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LM111: Frequency Limitation and Operation with ±800 mV Noise Signal Inputs (WAV File Test)

Anshuman Banik
Prodigy 20 points
Part Number: LM111


Tool/software:

Hi everyone,

I’m working with the LM111 comparator and testing it using two noise signals generated from .wav files. Both inputs swing roughly within ±800 mV and are connected to the inverting and non-inverting inputs of the LM111.

In the attached screenshot:

  • The top waveform (yellow) is the non-inverting input

  • The middle waveform (green) is the inverting input

  • The bottom waveform (red) is the LM111 output

What’s confusing me is the output — it’s showing a lot of fast transitions even when the inputs look fairly steady or only slightly crossing. I'm wondering if this is due to the nature of the noise signals, or if it's hitting some sort of frequency or response limitation of the comparator.

A few questions I have:

  • Is this kind of response normal for the LM111 with fast-changing or noisy analog inputs?

  • Could this be related to the lack of hysteresis, or maybe the input offset or slew rate limits?

  • Would adding external hysteresis or filtering help stabilize the output in this kind of use case?

Would really appreciate any guidance or suggestions. Let me know if sharing the .wav files would help.

Thanks!

  • +1
    TI__Guru**** 165566 points

    Ashuman,

    Lack of hysteresis seems to be the problem. The wav data could help.

    What is the desired output, [low, high, either if stable]

  • +1 in reply to Ron Michallick
    TI__Mastermind 26070 points

    Just to add to Ron’s comments, if you have the ability to show the difference between noninv and inv input, that would make it easier to analyze. But  if we assume the calm areas of the inputs being the same potential, it makes sense that the output would chatter as having a comparator’s inputs at the same potential is an undefined state (offset and noise will control the output).
    As Ron suggested, adding external hysteresis would solve that problem if my assumption is correct. 
    Chuck

  • 0 in reply to Ron Michallick
    Prodigy 20 points

    Hi Ron,
    I was testing the LM-111 comparator under various conditions as part of my attempt to implement a low-voltage current comparator using minimal components. However, I initially encountered significant chattering issues due to the absence of hysteresis. To address this, I added an external component to control the amount of hysteresis, and it seems to be functioning well now. Thanks again for your feedback!

  • 0 in reply to Chuck Sins
    Prodigy 20 points

    Hi Chuck,
    What you said was correct — a significant part of the waveform had low-level oscillations around the common mode, which resulted in noticeable chattering. To analyze it better, I simulated the transients across different time intervals. I’ve also added an external hysteresis component, which seems to have resolved the issue. Thanks again for your feedback!