

    Regards,
    Nic
  • 0 in reply to Nic Lin
    Prodigy 80 points

    Hello,

    We are using our own board, I am sending you the files attached (please tell me if you have access to them).


    Regards,
    Eduardo

  • 0 in reply to Eduardo Mendes
    TI__Intellectual 2020 points
    Hi Eduardo,

    I can access to them. I will try to figure it out and let you know later.

    Regards,
    Nic
  • 0 in reply to Nic Lin
    Prodigy 80 points
    Hello Nic,

    I realized that this forum is public, is it possible to switch to a private webcase?

    Eduardo
  • 0
    TI__Mastermind 43795 points

    Hi Eduardo,

    Could you explain how you are grounding and opening the inputs to the LVDT250?

    I believe the noise you are measuring is at least partially due to the input signal condition on the LVDT250.  If the input pair is grounded and even a small level of noise impacts the circuit, the voltage gain through the LVDT250 will result in some measurable output noise.

    Thanks and Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    Prodigy 80 points

    Hello,

    the input is not really grounded but connected to a PLL which is deactivated (and the outputs are low). The opened inputs are not connected to anything.
    The noise I observe is synchronous to the other input (the one which is not connected to the output).
    It would be helpful if I could have some characterization data of the crosstalk between inputs of the SN65LVDT250.


    Eduardo

  • 0 in reply to Eduardo Mendes
    TI__Mastermind 43795 points

    Eduardo,

    The noise you are seeing is the voltage gain of the LVDT250 from the 4:1 mux through to the output.  Since you have biased the input directly on its threshold, it only takes a small amount of internal noise to be visible on the output. 

    No input signal ( ~ 0V differential ) is not a valid input condition for most high speed LVDS devices.

    You are doing a worst case crosstalk test with your board.  The 10mV of noise coming out of OUT3 would be -40 dB if the input signal is 1000 mV.  This -40 dB number is a combination of the input-input crosstalk plus the voltage gain of the LVDS circuit from the 4:1 mux to the output.

    Regards,

    Lee