analogRead() adding noise to signal?

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using: atmega2560 microcontroller arduino bootloader

i have pressure sensor connected 2 separate voltage buffers, 1 voltage buffer fed arduino, 2nd goes on additional filtering (both signals read directly after buffer)

the signal going arduino seems have consistent period/amplitude , doesn't seem random noise, adding noise, while 2nd buffer continues additional filtering not have noise. added picture oscilloscope below depicting noise that's being produced signal being read arduino (the signal supposed centered @ .2)

i guess question is, there reason arduino's analog input load adding noise signal passes arduino

edit: tested 1 of our unused analog pins empty , read empty signal see if noise added well, , wasn't... also, in system shares common ground plane

quote
i guess question is, there reason arduino's analog input load adding noise signal passes arduino

i doubt it.
what scope connected to?
at guess signal being generated op amp , being filtered out later.
however @ 1mhz can't signal through a/d because can't sample fast.


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