24-Bit Analog-to-Digital Converter for Weigh Scales

[Thommyy]

Can you recommend a reliable ADC for strain gauge force scales ?
The project is to weigh an elevator car. Weighing accuracy is within +/- 1kg, in an industrial environment.
So far I have discovered the type: HX711
What do you think of it?  Alternatively, do you have any other types?
Thank you.

[selcuk]

I can recommend this one. It has two ports but you may use one of them for better noise filtering.
https://www.digikey.com/en/products/detail/nuvoton-technology-corporation/NAU7802SGI/2769782

[moffy]

This is the search results from Digikey for 24 bit ADCs sorted by price, it could offer some choices: https://www.digikey.com.au/en/products/filter/data-acquisition/analog-to-digital-converters-adc/700?s=N4IgjCBcpgrAzFUBjKAzAhgGwM4FMAaEAeygG1xYwBOAJmpAF0iAHAFyhAGU2AnASwB2AcxABfIgFpaSEKkh8AroRLkQsJhJCSG0OVCUrSkChEZitANln8AJp0lgADBFYdIIEETYBPFnk4MHFQLIA

[Thommyy]

How would it be better than the HX711?
I don't need a high transfer rate.

[selcuk]

I've never used HX711 before. Since NAU7802 has an I2C interface, you may connect it the the same bus with other sensors.

[coppice]

Don't assume anything from the data sheet of a sigma delta ADC. There used to be many that looked OK at first sight, but had a variety of quirks when you got into the details. In weigh scales you need a lot of resolution, so these quirks can be really important. Performance drifting over time affected quite a few, especially some of the early Chinese ones. A DC offset that drifted a lot, or moved in fairly sharp steps for no obvious reason, was common. Over time things have improved a lot, but when you have one of these problems it can take ages to properly diagnose. I'd try to find what everyone else is using successfully in your application, and look carefully at that part. If its a part that has been through a number of revisions you may find early ones were quirky, and newly produced parts are great. It can be a minefield.

[Overspeed]

Can you recommend a reliable ADC for strain gauge force scales ?
The project is to weigh an elevator car. Weighing accuracy is within +/- 1kg, in an industrial environment.
So far I have discovered the type: HX711
What do you think of it?  Alternatively, do you have any other types?
Thank you.

Hello

For heavy machinery as a car elevator ADC is not the first concern you need to carefully design your load cells / amplifier and the signal transmission ( I suppose you will have several load cell ) as frequency is low a 4/20 mA system is a safe way to send the data in a quite noisy shop

Except if you work on race car with a strict weight limitation , usual car can handle a quite large accuracy

an exemple of 4/20 mA interface

https://www.ato.com/load-cell-transmitter-output-0-5v-4-20ma

Regards
OS

[wasedadoc]

Are you seriously stating you need a 24 bit bit ADC? 1 in 16 million. 1 kilogram in 16,000 tons.

[Overspeed]

Are you seriously stating you need a 24 bit bit ADC? 1 in 16 million. 1 kilogram in 16,000 tons.

Hello

Accuracy of a scale in no only the ADC that also the bridges /  load cells concern and mechanical structure too

Regards
OS

[zapta]

If you want a low cost scale using a low cost load cell,  the HX711 and the likes are just fine.

If you want better accuracy, you need to start with the load cell configuration and use an ADC that supports it.  For example

1. High quality load cell.
2. Using Kelvin configuration where each of the load cell feed terminals as an additional sensing wire.
3. Operating the load cell on its full range to increase signal and reduce S/N ratio  (per the cell spec, e.g. 10V).
4. Use a ratiometric measure of output_voltage/feed_voltage rather than absolute voltage measurements.

Last year I designed something similar (one-of, not cost sensitive) and used a TI ADS1261BIRHBT.  You can see the diagram in the attached PDF. That board also has thermistor inputs but you can ignore them.  The load cell runs on +/-5V and the ADC on +/-2.5V for the analog and 3.3V for the digital domain. The ADC is configured to measure the voltage ratio (AIN0 - AIN1) / (AIN2 - AIN3) and the LDOs where selected to be low noise.

The ADS1261BIRHBT can measure a few thousands samples a second, which you don't need, but I would recomand to perform a large number of sample and average them to reduce the noise.

Bottom line, it all depends on your budget and requirements.

[zapta]

Are you seriously stating you need a 24 bit bit ADC? 1 in 16 million. 1 kilogram in 16,000 tons.

The ADC error is typically larger than the ideal +/-0.5 count.

[exmadscientist]

Nice ADCs are a lot cheaper and more common than nice load cells. What is your maximum weight? 4000kg or so? Even with class C6 load cells you are going to struggle a bit to get 1kg, precisely, accurately,  and repeatably, out of 4000kg....

You may want to buy some nice commercial kit and see how it does. Be sure to watch the drift for a week or so ....

[Fox_Alex]

Take a look on the ADS1255. It has nice analog front-end and can work in differential mode to use it with a bridge-type sensors.

[Poroit]

Thommyy,

What method are you proposing to install Strain Gauges/Load Cell.

This maybe outside your budget but I have used successfully for past applications.

https://defineinstruments.com/shop/panel-meters-indicators/pro-wei100-load-cell-controller/

[peter-h]

Aren't the last 2-3 bits of a 24 bit ADC basically noise?

For 22 bits I use the MCP3550.

For anything past 16 bits you need to be extremely careful with layout and shielding.

[coppice]

Aren't the last 2-3 bits of a 24 bit ADC basically noise?

You say that like its an inherent problem. As long as the noise is not structured digital noise there are many applications where it doesn't matter. If the measurement process requires taking a number of samples over time, AWGN just (de)correlates away. As long as the ADC had high underlying linearity things work out. Even in a weigh scale you can produce a quick displayed result from a single sample when the load changes abruptly, then average a number of samples to achieve a more precise result as things settle. It usually takes some time for the mechanics to fully settle, so this process can be transparent to the user.

[Terry Bites]

Sainbury's seem to know if I was breathing on the bagging area!

[coppice]

Sainbury's seem to know if I was breathing on the bagging area!

If you stop does a defibrillator pop out?