[TIP] Followings are measuring the weight using load cell sensor with AVR. It is usually said that accuracy of load cell sensor is approx. 0.01%.  Please see user manual or datasheet for load cell because exact accuracy is different according to each load cell.

First, apply 10V (recommended voltage) between input (+) and input (-), approx. 0~20mV will be generated in output (+) and output (-). For ground wire, please apply shield for earth, panel, chassis and cable. If possible, we recommend that you should not connect ground wire to input (-) at all.  That’s why it is likely to be affected by noise in real outer environment. For the principle, please look into calculation method of Wheatstone bridge. It must be G=0 but, the answer will not be 0 and minimal voltage is generated by distortion, we measure the extent.

Amplify 0~20mA which is the output using OP-AMP and make it 0~5V. Amplifier is available at retail shop and companies in the attached datasheet sell amplifier and company of UNIPULSE( http://www.unipulse.com ) in Japan is also famous for amplifier. But, you can obtain your desired voltage without mounting amplifier outside. If you see in AVR, there is a channel that can get ADC using amplifier with 200 times, from which you can get input from. For precise work, you can manufacture an amplifier by yourself. You may put the amplified voltage into ADC in AVR.

The available range of sensor that we recommended is 0~150kg, which has 0~20mA in linearity. You need to calibrate again whenever you replace sensor because each sensor has a slightly different value.  

And for calibration, 5mV will appear if weighing the object of 50kg, for example provided that you gained 100mV weighing the object of 100kg. You can use the sensor as it is after you see the extent of the voltage in linearity. And if you place top panel on the sensor, you need to calculate setting zero included the top panel.

We hope above will be helpful to you. Thank you.