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ADuM1401 input

We’ve been doing some tests on above, and get considerable oscillation on output when i/p risetime is 100us or so. Please see attached screenshot: yellow trace is i/p signal, 4.9v p-p, risetime of 100us (ignore Y axis scale for signal – x10 probe attached). Blue trace is o/p signal – as you can see, any amount of noise on i/p is giving rise to multiple o/p transients.

Is there any form of hysteresis on the input? The datasheet implies it’s Schmitt but it doesn’t act like it. Is there a pin for pin compatible device which does have Schmitt inputs?

  • Hi,

    ADuM1401 is standard CMOS input logic, it have hysteresis at the input. If the input signal rising time or falling time is not so slow, you will find the different judge voltage level between rising and falling. But in your case, the time last so long during the undefined input voltage level. This is like you put one middle voltage at the input. So the output is indeterminacy.



  • Hi,

    I am sorry I think I made misunderstanding for this issue at last reply. Please ignore it.

    Yes, the issue is because of the nosie of your input signal. What's the amplitude of the input signal noise? about 80mV?

    The input hysteresis of ADuM1401 is not so large. I tested one chip in my lab is about 70mV.

    Right now, we don't have large hysteresis parts. How large of the hysteresis do you think is enough?



  • Hi Jian,

    We don’t know how much noise. The signal is low freq TTL signal sent over long line so we have CR filter at input to reduce any noise picked up.

    We assumed standard TTL schmidt type hysteresis 300mV min (ADuM1401 data sheet does not spec it).

    However have found replacement Silicon Labs device Si8640 series with min 380mV hysteresis which works fine!

      Tony Haywood

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