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ADC value when ADC pin is not connected?

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banh

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i'm testing the ADC module of the microcontroller i'm using. 10-bit adc, so that maximum value is 0x3FF.

if there is an input volatge the read value will be changed accordingly.

however, one thing is that if i left the adc input pin unconnected, the read value is 0x3FF.

is there any reason for this value? why is it not 0x0?
 

Typically circuits are pull-ups... so you will either read totally random stuff and if not the maximum seems more reasonable
 

I wouldn't blame internal pullups ..
Usually microcontrollers have just one A/D converter module that is shared by all ADC pins and on top of that there is the S-H circuit .. see picture below ..
So, voltage levels that are present on the other inputs - after reading them - will affect this unconnected input reading as well ..

Regards,
IanP
 

    banh

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Thanks IanP,

However, in my case,
throughout my whole program i select just that single ADC input, and never read from the other inputs. Hence, there should not be any effect from other inputs!
 

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