Background: I'm working on a project that involves tapping into the load cells of a scale and reading the differential output of the Wheatstone bridge. I need to keep all of the original functionality of the scale intact. The scales internal circuitry is running at 5v and it's powering the load cells at 5v as well, with either node of the bridge outputting 2.5v.
The setup so far: I have an Arduino Mega communicating with a 16bit ADC over the I2C SDA and SCL pins that's taking a single_ended reading from the output of a TI INA125P (Inst. Amp) that is amplifying the load cell's bridge output. When I disconnect the load cell from the scale and I supply power to it via the Arduino's 5v pin I get perfect, almost full-scale readings (awesome resolution with little noise to boot).
The issue: When I reconnect the load cell to the scale and I let the scales 5v circuit power the Arduino and my small amplifying circuit (via the Vin pin on the Mega) the scale's digital display outputs perfect weight readings, but all my Arduino reads is bogus values with no meaning.
At first I thought it might be an issue with conflicting supply voltages 5v from my computer and 5v from the scales circuitry, but from what I understand the Arduino's on board power regulator will automatically switch to the Vin supply voltage and disregard the usb's supply voltage.
Does anyone have any thoughts on what might be going on here? I'm not all that experienced with electronics so hopefully I'm overlooking something pretty simple!