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ISO1211: ISO1211

Emirhan Köse
Prodigy 10 points
Part Number: ISO1211
Other Parts Discussed in Thread: ISO7720,

Tool/software:

Hello,

I have an input signal that is connected to ground by default and pulled up during normal operation. During EMI testing, the signal sometimes falsely detects as low (ground) even when it is being pulled up.

To mitigate this issue, I’m considering using a TI ISO121x device due to its robust sensing capability. However, these are isolated input devices, and in my system the return paths are shared — meaning the isolation is essentially broken.

Is it possible to use the ISO121x in a non-isolated configuration, leveraging only its sensing functionality while ignoring the isolation feature?

Alternatively, could you recommend a more EMI-robust input circuit design that doesn't rely on isolation?

Thanks in advance.

  • 0
    Guru 316510 points

    It is possible to connect the grounds, but then using an isolator would no longer make sense.

    To reduce high-frequency noise, add a low-pass filter. To make the signal more robust, increase the pull-up current.

    What are the input voltages? What is the highest signal frequency and the lowest noise frequency? Do you need overvoltage/ESD protection?

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Actually, my circuit is as shown in the attached continuity diagram. This is a basic continuity check circuit. I can improve EMI immunity by adding a filter or decreasing the pull-up resistor value to increase the pull-up current.

    However, circuits like this tend to look quite primitive in scenarios where robustness is critical. I'm planning to use a more reliable circuit for this purpose, but I’m still trying to find the most optimal solution.

  • 0
    TI__Mastermind 28625 points

    Hello Emirhan,

    In addition to Clemmens's question

    The decision to use ISO121x will depend on what the input is and application. If the input signal is a 24V input, then ISO121x makes the most sense. However, if it is a logic signal, then ISO7720 or an isolated transceiver may be more applicable. What level and standard of EMI immunity is being tested?

    To answer the question directly: 

    Emirhan Köse said:
    Is it possible to use the ISO121x in a non-isolated configuration, leveraging only its sensing functionality while ignoring the isolation feature?

    Yes, you can use an isolated device to leverage the functionality and ignore the isolation feature. The ISO1211 is designed to work with industrial 24V digital inputs which would increase the current. The ISO121x is intended to work in noisy industrial environments.

    ISO121x also does not require a field-side power supply. The application would benefit from maintain the isolation by placing the input switch on its own refence. 

    Best,
    Andrew