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A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING

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irmak
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2019/07/12 13:42:13 (permalink)
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A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING

 
A 10K resistor is sufficient for pull up / pull down connetion of digital input channels.
 
For A/D converter input channel small pull down resistors can cause sensor overloading and it can damage PIC MCU.
 
There are different suggestions in forums 4.7K-1M.
 
http://www.resistorguide....or_pull-down-resistor/
https://electronics.stack...d-a-floating-adc-input
 
 
My personal experience on loadcell and analog encoders 10k is not sufficient.
 
How to calculate exact pull down resistor value to overcome sensor overloading ?
 

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    ric
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    Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/12 14:33:33 (permalink)
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    irmak
    For A/D converter input channel small pull down resistors can cause sensor overloading

    That's totally dependant upon the sensor.
    If its output impedance is much more than 10k, then you need some external buffering.
     

    and it can damage PIC MCU.

    Who told you this rubbish?
     

    My personal experience on loadcell and analog encoders 10k is not sufficient.
    How to calculate exact pull down resistor value to overcome sensor overloading ?

    Explain more about what sensor you want to use that requires a larger resistor?
    100k should be enough, but again, if the sensor is that weak, then you're going to need a capacitor etc. to lower the AC impedance to allow the ADC to work anyway.
     

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    irmak
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    Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/12 15:07:29 (permalink)
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    e.g Loadcell input and output terminal resistance are 350R, excitation voltage 5V, 0.002 mV/V
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    NKurzman
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    Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/12 16:00:41 (permalink)
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    irmak
     
    e.g Loadcell input and output terminal resistance are 350R, excitation voltage 5V, 0.002 mV/V


    1.You are probably going to need a diff Amp for a load cell.
    2. You do not need a pull-up or pull down if the input is permanently connected.
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    irmak
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    Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/12 22:49:04 (permalink)
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    Loadcell is connected to an instrumentational amplifier. reference voltage of the instrumentational amplifer is sufficient to put output in 0-5V but when load cell is not connected to amplifier the output goes to Vss of the amplifer and this is harmfull for A/D of PIC. When I have used 10K pull down, output of amplifier stay in postive reference value but loadcell do not work properly.
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    ric
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    Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/12 22:54:49 (permalink)
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    irmak
    Loadcell is connected to an instrumentational amplifier. reference voltage of the instrumentational amplifer is sufficient to put output in 0-5V but when load cell is not connected to amplifier the output goes to Vss of the amplifer

    ... and you didn't think to mention these crucial details in your first post?
     
    Please show us your circuit. It is a strange amplifier if just a 10k load on its output makes such a difference.
    If just 10k can keep it below 5V, then it's incapable of supplying more than 0.5mA, in which case there is no way it's going to damage the PIC input.
     
     
    post edited by ric - 2019/07/12 22:56:44

    I also post at: PicForum
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    NEW USERS: Posting images, links and code - workaround for restrictions.
    To get a useful answer, always state which PIC you are using!
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    irmak
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    Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/12 23:10:57 (permalink)
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    I have used LT1168.
     
    I supply +9V, -9v, 2.5V reference.
     
    In spice simulation and when loadcell is connected everything work fine. But when loadcell is not connected there is excessive negative voltage. I do not have osciloscope and I am not sure if this is a saturation or another kind of problem
     
     
    I have attached the circuit in my first post
    post edited by irmak - 2019/07/12 23:13:00
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