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Hot!18F25K42 ADC Input Impedance Changed

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rob_d
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2019/10/14 15:41:27 (permalink)
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18F25K42 ADC Input Impedance Changed

We have a product that uses an 18F25K42 and I noticed that a new, no longer preliminary, data has been released. The preliminary data sheet listed a maximum recommended ADC input impedance of 10K and the new datasheet lists a maximum of 1K. The 10K number is what I have seen for most newer 8-bit PIC's. I would feel a lot better if I knew why the spec was lowered? I would have expected to see an increase in pin leakage current as well but it is listed the same in both datasheets.
 
I see an Errata issue that effects silicon A1 where they recommend 1K max impedance as a work around but, since A2 fixes that issue,  I can't imagine the datasheet would have been changed for that.
 
Does Microchip ever answer in these forums or do I need to submit a support ticket?
 
Rob
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4 Replies Related Threads

    jack@kksound
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    Re: 18F25K42 ADC Input Impedance Changed 2019/10/15 09:12:10 (permalink)
    +1 (1)
    As this is a "user" forum I suggest a support ticket is the most direct way to ask Microchip.
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    Kiwi Mike AZ
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    Re: 18F25K42 ADC Input Impedance Changed 2019/10/15 09:57:59 (permalink)
    +2 (2)
    The 18FxxK42 Family has a 12Bit ADC, the older parts are (mostly) 10bit ADC's.
     
    When releasing a new part the "Preliminary" data sheet is generally typical values based on previous parts, in this case most likely a 10 bit ADC. It takes about a year to collect enough data to do final electrical specs on a part.
     
    Sounds like  the 12Bit ADC needs the lower impedance than a 10bit. Possibly having a longer acquisition time you could still use a 10K.
     
     
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    rob_d
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    Re: 18F25K42 ADC Input Impedance Changed 2019/10/23 16:39:45 (permalink)
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    Mike,
     
    I certainly understand that the preliminary data is subject to change.
     
    I would like to point out that what bothers me is the pin leakage specification did not change and it is what drives the minimum input impedance requirement. They acknowledge this is note 3 at the bottom of page 594 in the latest 18F25K42 datasheet, pasted in bold below.
     
    3: The maximum recommended impedance for analog sources is mentioned in Parameter AD08 in Table 44-13. This is required to meet the pin leakage specification
     
    It is important to note that charging longer will not help with the error caused by current leaking from within the PIC's pin circuity which is known as pin leakage. The leakage current imposes a voltage change on the PIC's input pin and a given current will impose a bigger change at higher source impedance values.
     
    Easiest way to envision this is to imagine measuring the voltage from a simple two resistor voltage divider. Ohms law says V = I * R.  The pin leakage current changes the amount of current (I) flowing through the resistors (R) and that causes a change in the voltage (V) across the resistors. That voltage change translates to ADC measurement error.
     
    Rob
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    acharnley
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    Re: 18F25K42 ADC Input Impedance Changed 2019/10/25 16:28:04 (permalink)
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    It won't help with the error per say but it will allow the ADC cap to gain a better charge (is my understand of it).

    1k is low. I'm currently designing a dual PIC16 + PIC32 board and have found the 2.5k of the PIC32 to be restrictive in some cases.
     
    #5
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