Reading Loadcells at high sample rates - Page 1
Forum section: electronics / projects

author: nandoanalog
author_rank: Contributor
timestamp: July 29, 2023, 01:56:32 pm
content: Hello everyone,I hope you are all doing well. I come seeking advice and guidance on a topic that has been challenging me lately. Let me begin by stating that my level of expertise in this field is quite low, which is why I am reaching out for help in the most respectful manner possible.My current project involves acquiring data from a load cell at a frequency of at least 1000 times a second. So far, I've made some progress by overclocking an hx711 to achieve around 140 samples per second. Additionally, I discovered and successfully implemented the nau7802 chip, which allows me to take samples at a maximum rate of 320 sps. Both of these chips were relatively straightforward to use, thanks to the ample library support available.However, my knowledge and skills have hit a roadblock when it comes to understanding how load cells work and determining the appropriate amplifier/ADC combination for my needs. Despite my efforts to research and consult resources from Analog Devices and Texas Instruments websites, the technical jargon has left me confused and overwhelmed.I have been looking for an off-the-shelf IC that can amplify and convert load cell signals at higher frequencies, similar to the hx711 or nau7802 but with faster sampling rates. Regrettably, my search has not been successful so far. It's possible that such a chip exists, but my limited understanding prevents me from recognizing it.I considered using a separate amplifier/ADC combination for this purpose or even exploring the possibility of utilizing the MCU's integrated ADC if feasible. However, delving into this approach has only added to my confusion.At this point, I am feeling quite frustrated because I can't even articulate the right questions to ask. I sincerely request your patience and understanding as I navigate through this learning process.I am here to inquire if anyone knows of a chip, similar to the hx711 or nau7802, but with higher sample rates. Alternatively, if a separate amplifier/ADC combination is the way to go, I would greatly appreciate any recommendations or guidance you can offer.Thank you for taking the time to read my post, and I am looking forward to your kind assistance and insights.

author: gamalot
author_rank: Super Contributor
timestamp: July 29, 2023, 02:18:43 pm
content: I used to use ADS1220:https://www.ti.com/lit/ds/symlink/ads1220.pdf

author: MasterT
author_rank: Frequent Contributor
timestamp: July 29, 2023, 02:24:48 pm
content: Seems to me sigma-delta ADC with PGA and sampling >1kHz is what you search. There are many, but I name a few that I was evaluated recently: ad7124, cs5532A, isl26102.CS5532 has the best noise spec. characteristic at low sampling rate <10 sps. Things may be different at 1 ksps and above, anyway going higher in sampling would require special measure on spi bus be implemented, like slew rate control, line termination, opto-isolation etc.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 29, 2023, 02:50:41 pm
content: Quote from: nandoanalog on July 29, 2023, 01:56:32 pmWell then, it's time to do some basic research.Some kind of fancy chip is not going to magically fill that knowledge gap for you.Chances are you will apply it wrong, waste a lot of time troubleshooting, and then still have to do that research.

author: nandoanalog
author_rank: Contributor
timestamp: July 29, 2023, 03:59:12 pm
content: Thank you very much. Sometimes a nudge in the right direction is all we need.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 29, 2023, 08:30:12 pm
content: Quote from: nandoanalog on July 29, 2023, 01:56:32 pmVery few MCU have ADC of enough resolution, so you are best to focus on a single dedicated ADC/AFEYou did not say how many loadcells you need to read ?24b ADC sample rates have improved significantly, so do not search for exactly 1ksps.Create a spreadsheet with important details like sps,ENOB,PGA gains,interface,voltage supplies,price

author: jbb
author_rank: Super Contributor
timestamp: July 29, 2023, 09:51:26 pm
content: Lots of chips from Texas Instruments and Analog Devices could do this.I get the feeling - and I might be totally wrong! - that you only need a few units. If that’s true, I recommend reaching for the right chip, not the cheap chip.Analog Devices has a virtual evaluation tool which might help you think through your filter settings.http://beta-tools.analog.com/virtualeval/Depending on how much useful frequency bandwidth you need, analog bandwidth could be an issue. For example, load cells have moderately high resistance, and if you connect one to a long cable it acts like a capacitor. The resulting RC filter might be too slow for your application. I suggest you estimate max cable length, look up cable capacitance per meter and load cell output reaistance and calculate from there.Do you want to sample more than one load cell? If so:- I suggest you get oneconverterper chip. There are numerous parts with a nice big input multiplexer, but there is a big time penalty for switching from one mux channel to another.- You probably would need to synchronise all the devices to a common clock. An ADCs internal oscillator might be good to +-5%, so without a common clock you could get data from channel A coming in at 950 samples/s and data from channel B coming in at 1050 samples/sec.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2023, 03:18:34 am
content: Some high resolution delta-sigma converters support higher sampling rates at lower resolution. High resolution sampling ADCs are available which are much faster than 1000 samples per second but will require external amplification.Linear Technology application note 43 has a detailed discussion of signal conditioning for bridge circuits.

author: MasterT
author_rank: Frequent Contributor
timestamp: July 30, 2023, 03:43:11 am
content: Quote from: jbb on July 29, 2023, 09:51:26 pmNot always, AD717x has no time penalty when sequencing over 4 diff input pairs. My understanding that new design implemented 4 set of filters/ accumulators, so when MUX is changed simultaneously filter is swaped as well - no needs to re-flash/ re-initialize raw data pool.Curiosly, why it tooks decades to come up with this simple solution.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2023, 02:41:33 pm
content: Quote from: MasterT on July 30, 2023, 03:43:11 amIncreasing integration meant cheaper transistors until the point where the cost of the extra duplicated logic was insignificant.Delta-sigma converters with zero latency suitable for multiplexers have been available for a while, although I do not remember if any operate at higher sample rates.

author: mjkuwp
author_rank: Unknown
timestamp: August 01, 2023, 02:48:32 pm
content: I think you need 1) an instrument amplifier ic and 2) an ADCThe ADC should be ratiometric so you can reference the excitation voltage of the load cell as opposed to having a fixed or internal voltage reference.For example I usedAD8293G160 and MAX11201 in one project. However, that sample rate was very low.You didn't state how many bits of resolution are required. That is a very important question.