I'm planning (or dreaming) to make 100g/1mg repeatability scale using Arduino and HX711. I ordered few modules and load cells from ebay, and started researching stuff.
Reading datasheets I noticed that HX711 has 20mV full range ACD with 128 times gain, that means, ideally load cell should produce 20mV difference at full scale.
However, it seems that typical load cells only produce between 0.6 - 1.0mV/V. Given that HX711 modules tend to have 4.3V excitation voltage, this only gives 4.3mV max reading from the load cell, which is less than 1/4 of the HX711 range. This is not ideal, because essentially ADC range is underused and you instantly lose 2 bits of resolution, and realistically even more as least significant bits are noisy.
Load cells supports higher excitation voltage, typically 10V or more.
So I would like to use higher external voltage for my load cell while still utilizing the HX711. Is it possible to do so?
Bad idea I fancy, since the ADC in the HX711 is ratiometric.
I have found that somebody made nano amp meter with hx711, it does not share ground with the test circuit, so I guess this would work.
A higher excitation voltage may violate the common mode input voltage limit.
I'm afraid both are right.
A measuring ratio of 1:20,000 could be a challenge.
Specialized chips, shielding, and noise-free power.
I'm not sure what common mode voltage is. definitions online seems to be not explaining much.
I was thinking of doing it like in the schematic below
However looking at application notes for scales, nobody is doing that, so I don't have high hopes
By 1:20 000 ratio I assume you mean achieving accurate 20000 count scale. Again reading appnotes seems to indicate that to be hard, but then HX711 is not a bad chip hopefully.
And I found this paper paper where they achieve 0.0163g accuracy on 5kg cell, which would be equal to 300,000 count scale after post processing. No shielding or fancy pcb layouts, just hastily soldered together.
Although, they cannot achieve similar counts with smaller load cells.
The common mode voltage, the average on the two inputs, will be 5 volts. NG.
See the HX711 datasheet.
Voltage of both inputs of the HX711 can be a maximum of AVDD-1.3,
so with the 4.3volt regulated AVDD of the analogue part of the HX711, that would be 3volt max.
Your diagram, with a 10volt loadcell supply outputs 10/2= 5volt.
With un-linked HX711 supply and load cell supply, you also will have stability issues.
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