24-Bit Analog-to-Digital Converter for Weigh Scales - Page 1
Forum section: electronics / projects

author: Thommyy
author_rank: Newbie
timestamp: August 18, 2024, 08:49:22 pm
content: 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: HX711What do you think of it? Alternatively, do you have any other types?Thank you.

author: selcuk
author_rank: Frequent Contributor
timestamp: August 18, 2024, 09:15:15 pm
content: 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

author: moffy
author_rank: Super Contributor
timestamp: August 18, 2024, 09:17:24 pm
content: 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

author: Thommyy
author_rank: Newbie
timestamp: August 19, 2024, 10:20:37 am
content: How would it be better than the HX711?I don't need a high transfer rate.

author: selcuk
author_rank: Frequent Contributor
timestamp: August 19, 2024, 11:17:01 am
content: I've never used HX711 before. Since NAU7802 has an I2C interface, you may connect it the the same bus with other sensors.

author: coppice
author_rank: Super Contributor
timestamp: August 19, 2024, 11:44:21 am
content: 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.

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 19, 2024, 12:14:04 pm
content: Quote from: Thommyy on August 18, 2024, 08:49:22 pmHelloFor 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 shopExcept if you work on race car with a strict weight limitation , usual car can handle a quite large accuracyan exemple of 4/20 mA interfacehttps://www.ato.com/load-cell-transmitter-output-0-5v-4-20maRegardsOS

author: wasedadoc
author_rank: Super Contributor
timestamp: August 19, 2024, 03:23:14 pm
content: Are you seriously stating you need a 24 bit bit ADC? 1 in 16 million. 1 kilogram in 16,000 tons.

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 19, 2024, 04:15:28 pm
content: Quote from: wasedadoc on August 19, 2024, 03:23:14 pmHelloAccuracy of a scale in no only the ADC that also the bridges / load cells concern and mechanical structure tooRegardsOS

author: zapta
author_rank: Super Contributor
timestamp: August 19, 2024, 04:55:40 pm
content: 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 example1. 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.

author: zapta
author_rank: Super Contributor
timestamp: August 19, 2024, 05:17:11 pm
content: Quote from: wasedadoc on August 19, 2024, 03:23:14 pmThe ADC error is typically larger than the ideal +/-0.5 count.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: August 19, 2024, 05:38:06 pm
content: 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 ....

author: Fox_Alex
author_rank: Newbie
timestamp: August 19, 2024, 11:25:00 pm
content: 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.

author: Poroit
author_rank: Frequent Contributor
timestamp: August 20, 2024, 03:22:08 am
content: 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/

author: peter-h
author_rank: Super Contributor
timestamp: August 20, 2024, 11:29:57 am
content: 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.

author: coppice
author_rank: Super Contributor
timestamp: August 20, 2024, 11:51:16 am
content: Quote from: peter-h on August 20, 2024, 11:29:57 amYou 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.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 20, 2024, 04:28:59 pm
content: Sainbury's seem to know if I was breathing on the bagging area!

author: coppice
author_rank: Super Contributor
timestamp: August 20, 2024, 04:37:29 pm
content: Quote from: Terry Bites on August 20, 2024, 04:28:59 pmIf you stop does a defibrillator pop out?

author: graphtronics
author_rank: Contributor
timestamp: November 19, 2024, 03:07:13 am
content: HelloA simplistic perspective because it is too long to document.A weighing scale "accuracy" depends on many factors.Some of them are:1) Sensor2) Instrumentation3) Electromechanical assembly4) Adjustment5) Heat, EMI, Humidity, Vibration, ....SENSORAuthentic quality load cells can render 10,000 divisions or 0.01 ppmThe strain gauge type depends on the intended force: bending, shear, torsion, compression/tension...Although they can cost $ 25 at distributor, expect to pay over $ 100 at local store.Building your own sensor with 4 strain gauges might be better, depending on...Consider your local weighing company and if in doubt write me.INSTRUMENTATIONThis i the part of your ADC that works on GIGO or quality of strain gauge or sensor.Filtering is a problem since it operates in 3 mV/V signal.Industry is set on 5 V excitation, but 10 V works best.Remember it is 10,divisions on a 30.000 mV or 3 µV per division.A 6½ or 7½ Bench DMM is a must.ASSEMBLYIt does not matter if your sensor and instrumentation are better than the Kibble Balance.if the electrical connections are faulty and/or the mechanical coupling can't transfer the applied force properly.ADJUSTMENTEven the Kibble balance can deliver inaccurate results if adjusted improperly.A quality test mass is a must, but they are extremely expensive.Your "accuracy" ultimate depends on this.Keep in mind even seasoned weighing pros improperly refer to adjustment as calibration.They also spell lbs (which reads pound second) instead of lb (because units have no plural)EXTERNAL INFLUENCESHeat, radiation, humidity, vibration, age, have a detrimental effect on your reading.WarningDesign mus consider protection against: overload, side load, shock load and more.ConclusionThe desired ± 1 kg "accuracy" can be accomplished if max capacity is 10 metric tons.For lower quality load cells, consider a 1,000 divisions or 1 ton by 1 kg resolution.Familiarize with the meaning, differences and relationship of the following:Accuracy, precision, resolution, repeatability, reproducibility, eccentricity,

author: Postal2
author_rank: Frequent Contributor
timestamp: November 19, 2024, 04:25:01 am
content: You can cascade 2 12bit ADCs using an op amp for the second one driven by a 12bit DAC.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 19, 2024, 11:49:47 am
content: The HX711 is a common cheap Chinese solution. It would be likely good enough. There are laternatives from AD and Ti. Which to choose may depends on the availability at your distributor or shop. The ADC is one of the easier and more reliable parts.Those 24 bits are more like the digital data format and one can usually get no missing code up to this resolution. The actual resolution will usually be less.The SD ADCs are cheap and no need to complain a few extra bits that are noisy.With a strain gage the issues are anyway more with the sensor and amplifier part (is included with some ADCs like the HX711), not the actual ADC part.Also the mechanics can be relevant with forces or torque in other directions as to measure.