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SN75LVDT390: When I use LVDT390, the differential input P-impedance changes from 180KΩ to 453Ω

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Replies: 8

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Part Number: SN75LVDT390

When I use LVDT390, the differential input P-impedance changes from 180KΩ to 453Ω, which leads to a high single-ended signal. I would like to ask under what circumstances this situation occurs, the use of chips will break down?

In addition, I have another question. Does the load input current of the output terminal, which is higher then 10 mA, affect the differential input?

The voltage supply is 3.3V, well grounded, and the differential input is connected to the printed circuit board by cable.

  • Hello,

    Please provide more details about the use conditions. What is driving the LVDS signal? How do you measure "P-impedance before and after? A diagram would be very helpful.
    Also, please clarify your second question (input to what and output from what)?

    Regards,
    Yaser
  • In reply to Yaser Ibrahim:

    Thank you for your reply. The other end of LVDT390 is LVDT391, which is provided by another PCB. The cable is 2 meters long in the middle. The differential signal is twisted and the single-ended signal is sent to the FPGA.
  • In reply to user5993392:

    Hello,

    Thanks for providing the details.

    Regarding first question, please provide details of what you mean by "P-impedance" and how you measure it (between which terminals?). What are the 2 conditions under which you measure the 2 impedance values. Again, a diagram would be very helpful.

    Regarding second question, the current from LVDS driver (LVDS391) should be ~3.5 mA, so I am not clear on where the 10 mA is coming from? Please explain.

    Regards,
    Yaser
  • In reply to Yaser Ibrahim:

    Thank you for your reply. So far, six LVDT390 pieces have been out of order. Four of them are the same as the normal differential terminal impedance. The impedance of the differential P and N terminals and the ground are measured with a multimeter. The impedance of the other two differential P and N terminals and the ground have changed.
    What I want to say is that there are four LVDS signals coming out of a PCB, which are produced by LVDT391. There are also four differential TTL serial signals and five local signals on the same line. The differential signals are twisted and shielded on the outermost layer of the cable. After four connectors, the total length is 2.5 meters. Finally, the differential LVDS signals are connected to my PCB. The differential LVDS signals are connected to LVDT390. Four differential TTL serial signals are connected to four MAX391 chips. Will this damage LVDT390?
  • In reply to user5993392:

    Hello,

    I am not sure, as I still don't fully visualize the system (a diagram or partial schematics would be very helpful). However, if it seems these particular LVDT390 devices became damaged, then one possibility is an EOS event (ESD). How many LVDT390 devices total have you tried and how many became damaged?

    Regards,
    Yaser
  • In reply to Yaser Ibrahim:

    Thank you very much for your reply. This is part of my schematic diagram and PCB. In the PCB layout, most of the LVDT390 used before was broken. The capacitor C300 was not placed near LVDT390. Is this the reason why the LVDT390 burned out?

    Also, you think it might be ESD. How can I design to avoid ESD? Thank you and look forward to your reply.

  • In reply to Yaser Ibrahim:

    Thank you for your reply. The last picture was not shown, so I am sending it again.

  • In reply to user5993392:

    Hello,

    The SN65LVDT390 has higher ESD rating (15kV vs. 4kV for SN75LVDT390), so that's one option. If that's not enough protection then using an external ESD protection device would help. This System-Level ESD Protection Guide has more details, including specific device recommendation for LVDS (just search for LVDS): www.ti.com/.../sszb130c.pdf .

    Regards,
    Yaser

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