This is typical for ADC's, and you should never use one and expect a reading which doesn't flutter. If you do, you're naive.

So how to get rid of this? Most people will tell you you should apply averaging or such. Essentially this will act like a low-pass filter. But the problem is that the flutter is essentially noise, and contains very low frequency components. So averaging will get rid of quick fluctuations, but it will still give you slow ones.

I once had a chat with a manufacturer of digital mixing consoles, and we talked about how to read the faders, and how to get rid of ADC flutter there. Surprisingly, he had devised a mechanism which was identical to a mechanism I had also developed and applied in some device of my own. To give an analogy, this mechanism imitates the slack you have on an analog potentiometer. Turning the shaft a little will not move the wiper. Only if you turn past the slack, the wiper will follow. Remember that this algorithm works miracles when it comes to clean up flutter from reading potentiometers and such, but it should never be used as a noise suppression algorithm on signals.

; if |current-previous| > slack
; 	if (current-previous) > 0
; 		previous = current - slack
; 	else
; 		previous = current + slack

			bseg

dir:			dbit	1

			dseg

current:			db	1
previous:			db	1

			cseg

			mov	a,current		; current should be a buffered reading
			clr	c
			subb	a,previous		; current - previous = change
			mov	dir,c			; dir set if change was down
			jnc	absignend		; calculate absolute value of change
			cpl	a
			inc	a
absignend:		cjne	a,#slack,tresadc	; Compare change to slack
tresadc:		jc	endadc			; if not changed more than slack, ignore
			mov	a,current		; remember current value
			jb	dir,addtresh		; If change is up, compensate down
			clr	c
			subb	a,#slack
			sjmp	storeprevious
addtresh:		add	a,#slack		; If change is up, compensate up
storeprevious:		mov	previous,a		; Store calculated value

endadc:							; Your new value now in a.