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    What is a roughly realistic ambient temperature speed of fluctuation?

    Hi,

    To implement an RC low pass filter at a (window) comparator input that is buffered by a voltage follower op amp, the window comparator purpose is to control an LDO enable pin and only take it high between 0C and +45C, what might be a realistic RC LPF value to avoid comparator chatter if hysteresis isn't used?

    Would ~3.3 Hz (0.29s) be okay/suitable to filter out fluctuations around the trigger points? Let's say the temperature sensor is in still air.

    The simulation showed it to be effective but maybe ambient temperature fluctuates faster or slower than my guess.

    Attached is the schematic, the upper buffer, difference amplifier and comparator are sort of not important to this question as it's the lower buffer and window comparator the transient results were taken from.

    FWIW, the transient shows fast fluctuation at first where the LPF seems effective at filtering out spurious, momentary rise and fall around the upper trigger point and then a longer rise over +45C where the window comparator triggers as required.

    Also, can I feed one LPF into both buffers or is it wiser to use one for each?

    Thanks.

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    Re: What is a roughly realistic ambient temperature speed of fluctuation?

    Hi,

    No hysteresis: Then I'd say stability is problematic and mainly depends on PCB layout and schematic...

    I'd use hysteresis to get reliable and calculable function.

    Klaus
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    Re: What is a roughly realistic ambient temperature speed of fluctuation?

    The measured temperature fluctuation speed depends from many factors, including the sensor inertia. Furthermore I don't know were you sensor is placed indoor, in a box, outdoor... it could be, for instance, cooled by wind or by a fan so that the speed of the temperature variation is faster as in other conditions. As already said by Klaus using hysteresis should be a preferable choice.


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    Re: What is a roughly realistic ambient temperature speed of fluctuation?

    Hi Klaus,

    Quote Originally Posted by KlausST View Post
    Hi,

    No hysteresis: Then I'd say stability is problematic and mainly depends on PCB layout and schematic...

    I'd use hysteresis to get reliable and calculable function.

    Klaus
    Okay, great thanks. I'll take your word for it, this thread is basically solved. One doubt about your answer: Where you say 'stability is problematic', are you referring to the filter output voltage changing with the speed of fluctuation and possibly falling below a valid comparator input threshold or to pcb layout or to something else?

    Regards.

    - - - Updated - - -

    Hi,

    Quote Originally Posted by albbg View Post
    The measured temperature fluctuation speed depends from many factors, including the sensor inertia. Furthermore I don't know were you sensor is placed indoor, in a box, outdoor... it could be, for instance, cooled by wind or by a fan so that the speed of the temperature variation is faster as in other conditions. As already said by Klaus using hysteresis should be a preferable choice.
    You have a point, from memory I think the LM335 datasheet mentions its 'inertia' - which I take as speed of response to change, I''ll re-read it yet again to have that information. The sensor would be in a plastic enclosure, i.e. still air. Hysteresis it will be then. I looked a little through resources and found nothing much against input signal filtering vs hysteresis (except for large frequency variations of fluctuation affecting filter output voltage), which is why in this case I thought it may be okay to use.

    Thanks and regards.



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    Re: What is a roughly realistic ambient temperature speed of fluctuation?

    Hi,

    One doubt about your answer:
    Without hysteresis..
    * supply voltage noise and fluctuations will be coupled to the comparator analog inputs
    * comparator output will be coupled back to the comparator inputs causing unknown either positive or negative HF feedback
    * analog input noise will be amplified and will cause high noise at the comparator output ... expect the digital value to randomly jump when the analog input signal is close to the switching level.

    If you use a microcontroller, then I recommend to use the ADC instead of the comparator. Then you are most flexible to adjust the algorithm to your needs without the need for a soldering iron...

    Klaus
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    Re: What is a roughly realistic ambient temperature speed of fluctuation?

    * When I grip a component (temp sensor, diode, transistor) between my fingers, it shows a change after one or two seconds. Then it takes several seconds to reach a stable reading. When I release the component, the reverse behavior happens although it takes a bit more time.

    * A sample-and-hold circuit might work as well as hysteresis. Hold a value for a while, then compare it to the live reading. A reasonable waiting time can be the point where the sample-and-hold starts to lose its reading.


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    Re: What is a roughly realistic ambient temperature speed of fluctuation?

    Hi Brad,

    Thanks for your observations about real-world component temperature behaviour. The sample-and-hold is an interesting take on implementing that kind of function, nice.



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