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SN65LVDT125A: 8 x 6:1 LVDS MUX / Crosspoint switch solution

Chris Stubbs
Prodigy 10 points

Part Number: SN65LVDT125A

Hi,

 

I am looking to create an LVDS multiplexer to switch 6 devices under test (8 pairs each) to 1 acquisition card. 7 pairs are outputs from the DUT, 1 pair in an input to the DUT.

The signalling rate is <75Mbps. The data is not necessarily DC balanced and may contain log chains of identical bits.

 

My plan so far is to use one and a half SN65LVDT125A for each pair (4+2, allowing for 6 DUTs), cascaded in to another layer of SN65LVDT125A to select between them.

SN65LVDS125A example extract.png

[Extract showing only 2 pairs from the 6 DUTs]

 

Is this the most appropriate solution from your product portfolio? I can only find larger switches/muxes which do not directly support LVDS and would require layers of translation.

 

Would cascading to a second layer of switching be required as shown, or could the outputs of the first layer be connected together in parallel instead?

 

Would termination resistors be required on the unused outputs, as it looks like they would always need to remain internally connected to one of the inputs by selection logic?

 

This would need 16 IC’s, is there an alternative that uses I2C/SPI etc for the selection control pins, otherwise this could need a lot of IO?

 

Thank you for the support.

  • +1
    TI__Genius 16656 points

    Hi Chris,

    To make sure I'm understanding your switching scheme correctly, each DUT will have 7 LVDS output and 1 LVDS input. You want to mux between DUTs and send the 7 output and 1 input LVDS signals of the selected DUT to the acquisition card. For example, when DUT2 is selected, the acquisition card will receive Data Out 1-7 all from DUT2 and one Data In signal. You will not be reading signals from 2 different DUTs at the same time, and each output from a single DUT will only be connected to one mux IC channel. Is that correct? 

    If so, then your current solution would be the best if you do not want to do any translation. DC balancing should not be of a concern if you are not AC-coupling these signals. 

    Parallel LVDS connections for a multi-drop solution is technically possible if you don't want to cascade, but this could potentially be more of a design challenge depending on you cable/trace length. You would also need to change the IC to SN65LVDS125A without the integrated termination resistors since and LVDS bus can only have one 100Ohm termination, You can read more about it in this app note: https://www.ti.com/lit/an/slla054a/slla054a.pdf?ts=1767895356722&ref_url=https%253A%252F%252Fwww.google.com%252F 

    Regards,

    Matt

  • 0 in reply to Matt Smith
    Prodigy 10 points

    Thanks Matt! That's right, I would only be reading from one DUT at any time.