Arduino uno Interfacing with IC problems

Everytime I try to read an ADC output voltage from an IC, my Arduino seems to start outputing 5V instead of reading the voltage values. If I hook IC output to a multimeter I can see the right value, but as soon as I connect it to Arduino pin it starts reading a value around 4-5V.
I did connect the ground of arduino to the ground of the powersupply. Even tried to use pinMode command but nothing works. PLease help as soon as anyone can.
I am trying to read output from LM35, ACS 712 current sensor, MAX4173 current sensor.
Thanks

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