ADC not giving 0 value when grounded

[Anik96 1]

When using analog read function, ADC never gives 0 value even when it is directly connected to the ground. But when I connect it to 3.3V it gives constant 4095 value. What is the problem? 

[Rei Vilo 695]

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[Anik96 1]

I am using TM4C123GXL launchpad ....MCU TM4C123GH6PM.... when I am trying to read ADC value using analogRead() it is fluctuating very much....even if I directly connect it to the GND pin it is giving random value between 0 to 50( 50*0.8mV = 40mV). 

[dubnet 238]

Sounds like noise.  How is the ADC input physically connected to ground? If it is a long jumper wire you may want to shorten it as much as possible. A long jumper wire will act as an antenna, picking up noise from the digital clocks on your board. Another thing you can try is placing a small capacitor right at the ADC pin to ground.

[Anik96 1]

9 hours ago, dubnet said:

Sounds like noise.  How is the ADC input physically connected to ground? If it is a long jumper wire you may want to shorten it as much as possible. A long jumper wire will act as an antenna, picking up noise from the digital clocks on your board. Another thing you can try is placing a small capacitor right at the ADC pin to ground.

May be it is noise but not external.... cuz if i use internal Pulldown resistor , I am getting the same result. But If I connect it to 3.3V ....I am getting almost constant 4095 value. I have also tried using hardware avg and dither enable. When using hardware avg it is taking avg over the noise and thus giving a value between 10 to 15. But I need 0 output when connected to GND.