ANother sensor issue.

I'm working with arduino nano and taking pressure sensor readings from A0 ( I used a differential amplifier using BC547 transistor circuit and it responds to the changes in pressure while checking with a DMM ). But, the values in serial monitor always fluctuates very much. I used internal reference (1.1V). But, when I checked the reference pin using DMM, the voltage is just below 1.1 V. How to solve this problem.

I have another question : If the problem is with the power using USB jack. Can I use an external DC supply feeding to VIN pin of my nano board while there is a USB connection with computer?

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