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CD4071B: Slow CMOS input problem

Marshall Folkman
Intellectual 720 points
Part Number: CD4071B
Other Parts Discussed in Thread: CD40106B

I have made the mistake of doing what Ethan proposed in his e2e post:     <https://e2e.ti.com/support/logic/f/151/t/536693>   

Which is to filter an input signal line of an CMOS OR gate using an RC filter. After reading SCBA004 <www.ti.com/.../scba004d.pdf> I am now understanding why this is a bad idea.

But, I am having a hard time solving the problem for my application. My condensed circuit is shown here:

It seems that I should be able to insert a schmitt trigger between the RC filter and pin 1 of CD4071B, but being that I have never used a schmitt trigger before I was hoping to confirm with an expert if this is a good idea or not. If this is a good solution, how should I pick my schmitt trigger? I am unable to find one online that can support my voltage requirements. The best I have found (I think) is one by TI that can support up to 5.5V or so (SN74LVC1G17DBVR). but this means that I would have to add a voltage divider to bring the voltage down, which I don't want to have to do because I am trying to keep my parts count at a minimum (if possible).

There must be a simple solution to this that I am not considering, as I am sure that it is often done in industrial applications. 

Also, I know what your probably thinking. Why not just spare an extra processor pin and OR them in software? I can't do this because part of this circuit is embedded into a cable exterior of the electronic control box and I am limited on how many wires I can use.

Thank you!

Marshall

  • 0
    TI__Guru* 97046 points
    Hi Marshall,

    May I ask why you are doing logic at such high voltages? It's rather uncommon to see.

    Also, are you really leaving all the inputs to the unused OR gates floating, or is that just a mistake in the schematic? Those need to be terminated!

    Here's a part that operates at the correct voltage and contains 6 Schmitt-trigger inverters: CD40106B (just put 2 in series for a buffer).
  • 0 in reply to Emrys Maier
    Intellectual 720 points
    Emrys,

    Thank you for your help. I have just purchased CD40106B and will give that a try.

    To answer your questions:

    1. Like I said I am embedding most of this circuit into a cable exterior of the control box because I have run out of wires in my cable and my company hates changing cable connector configurations. In other words, I am trying to condense 2 inputs into just one input to save a wire in my cable.

    2. No, I am not leaving the other OR gates floating. I left that out to not distract from the overall point of the question.

    Thanks again!

    Marshall