RBE 2002 Lab 2 appendix

Differential Amplifiers

A differential amplifier amplifies the difference between two input voltages by a gain factor. This can be modeled by the equation

Strain Gauges

When putting together the strain gauge circuit, remember to reference the AD620 datasheet provided to you to figure out the correct placements of the wires. Also remember to set the -Vs at the ground of the circuit.

Sparkfun load amplifier

The Sparkfun load amplifier, has a 24 bit register unlike the Arduino which only has a 10 bit register. This means that the Sparkfun board is much more accurate than the Arduino by several factors of 10. Using the Serial example under the HX711 library, you should be able to see the difference that a higher resolution board makes, and compare it to our on AD620 circuit.

The calibration constant is what makes the load cell amplifier output the correct numbers that we are looking for. In our case, this calibration constant is 88140. This was found through utilizing the calibration code found on the Load Cell Amplifier page. By first zeroing the strain gauge, and then putting on a known weight of 1kg, it was possible to simply increment the calibration constant until the strain gauge output the correct reading of 1kg.

When calculating the output in terms of mV to grams, much of the information needed can be found in the load cell amplifier datasheet. Use unit conversion to find the solution.

Spring Scales

It is important to note that due to the spring scales that we use in the lab, you will be getting very inconsistent results. This is due to the increased friction on the potentiometer from the plastic casing. As such, this will give you variable results based on how you put the weight on the spring scale.