ADC input impedance?

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ADC input impedance?

By cerebrate
- Sat Jan 21, 2017 5:16 pm #61322 Quick question, if I may - is the input impedance, or maximum input current, of the ESP8266-12 ADC pin documented anywhere? I haven't been able to find it in any of the spec sheets I've dug up.

Thanks in advance,

-c
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Re: ADC input impedance?

By DonTone
- Fri Jan 27, 2017 7:38 pm #61642 The adc pin reports from 0-1,0 Volt. Your typical sensor will return 3.3v, so remember to add some resistors to divide the voltage.

/tony
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Re: ADC input impedance?

By g6ejd
- Mon Jan 30, 2017 5:04 am #61759 The espressif data sheet does not give a specification but it says all inputs have a leakage current to maintain an input state of 50nA and you should expect input impedance of any ADC to be high. using 50nA and 1volt maximum input allowed yields 20Mohm which sounds about right to me.
However which ESP8266 implementation are you using because many boards like NodeMCU or WemoS D1 have input resistor diver chains to allow 0-5 inputs and effectively divide that input range down to 0-1volt for the ESP, in which case the input impedance of these boards would be much lower. The Wemos D1 mini for example has a 220K and 100K series divider giving you an input impedance of < 100Kohm.
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Re: ADC input impedance?

By cerebrate
- Mon Jan 30, 2017 1:46 pm #61789
g6ejd wrote:The espressif data sheet does not give a specification but it says all inputs have a leakage current to maintain an input state of 50nA and you should expect input impedance of any ADC to be high. using 50nA and 1volt maximum input allowed yields 20Mohm which sounds about right to me.
However which ESP8266 implementation are you using because many boards like NodeMCU or WemoS D1 have input resistor diver chains to allow 0-5 inputs and effectively divide that input range down to 0-1volt for the ESP, in which case the input impedance of these boards would be much lower. The Wemos D1 mini for example has a 220K and 100K series divider giving you an input impedance of < 100Kohm.


Thanks, much appreciated!

I'm using the Adafruit Feather HUZZAH, which claims a ~1.0V maximum voltage for the analog pin, so that would suggest the 20 Mohm figure, I suppose.

-c
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