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Helpful ReplyHot!Definite maximum source impedance for ADC module in PIC18F45K22

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sxanthony06
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2019/07/09 16:14:33 (permalink)
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Definite maximum source impedance for ADC module in PIC18F45K22

I have come across two varying values for the maximum impedance for analog sources connected to the PIC18F45K22. I have downloaded the datasheet for this PIC in particular directly from Microchip's website, so I know it's trustworthy. On page 299, chapter 17.4 it states: 
The maximum recommended impedance for analog
sources is 3 kΩ.

In the acquisition time example in Equation 17.1, a 10kΩ impedance is used, in the next chapter about Comparator, 10kΩ is the maximum source impedance and there is also a webpage on microchip's website that states that for 8-bit and 10-bit ADCs the maximum source impedance is 10kΩ[1].

So Microchip, what is it then?
 
 
[1]: https://microchipdevelope...c:adc-acquisition-time
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    mbrowning
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    Re: Definite maximum source impedance for ADC module in PIC18F45K22 2019/07/10 08:43:27 (permalink) ☄ Helpfulby sxanthony06 2019/07/10 11:07:49
    +2 (2)
    directly from Microchip's website, so I know it's trustworthy.

    Thanks, I needed a good laugh this morning.
     
    The Electrical Specifications section has actual contractual specifications. There you'll find parameter number A30 which says 3Kohm max. This is a real fuzzy number however. You can easily exceed it by adding some external capacitance to lower the AC impedance, which is what the ADC actually cares about. I typically use 1-10nF with voltage dividers of several Kohms.
    Of course this filtering slows the ADC response but that's fine for many applications, so your mileage may vary.
     
     

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    sxanthony06
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    Re: Definite maximum source impedance for ADC module in PIC18F45K22 2019/07/10 11:13:17 (permalink)
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    Hahaha, I get what you took as a joke but I think you misunderstood me. What I meant was that since I got the datasheet in pdf form from Microchip's website, I know it's legitimate. Maybe not the latest version, but at least legitimate.
     
    Could you elaborate on what a lower AC impedance means for the ADC? How does it affect it and also could you explain the purpose of the voltage divider? I can't get a mental picture of how it's supposed to be connected to the ADC input.
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    mbrowning
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    Re: Definite maximum source impedance for ADC module in PIC18F45K22 2019/07/10 11:42:20 (permalink)
    +1 (1)
    When the adc initiates a sample it switches in a small capacitor. High source impedance means longer capacitor charging, longer acq time required. A cap at the adc input stores charge from the source and transfers charge to the sampling cap. Less acq time needed, but longer RC time constant on the external circuit.

    A voltage divider is needed to measure voltages outside the range of the ADC like measuring 15V supply with a 3.3V ADC. A simple resistive divider is usually fine but low value resistors waste power but high values require longer acq time and can cause other ADC measurement issues. A cap will allow larger values by making the voltage divider impedance seem smaller to the ADC. If you need really higher resister values then it’s best to use an opamp to buffer the divided voltage

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