Arduino Mega 2560 Analog input data changes by device?

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hello

i taking sensor data analog input. simple voltage data.

so data constant 90 bit +-5 bit (441mv +- 20mv).

i using desktop measurement.

however, tried take sensor data laptop same setup , same code.

what did plug out usb desktop , plug in laptop.

than, data not consistence @ all, voltage fluctuate 100~200mv.

i used ina129 instrumentation amplifier voltage output.

ina working fine have no idea why signal fluctuate

when connect arduino laptop.

does analog input affected power supply??

i wondering laptop power not enough, powered arduino 2.1mm adapter

but result same.

does know problem?

thanks help.

it's reference. read reference of analogreference: http://arduino.cc/en/reference/analogreference

normally 5v default reference analogread(). if 5v power changes 4.8v, analogread return higher value. can use power supply of 7.5v or more dc barrel jack. should result in same analog values.

when internal 1.1v or 2.56v reference selected, should tuned every atmega2560 chip, since value never 1.100 or 2.560v.

there of course more it. ground currents, or mains noise, , on.


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