Anyone used the HX711 24b strain gauge interface iC? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: June 16, 2018, 08:51:35 pm
content: As per title really! Has anyone used this IC and found any issues? How many real bits does it return in a typical application? I'd like to use it with a 100N load cell and i am wondering what resolution would be sensible to expect (note resolution, not accuracy!)Datasheet here:http://www.mouser.com/ds/2/813/hx711_english-1022875.pdf

author: MasterT
author_rank: Frequent Contributor
timestamp: June 16, 2018, 10:37:04 pm
content: Herehttps://forum.arduino.cc/index.php?topic=365107.0someone estimates 14-15 bits noise free. I'd say 15 for short term just right after re-calibration, and 12 -bits or so in continuous mode. Essentially IC is a crap, look for alternatives, mcp34xx/35xx, ads1232, nau7802 etc.

author: max_torque
author_rank: Super Contributor
timestamp: June 17, 2018, 10:43:01 am
content: ah ha, i had a suspicion that a true 24b from a no-name Chinese outfit might have been a bit of overselling!I'd like a genuine 16b result, and i can low pass filter because i don't need a high update rate, so rolling my own front end might be more sensible and then sticking the output into a decent 16b ADC?

author: wraper
author_rank: Unknown
timestamp: June 17, 2018, 10:49:33 am
content: Quote from: MasterT on June 16, 2018, 10:37:04 pmRecent thread about crap modules cases me doubting those results.https://www.eevblog.com/forum/projects/how-can-a-chip-(hx711)-work-without-a-ground-pin-connected/msg1605814/#msg1605814

author: Siwastaja
author_rank: Super Contributor
timestamp: June 17, 2018, 11:04:10 am
content: It's the Arduino forums. It's for artists, by artists (by their own definition).The test is completely meaningless as there is no picture of the layout nor schematic.The authorseemstoalmostunderstand that the limiting factor is not the IC but the layout and other circuitry, by saying: "Maybe 16  if you're careful with grounding and shielding and use a very low noise power supply and sensor." But accepting the consequence of this understanding, that the test is completely meaningless, is left for the reader.But hey, when working with 24-bit ADC's, youhaveto be careful with grounding and shielding and use low noise power supply and [low noise] sensor!If not, the ADC magically cannot clean up after your mess; you are just measuring the noise you put in. Actually fairly benchmarking 24-bit ADCs would require quite some expensive gear and massive knowledge.And, if the end user doesn't have such gear and knowledge, the results is limited to around 13-14, maybe 15 bits maximumregardlessof the IC.The arduino forums can sometimes be a good source of information, but there is a nonobvious, cultural gap in the nomenclature. For example, when they talk about an IC using its specific name, they are almost always talking about the combination of the IC, a poorly designed module (connected using coat hanger like air wires), and a poorly written software library for it. Youcanapply the information if you are going to use the same module and software library, but you need to know this.

author: MasterT
author_rank: Frequent Contributor
timestamp: June 17, 2018, 02:46:59 pm
content: Quote from: max_torque on June 17, 2018, 10:43:01 amLow passing does not help much against non-linearity, thermal drifting or low frequency noise.You don't need your own front-end, since there are plenty of ADC that are specifically designed to work with load cells.Quote from: Siwastaja on June 17, 2018, 11:04:10 amYou 'd be surprised, but sigma-delta ADC does exactly that, magically cleaning up interference and noise from power lines. Power rejection is a parameter strictly specified in the data sheet.

author: max_torque
author_rank: Super Contributor
timestamp: June 17, 2018, 07:26:34 pm
content: In my application i am weighing a fluid tank, and that only ever gets lighter as the fluid is used, so i can probably come up with some sort of digital filter to achieve a decent SNR i suspect?Also, if i use the same voltage ref to drive the strain gauge excitation, and as the ADC ref, then a lot of the major thermal drifts disappear (assuming of course the pcb and components are broadly isothermal) as it ultimately is a ratiometric system.If my strain gauge is nominally 2mV/V and i use a 10V excite, that 20mV output span, so a front end gain of 100, and a suitable 2.096V reference might work?

author: MasterT
author_rank: Frequent Contributor
timestamp: June 17, 2018, 10:13:52 pm
content: Design analog front end x100 that has a few uVolts zero offset and a few ppm FS gain error in full industrial temperature range is not easy task.There is no ratio-metrics, all your caps, resistors and OPA gonna to drift with temperature on a pro-cents scale, hardly provide 7-8 bits accuracy. I'm not saying it's not possible, but why bother if you can buy an ADC for $2 with all features included? With auto-calibration, auto-zero and internal temp sensor?There are engineering who has done work for you, just buy right chip. Ads1232 module that I've got from china for about $3 CAD makes me very happy.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 18, 2018, 02:02:59 am
content: You can design the electronics perfectly, but you'll get error in a milligram precision scale from a draft of warm air. Or just a draft. You have to put a cover over the scale and hold your breath...That's what I experienced using the Chinese scales I bought a few years ago to weigh quadcopter parts.There wasn't any need for the 0.001g scale for this kind of work but I couldn't resist. I don't use it anymore because who knows what those digits mean?I liked the 0.1g and 0.01g scales a lot more.

author: djnz
author_rank: Regular Contributor
timestamp: June 18, 2018, 05:04:14 am
content: Just out of curiosity, how are you dealing with the load cell creep?

author: max_torque
author_rank: Super Contributor
timestamp: June 18, 2018, 08:13:45 pm
content: The device doesn't' need to be across the full industrial temp scale, and in fact, i'd probably put the whole lot in a temp controlled enclosure simply for the sake of simplicity (ie, keep everything at say 40degC, except the strain gauge itself)