The datasheet of HX711 is found here:
https://www.digikey.com/htmldatasheets/production/1836471/0/0/1/hx711.html
Furthermore, I have looked at some of the cheaper load cells with an aluminum bar as you see here, and I found similar data on all of them:
https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/TAL220M4M5Update.pdf
The HX711 integrated circuit is mostly sold mounted on a small PCB together with cells. When load cells are described in this forum the HX711 is used at least half the times. I have no practical experience with them, but when I look at the data sheet, I am not that impressed. I think that it normally be only a small part of the AD-converters dynamic range that is utilized, and therefore I expect more noise on the measured values is introduced compared to better alternatives. Am I right?
This is a theoretical approach from reading – I know.
The AD-converter in the HX711 does have a huge dynamic range of 24 bits, but I guess that a significant amount of noise will be there on the less significant bits including some other errors. 24 bits AD-conversion are normally very difficult to achieve, and this is a low-cost integrated circuit. Therefore, I guess that the results are far from that accurate.
Assume that you have got an excitation voltage to the wedstone bridge of 3 V, and the same voltage is used for AVDD of HX711. Then the expected rated full scale output range is +/- 3 mV for both positive and negative forces on the load cell. The internal amplifier can be set to a maximum amplification of 128, and therefore the input to the AD converter is +/- 384 mV. Consequently, you use only 100 * 2 * 0.384/3 = 25,6 % of the AD converters dynamic range. When you use the load cell with only zero and positive loads, then you only utilize half of that eg. 12,8% of its range.
The load cell will typically be damaged if you apply above 150% of the rated load for its full-scale output. In most applications you can expect significantly higher short-term dynamic loads due to blows or some perhaps unexpected things happening.
As an example, I got an application to measure the pressure applied to a pedal to be used for speed control of a sewing machine. I think you might expect a down force from a foot up to 30 kg (300 N) in some few lifetime cases. But in normal use the max speed would be, when you press 4 kg or 40 N. Consequently, I like to choose a load cell specified to 20 kg. But then only 10 % of the full-scale range of the load cell will be utilized in this application. And it will then be only about 2.5 % of this AD-converters range.
With the HX711, you can choose to measure the input every 12.5 ms or every 100 ms. The datasheet does indicate that you will see more noise when you choose 12.5 ms. I know that you will need a fast response for the pedal, and 100 ms will not be acceptable. In this application you cannot apply digital filters to reduce noise.
As an alternative for this application, I should design a circuit with an op-amp with an amplification of about 2500 with manual zero-point-adjust. The output can then be measured by a normal analog input of a microcontroller.
Have you been able to achieve fast and accurate measurements with load cells using the HX711? How accurate is this AD-converter? (I do not find data on accuracy in the datasheet).