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ISO7741: Safety limiting values vs. signal integrity and termination for high-speed signals

Part Number: ISO7741

Hi TI Experts,

I have an application where we are planning to use multiple ISO7741DW isolators.

The isolators will serve a couple of purposes:

  1. Isolation barrier for "standard" product safety (e.g. UL/CSA/EN 61010-1)

  2. Measure to demonstrate independence in terms of Functional Safety (e.g. IEC 61508-2)
    1. To block cascading failures (e.g. over-voltage) from propagating between redundant channels that are supposed to be "independent" (e.g. failure in one channel should not propagate to redundant channel or else system is no longer HFT>0)
    2. To block cascading failures from propagating between safety and non-safety circuits, otherwise the non-safety circuit needs to be treated as a safety circuit (e.g. non-safety circuit FIT rates need to be included in PFD and SFF calculations, and included as part of the safety function).

I have read about "safety limiting values" and understand the need to keep parameters (e.g. current) within the safety limiting values even under fault conditions to maintain the insulation barrier (and thus maintain independence in terms of Functional Safety).

https://e2e.ti.com/blogs_/b/analogwire/posts/understand-and-apply-safety-limiting-values-for-digital-isolators

My question is, does TI have any recommendations / app notes / techniques for how to balance techniques to conform with safety limiting values with also maintaining signal integrity / line termination techniques for moderately high-speed signals?

My searches didn't come up with anything.

We are worried about inserting series resistors in the 100s of ohms to limit fault current to conform with safety limiting values and how that might affect signal integrity and proper signal termination for some serial communication links (e.g. 5-50MHz SPI, 100-500Mbps LVDS).

Are there any recommendations TI could provide? That would be much appreciated.

Best Regards,

USO

  • Hi USO,

    Thank you for your inquiry.

    Please give us some time to review and we will get back to you in 24 hours.

    Thanks,
    Aaditya Vittal

  • Hi USO,

    Thank you for your patience and your question.

    The document you referred to is the main document that describes safety limiting values and also gives an example approach to achieve those limits. The implementation of the approach depends on the application itself, so you are free to choose any approach that you may find suitable to meet safety limiting values while also maintaining signal integrity.

    One other way I can think of to limit current to use load switches or signal switches. for more details on types of switches available, please post your question on the switches and multiplexers forum: https://e2e.ti.com/support/switches-multiplexers-group/switches-multiplexers/f/switches-multiplexers-forum

    Regards,
    Aaditya Vittal

  • Hi Aaditya,

    Thanks, I will post my question in the forum you suggested.

    I had previously found a device that might be suitable, but it's from ADI.

    https://www.analog.com/en/products/adg7421f.html

    I'll have to see if TI has similar types of devices.

    So no specific recommendations from TI regarding SI line termination techniques when also needing to maintain SLVs for isolation?

  • Hello,

    Thank you for your reply.

    In terms of recommendations, the document you linked in your original post is the primary document regarding faults and safety limiting values.

    The switches and multiplexers forum I recommended in my earlier post will definitely be able to help you to find a similar TI device to the part you have listed. 

    Regards,
    Aaditya Vittal

  • Hi,

    I understand the primary document regarding faults and safety limiting values, but there's no mention regarding signal integrity or transmission line termination together with it.

    Adding series resistors to limit current makes sense for the safety limiting values, and I get that, but it could negatively affect typical things done for SI, like transmission line termination, so I had wondered if TI had any recommendations with respect to doing both.

  • Hi USO,

    I understand your concern with keeping safety limiting values while also having signal integrity. Unfortunately, we do not have any recommendations to incorporate both safety limiting values and signal integrity.

    Regards,
    Aaditya Vittal

  • Fair enough. Thanks for your support regardless!

  • No problem!

    If you have any questions in the future regarding isolator devices, we would be more than happy to help!

    Regards,
    Aaditya Vittal