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ISO7821LL: LVDS and EFT

Part Number: ISO7821LL

HI,

     When I use ISO7821LL to isolate LVDS signal,I want to consult some questions,

there are some points you can attention in my device:

1.Ethernet cable and RJ45 are used to connect my two devices.

2.LVDS signals are transmited through Ethernet cables;

3.EFT is coupled to the Ethernet cable by coupling clamp

I want to test  the immunity ability of my device to EFT,Can you give me some  advice?

Thanks!

  • Hi Teck,

    Thank you for posting to E2E!

    EFT tests are powered tests complying with one of four different criteria specified in IEC 6100-4-4: criteria A, B, C, or D at test levels from 1kV to 4kV. The EFT waveform and pulse sequence is described in the "Electrical fast transient immunity" section of Understanding electromagnetic compliance tests in digital isolators (SLYY064), as well as the standard itself. 

    Testing your device to ensure that it meets one of the four criteria for EFT, described in the "Pass-fail criteria" section of the document linked above, requires monitoring the device and ensuring proper operation during or after the test -- depending on the applicable criteria you would like to meet -- and that the device is not damaged. Proper operation during and after tests can include expected data transmission, normal power consumption levels, and no latch-up conditions.

    A comparison of a TI transceiver passing criteria A EFT tests and a competitor device passing criteria B for the same EFT test can be found on this E2E blog post (What is an EFT? Part 2) for your reference. More background on standard EFT tests are also included in Part 1 of the blog series.

    EFT immunity tested per IEC 6100-4-4 is well-known, thus, more information is available for understanding and improving performance if needed. I hope this is all helpful information. Please let me know if this a test you can complete using the EFT clamp for your device and if you have any additional questions.


    Thank you,
    Manuel Chavez

  • Hi ,We all know that RS485 has excellent EMC performance because it has a wide Common Mode Voltage Range; We can easily realize 4KV EFT and performance criteria A.

    But LVDS's  Common Mode Voltage Range  is very narrow. We expect to know the maximum EFT voltage that LVDS signal can withstand in engineering application if we want to reach the performance criteria A.

    Is it possible to optimize the design to make the signal immunity of LVDS reach EFT 1kV and performance criteria A.,It can't be achieved in our circuit at present. Can you give us some suggestions.

  • Hi Hui,

    Thank you for following up! Are you and Teck referring to the same project?

    1kV EFT is high to meet for LVDS. Even RS-485 systems can have challenges at 1.5kV or 2kV. Since most clamping diodes and protective devices are tailored to RS-485, and the abs max ratings of LVDS transceivers can be easily exceeded, there is not much we can recommend to improve EFT performance and meet criterion A for LVDS signals. Nonetheless, implementing error-checking in software can help the system meet sufficient performance and pass criterion B.

    If you'd like, feel free to share a block diagram of the test setup and details about the coupling clamp. Is it a capacitive clamp? Which levels is the system currently passing? We can discuss this via Personal Message if you prefer.


    Thank you,
    Manuel Chavez

  • Thank you for your reply. How do you evaluate the performance of Ethernet/Ethercat in EFT test, relative to LVDS or RS485 signal.We can easily  meet  2KV Ethernet/Ethercat  in EFT and performance criteria A..

  • Hi Hui,

    Your understanding of RS-485 being more robust against EFT compared to LVDS is correct but please do note that even RS-485 needs either internal (to device) or external necessary protection to be able to meet required EFT levels. The same explanation goes for LVDS as well.

    The RS-485 or an LVDS bus itself cannot be said to meet any given EFT levels, the actual levels depend on overall system and can vary widely between one design to another. For example, one RS-485 device could only meet 1kV EFT while other device could meet 4kV EFT because of the internal protection.

    How do you evaluate the performance of Ethernet/Ethercat in EFT test, relative to LVDS or RS485 signal.

    This is something that has to be decided by you, OEM, end customer or the government EFT standard. If you have already performance EFT testing on Ethernet / EtherCAT, then you can follow the same approach even for the LVDS/RS-485 signals over Ethernet/EtherCAT.

    Please refer to the below RS-485 reference design that implements various protection components for various EMC tests. You can similar approach to further improve EFT performance for LVDS as well. Please make sure you choose the protection component ratings according to LVDS voltage levels. As already mentioned by Manuel Chavez, it might to difficult to find protection components that are tailored for LVDS bus. Thanks.

    https://www.ti.com/tool/TIDA-00731


    Regards,
    Koteshwar Rao

  • Hi Hui,

    EFT tests and passing criteria should be the same regardless of the protocol being tested. A summary of EFT test setup and conditions can be found in IEC 61000-4-x Tests for TI’s Protection Devices (SLVA711).

    Each protocol has its own advantages, including EFT and immunity performance.


    Thank you,
    Manuel Chavez

  • Thank you for your reply I learned a lot .
    In differential signal anti-interference, we usually use the common mode choke, TVs , shielded wire, and how the shielding layer  is connected to the earth. We have accumulated a lot of experience in the peripheral circuit of RS485. Let's also build a system that can meet EFT-4kv and performance criteria A. We have also accumulated experience in EtherCAT, which can meet EFT-2kv nd performance criteria A.
    But none of these measures worked in LVDS. Our LVDS cannot reach EFT-500v. At present, we hope to know whether there is a real case to make LVDS signal reach a higher EFT test index. Have you ever built a case to try this chip to achieve higher EFT in the experiment? Can you give it a try?

  • Hi Hui,

    I'm glad our responses have been helpful. LVDS devices are inherently weaker when it comes to EFT performance, so it hasn't been something our customers check for or that we have pursued. In short, we haven't found a case to pursue EFT performance improvements of LVDS systems, and we will likely not pursue this in the near future.

    I hope this answers your remaining questions. If there is additional information we can provide for you, please let us know or feel free to create a new thread later using the red and yellow buttons in the upper right corner of this window.


    Respectfully,
    Manuel Chavez

  • Your reply is very helpful to my team.I think your reply is the answer to this topic. I want to ask the last question. What are the characteristics of Ethernet signal that determine that it has higher immunity than LVDS? I We look forward to your comments

  • Hi Hui,

    Thank you. Please allow an expert from our Ethernet team to follow up regarding this question by early next week.


    Respectfully,
    Manuel Chavez

  • Hi Hui,

    Ethernet provides a certain standard as defined in IEEE. 

    Ethernet has significant isolation capabilities via the design of the MDI network, which provides isolation to magnetics. 

    In turn, this offers higher robustness with regards to EFT. 

    Hope this helps. 

    Best,

    Alon