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Hot!PIC15313 ADC Input Impedance

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anandkulkarni88@gmail.com
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2020/02/20 08:56:18 (permalink)
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PIC15313 ADC Input Impedance

I am using ADC module in PIC15313 to detect voltage levels... With respect to Datasheet...FIGURE 20-4: ANALOG INPUT MODEL... Rs is in series to Analog pin... Does it mean Series resistance to input pin should not > 10K.... In my design, i have Potential divider 13.7K to 5V, and 13.7K to GND, and 10K in series. Should 13.7K be less than or equal to 10K also?

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    ric
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    Re: PIC15313 ADC Input Impedance 2020/02/20 12:26:39 (permalink)
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    anandkulkarni88@gmail.com
    In my design, i have Potential divider 13.7K to 5V, and 13.7K to GND, and 10K in series. Should 13.7K be less than or equal to 10K also?

    That circuit has an effective impedance of [1/(1/13.7+1/13.7)]+10 = 16.85k which is more than 10k.
    You can lower the effective impedance by putting a low ESR capacitor between the ADC input and ground. Say 100nF.
    Why do you have the 10k in series with the pin at all?
     

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    anandkulkarni88@gmail.com
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    Re: PIC15313 ADC Input Impedance 2020/02/21 10:23:47 (permalink)
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    Thank you.... 10K was in old design for dedicated Comparator IC, i just carried over... and programmed to PIC ADC..... So if  i just add 0.1uF cap ADC to GNd, without any other modification, how cap ESR will help see reduced impedance to ADC... 16.85K parrel to ESR value (0.01 approx)...?
    post edited by anandkulkarni88@gmail.com - 2020/02/21 11:32:06
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    ric
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    Re: PIC15313 ADC Input Impedance 2020/02/21 13:37:24 (permalink)
    +1 (1)
    Remember that "impedance" is an AC property, it's "resistance" can vary with frequency.
    If only resistors are involved, then impedance = resistance at all frequencies.
    As soon as you add an active element (like a capacitor), the impedance now has an active component, and the resistamce component varies with frequency.
    You can regard the ADC sampling cycle as one half of a waveform to estimate the equivalent frequency.
    Have you studied circuit theory? Are you familiar with these concepts?
     
    post edited by ric - 2020/02/21 13:39:47

    I also post at: PicForum
    Links to useful PIC information: http://picforum.ric323.co...opic.php?f=59&t=15
    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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