load cell wheatstone bridge + instr. amp question - Page 1
Forum section: electronics / projects

author: rl4
author_rank: Newbie
timestamp: May 07, 2023, 05:10:19 pm
content: Hello,I am currently building a resistive load cell with simple analog voltage output (mV/V, 0-5V). Basically, it's just a simple wheatstone bridge in an active half-bridge layout connected to an INA126U instrumentation amp.However, I just drew out the schematic of an older design (by someone else) doing the same thing, which I can't quite make sense of. Why is there a voltage divider added to the wheatstone bridge, am I missing something? That Cap on top also doesn't make any sense to me, I am guessing it may just be a misplaced decoupling cap because someone misread the datasheet. First schematic is the old one and second my new one (same circuit twice). Click to enlarge the pics.Thank you very much for any help!

author: MasterT
author_rank: Frequent Contributor
timestamp: May 07, 2023, 06:43:40 pm
content: Cap - noise reduction.Pot - offset adjustment. Requirements of this feature depends on the post processing, if microcontroller is in use than offset adjustment makes no sence, better to do in software. For old design with dual slope integrating adc it would be necessary.Settng pot into signal path like you did - is no go. Better way is to place pot in parrallel with existing network, so only small fraction of the signal flows via not reliable potentiometric's contact

author: rl4
author_rank: Newbie
timestamp: May 07, 2023, 07:59:55 pm
content: Thank you for the input!So if I understand correctly the trim pot in the old design offsets/ shifts the complete voltage range? If that's the case, does it even matter if the wheatstone bridge is perfectly balanced? While it can be done in Software, I'd like to be able to center the output at half the supply voltage (~2,5V).How can I calculate the cap value for noise reduction or is it just thumb rule? I thought about adding a low pass filter on the output to reduce noise.

author: MasterT
author_rank: Frequent Contributor
timestamp: May 07, 2023, 10:04:09 pm
content: No, not the voltage range, rather "0" adjustment. Capacitors value sets cut-off frequency with simple RC equation, where R is output impedance of the bridge, 350||350 + 350||350 . It's quite posiible to use LPF somewhere in between bridge & adc, but careffully choossing cap's size, since 5%-10% tolerance may degrade CMRR

author: MasterT
author_rank: Frequent Contributor
timestamp: May 08, 2023, 01:29:35 am
content: My recent research for good quality instrumental op-amp turns disappointing. There are some very expensive parts from AD, but high voltage and not R-R, so interfacing with 3.3V microcontroller quite complex . I tested ad8223 and ad8237 - low voltage, but find out that noise level is enormous, and in summary 12-bits ADC performs better w/o this noisy front-end. It's possible to built discrete instumental 3-OPA circuits, the same time to buy low price 24-bits SD adc is more practical way.|| - means parallel.

author: hubi
author_rank: Regular Contributor
timestamp: May 08, 2023, 02:17:33 am
content: I've used some cheap HX711 modules to read load cells with a microcontroller and they have worked really well for me. The chip is available from lcsc.com for less than fifty cents and the modules go for less than $2 on amazon in qty. 5 including shipping. A similar device might be the ADS1231 for around $7. Not sure if either of those fit your requirements, but might be worth looking into, especially if you are using a microcontroller with a poor ADC (I was using an RP2040).

author: David Hess
author_rank: Super Contributor
timestamp: May 08, 2023, 02:36:00 am
content: Quote from: rl4 on May 07, 2023, 05:10:19 pmThat is a zero trim for the bridge, which may be required even when zero is handled digitally to maximize the dynamic range of the ADC. Internally a load cell has several electrical trims.QuoteThe capacitor is across the differential output but because of the low impedance, is only useful for suppressing RF. I have never seen that located at the bridge itself. RF suppression should be located at the input of the amplifier and low pass filtering done after because it is difficult to do it differentially at the input without adding noise and drift.Quote from: rl4 on May 07, 2023, 11:45:49 pmHigh common mode rejection is not very important when the common mode voltage varies so little. Low frequency noise and drift are important when dealing with the low signal levels from a strain gauge bridge.QuoteMake sure to use the same source for the bridge excitation and ADC reference so that drift and noise from the supply cancel out in the ratiometric conversion.Quote from: MasterT on May 08, 2023, 01:29:35 amThere are alternatives to using an instrumentation amplifier. Check out Linear Technology application note 43,especially page 7, which has some single supply examples which do not require an instrumentation amplifier to produce a single ended output. Anybody who figures out why Q2 is necessary in Figure 10 wins an internet.QuoteSome instrumentation delta-sigma converters have differential inputs and good enough sensitivity and low enough noise to digitize the signal from the strain gauge bridge directly.

author: MasterT
author_rank: Frequent Contributor
timestamp: May 08, 2023, 03:32:29 am
content: Quote from: David Hess on May 08, 2023, 02:36:00 amI see an explanation for Q2 in LT1054 p.23 DS,"Special care must be taken inLT1054 circuits to avoid making VOUT positive with respect to any of the other terminals. For circuits with theoutput load connected from VCC to VOUT or from some external positive supply voltage to VOUT, an externaltransistor must be added (see Figure 30). This transistor prevents VOUT from being pulled above GND duringstartup. Any small general-purpose transistor such as a 2N2222 or a 2N2219 device can be used. Resistor R1should be chosen to provide enough base drive to the external transistor so that it is saturated under nominaloutput voltage and maximum output current conditions."Still, overall complexity and cost of those single-ended alternatives to INA not really competitive. Also CMRR is only good tillsignal is delivered by TWO wires to the other end, especially over long distance. Power and enviromental condition may not be so favourable to buld low noise preamp close to the load cell.

author: rl4
author_rank: Newbie
timestamp: May 11, 2023, 10:49:40 pm
content: Quote from: hubi on May 08, 2023, 02:17:33 amThanks, I was experimenting with a few HX711 modules I had laying around initially but the software I'd like to use sadly doesn't support these (yet).Quote from: MasterT on May 08, 2023, 01:29:35 amI gave calculating the cutoff frequency of the RC filter in my OP a shot and ended up around 3.6kHz. Looking at the datasheet the recommended RFI suppression layout is different. As I understand the single cap (called CD in pic) between the diff inputs filters the diff. signals but provides no common mode signal filtering. Adding caps (CC in pic) to both diff. inputs adds common mode filtering, but tolerances in the caps can add imbalances.Quote from: David Hess on May 08, 2023, 02:36:00 amThank you for your help! The cap is placed directly on the amp inputs, I just misplaced it on the schematic.Question, does common mode filtering benefit me in this application case or can I get away with the single cap between diff. inputs? Does it make sense to add additional Rs (like 4k in the pic) to the diff. signal lines between bridge and amp, it seems to me it wouldn’t.

author: MasterT
author_rank: Frequent Contributor
timestamp: May 12, 2023, 12:15:37 am
content: Quote from: rl4 on May 11, 2023, 10:49:40 pmDepends on ikind of ADC selected.If you go with 12-bits ADC, than antialliasing filter is required and the more filtering the betterHX711 what I tested (bought on china market) is a crap. 13-bits noise free at most.Get Microchip mcp3550/1/3 series, or max11201/.I'm having fun right now with AD7172-2 - the best SD adc I've ever seen. Despite higher price compare to competitors noise performanse is really outstanding. I could not find OPA that capable to drive this ADC w/o SNR degradation. Good things, adc hasreal internal buffers and doesn't require any pre-amp. Evaluation module from AD shows ad4528-2 , and still I think better without

author: rl4
author_rank: Newbie
timestamp: May 12, 2023, 12:32:56 am
content: Tbh I'd be quite happy with 13bit noise free. I was planning on using an MCP3302 13bit ADC.

author: MasterT
author_rank: Frequent Contributor
timestamp: May 12, 2023, 02:05:14 am
content: Quote from: rl4 on May 12, 2023, 12:32:56 amIt all boils down to dynamic range. For example, to measure weight 1 kg -100 kg and have 1% accuracy at the lower end you need ~14 bits.Slow SD ADC, about 10 sample-per-seconds have no capacity for averaging . Fast mcp3302 - 100 kHz or so would provide about 7 extra bits if sampling rate averaged down to 10 Hz. Overall up to 20-bits. Theoreticaly. But than linearity would be likely much worse compare to mcp3550, and good pre-amp or PGA is necessary, plus sone switches for zero & full scale continuos re-calibration. Though, my advise get sigma delta adc, even HX711 and forget about INA instrumental amp.

author: David Hess
author_rank: Super Contributor
timestamp: May 12, 2023, 05:52:13 am
content: Quote from: rl4 on May 11, 2023, 10:49:40 pmCommon mode filtering is a real problem because if there is any mismatch in the time constants, then common mode noise will be converted to differential and end up in the signal. This means that common mode filtering should only be used for RF suppression.