HX711 Load Cell application strange behaviour

Hello everyone,

I have an application where an HX711 ADC is used with 4 load cells. (in wheatstone bridge configuration, rated at 50kg each).

The thing is that I am trying to sense 30~ish gram changes with it accurately, while the cells will be loaded with 10 to 40 kilograms.

I have built up the circuitry and placements properly, calibrated and tested etc. but I have 2 questions that I couldn't come up with a solution.

1st question: When powering arduino from USB cable the readings are much more stable (they are fluctuating like +-5 grams only) but when I connect a 12V power source (with 5A output) to the arduino, the readings start to fluctuate much more. (+-30 grams or so). The HX711 is powered from arduino 5v out pin but even though i tried to power it from a 5v adapter the fluctuations seemed to happen.

I should also note that the HX711 is connected to the analog pins A0 and A1, and there are 8 other sensors connected to the pwm pins of the arduino.

Now any ideas why does that happen? Any way to prevent this fluctuation?

2nd question: If I calibrate with a load close to the max. weight (as suggested in many papers) the small changes seems to be very unreliable. If I calibrate it with a small weight (like around 200g) I can sense small weights properly but this time when a 2 kg weight is put it is read like 2.3kg or something. Is this normal?

I know that my load cells are not the proper ones for the application but I would like to push them as much as i can.