voltage divider ground problem

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hello all!

i have 12v power source, powers arduino mosfet controls 12v computer fan.
i wanted measure voltage after mosfet in relation pwm signal.
i created voltage divider on mosftet output using 100k (+ side) , 47k resistors (- side) of output runs analog pin on arduino.
when output of divider not connected arduino, expect, 3.8v when output of mosfet 12v.
however, when connected arduino analog pin, voltage soars mosfet output voltage goes down. think i'm lucky haven't fried it.
i not understanding how ground working through arduino.

can lend hand?
thanks time!
-andrew

is -ve side connected arduino's ground?


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