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SN74CBTLV3245A: Can the SN74CBTLV3245A be used with LVDS?

Part Number: SN74CBTLV3245A

Hi,

I am wondering if the SN74CBTLV3245A 3.3V 8-Channel 1:1 Bus Switch can be used with LVDS ?

Thanks,

Elliot

  • Elliot,

    The SN74CBTLV3245A device is a passive FET switch that will simply pass any voltage or current through the device. This means it can support any signal as long as it is within the recommended operating conditions of the IC.

    The SN74CBTLV3245A device can support up to 3.3 V and 200MHz signals.

    What is your concern that the SN74CTBLV3245 device will not support LVDS?

    Thank you,
    Adam
  • Thanks Adam, Just wanted to make sure there wouldn't be any issues with the timing and impedance. 

    Elliot

  • Hi Adam, It seems the Impedance changes with the voltage for example with a VCC of 3.3V and an input voltage of 2.4V, and a current of 15mA I see a typical Ron of 10ohms. What would it be for a voltage of 1.3V to 1.9V and current of 3.5mA? and can two pins (A1 and A2 for example) have a differential impedance specification (LVDS requires 100 ohm differential impedance)?

  • Elliot,

    The SN74CBTLV3245A is a FET switch which is non-linear such as a mechanical switch.  The on-state resistance of the FET changes as the input on the source of the FET changes which varies the FETs Vgs (gate to source voltage)

    The graph below is the Ron as the input voltage sweeps for an SN74CBTLVXXXX family of switches.  The 1.3 V and 1.9V look to be about 3-4 ohm Ron. 

    There is more information on switch performance in this application note

    I believe most LVDS receivers have the 100 ohm differential impedance integrated into the IC and you will just have the 4ohm losses on the transmission line due to the switch Ron.

    Thank you,

    Adam

  • Thanks Adam,

    Yes LVDS requires 100 ohms at the termination end, and that may or may not be included on the chip, but it also requires 100 ohm differential transmission line impedance.

    and when adding the chip we get a discontinuity in that transmission line differential impedance. 

    That is the concern I have.

    Thanks again,

    Elliot

  • Elliot,

    You are correct anytime your transmission line encounters and IC there will be a discontinuity due to the IC pad capacitance and cause a signal reflection. At signals <100MHz this should not be an issue as your signal will not be attenuated substantially and still be seen as logic high or low on the receiver.

    There are some layout things you can due like remove the ground plane right underneath the IC pad to reduce the pad capacitance but at these low frequencies it shouldn't make much of a difference.

    Are you currently seeing issues on your signal path?

    Thank you,
    Adam
  • Sorry for the delay in response.... I haven't tested it yet just more of a design question looking forward. I will test the board soon and then let you know how it looks. Thanks again, I appreciate the support! Elliot
  • You're welcome. We're looking forward to the results.

    Adam