Not specific experience with strain bridges, but high precision op amps often use a linear regulator followed by an lc filter to get cleaner power to the op amps. In theory there is high power supply variation resistance in most op amps, but cleaner power is always better (especially if your input power source is a switching power supply which generally has high frequency noise in it). I’d be tempted to use a 78lxx non ldo regulator from a higher supply voltage (if you have one) to give the regulator more headroom, but if you only have 5V a ldo linear regulator should still give you a cleaner supply voltage.
Is it necessary to have an IC filter following an LDO? i was under the impression that i could perhaps use a low noise 250mA LDO - 3V to power the bridges directly. And IC filters just before the IN+ and IN- of the op amps. Design wise i am not sure if this is a stupid thing to do in my case. As my XR18910 (8:1) amplifier doesnt seem to be linearising the output very well. And i suspect that it could be due to a bad voltage regulation stage. Since i am currently powering my bridges directly, from DF robot power supply that has a AMS1117 LDO. I would suspect that i would need to add another LDO after this stage, preferably the https://MAX8902Awww.maximintegrated.com/en/products/power/linear-regulators/MAX8902A.html
to power the bridges. As for the filters before IN+ and IN-, was thinking some type of 50/60Hz rejection filter?
Looking at the XR18910 data sheet (which I didn’t before) I’m not sure the ldo is going to buy you much. There already is one in the chip that appears to be used to drive the bridge power so Exar seems to have already dealt with this. How long are the wires from the chip to the sensor? If you are having noise problems that’s where I would look and perhaps add extra bypass capacitors to the bridge power wires out at the sensor to see if that helps if the wires are long. Putting a scope on the bridge power wires at the sensor to see if there is ripple or noise would be a good bet too. Are you sure the linearity problem is in the Exar chip and not your A/D setup? Either one could be the cause which makes life exciting . I’d expect the differential input from the bridge to be pretty much cancelling 60hz noise on the input as it should be pretty much equal on the two input wires (assuming they are running parallel to each other and perhaps twisted pair or shielded, so the induced noise is the same and gets cancelled), that’s why the inputs are usually differential (as in this case) from a bridge. If you have a high accuracy voltmeter and a lab supply you could replace the bridge with the lab supply and check the linearity of the XR18910 A/D combination to see if that is the problem or if the non linearity is coming from the sensor. If it is the Exar/ A/D that is non linear them move the power supply to the A/D input and see if it is the Exar of the A/D which has the problem (on a board this may not be easy or even possible to do though). This is why noise problems in an analog circuit are a nightmare because there are so many possible causes and measurements have to be made very carefully so as to not add new unexpected errors to mislead you.
Yes I did infact consult to use the LDO built-in the chip. but this only delivers 20mA maximum . And i want to currently use 4x120Ohm strain gauge excited by 3V which would consume 100mA. So this option is unfortunately ruled out. Other wise it would have been terrific to use the XR18910 to the fullest.
The wires are about 6 to 10 meters. That is the gauges are kept about 6-10 meter away from the bridge.
I would assume the linearity problem would be in the A/D setup. The ADC was not fit with a proper reference, hence this could be a reason where the non-linearity arises.
Your reasoning is extremely right behind the 60Hz noise being differentially inputted. Which is why i am confused that there is infact this noise. I am unsure as to where it arises from. Perhaps ill add a few by-Caps atop the bridge.
Currently here, goes my design. It looks a bit. Lame. I will add the by caps for the xr18910 as recommended by the data sheet.
Ah, I hadn’t considered current (assuming the gauges were low current) the low resistance gauges are an advantage in this case as they reduce the induced noise (it has to be much stronger to affect the readings).
That’a a fair distance and noise will likely be an issue, however my guess is going to be that the problem is in the 3v power supply to the
bridge. Noise such as 60hz on there is likely to be more of an issue as it isn’t differential so bypass caps at the bridge may help (possibly a lot). The balance of the bridge should reduce that but perhaps not by enough.
That’s also a good bet, the more bypass caps the better in general.
While it is likely too late, had you considered the hx711 chip?
If you could put this module at the bridge somehow your 10 meters of wire would have digital signals not analog and a much higher res A/D (24 bit against 10 or 12 for most micro A/Ds). Note I’ve only seen these on Ebay not actually used one so I don’t know if they are good in reality, but Sparkfun has a breakout board and they are usually pretty careful. Another thing to try would be to move a sensor to within a foot or so of the board and see if the same 60hz noise appears without the 10 meters of cable. If it doesn’t then the cable length is the issue, if it does then its board/chip/ad which helps to isolate the problem.
By induced noise do you mean that, since its low resistance, it draws more current and hence better signal? So which means inorder to have the same relative quality of signal for a higher resistance gauge like the 3000Ohm gauge, my excitation voltage must high enough to allow the same amount of current that would otherwise flow through a 120Ohm gauge?
Yes, it would indeed be an issue, in the future i would want to keep the signal conditioning as close as possible to the sensors, but for now it seems unlikely. Bypass caps between IN+ and IN- before the preamp stage could possibly reduce this noise. The frequency of the signal i would be measuring wouldnt exceed 50Hz. so an RC circuit would be ideal as a filter. I would assume that i must keep the R part as small as possible otherwise i would get even lower current to the op amp.
This was infact the dream IC to use. Hx711. But the biggest problem i have here is the low sampling rate. I can only get a sampling rate at a maximum of 80 SPS. My requirement here is at 1000SPS unfortunately. Otherwise i did consider the Hx711 being a very viable solution.
With the warning that I haven’t actually done much analog after about 30 years ago , yes that is the theory. You are basically in the area of signal to noise ratio, and the more power that the interference has to provide to change your signal level the better off you are.
A signal that close to the pass band of the filter is going to be a problem. I expect you may be better to look at an active 60hz notch filter (an rc filter is unlikely to be able to attenuate much that close to the corner frequency). The problem is (although technology after 30 years may have fixed this ) getting an active filter that doesn’t add undesirable dc offsets and/or temperature effects. Analog has always been difficult and I don’t think it has gotten all that much easier over the years.
Yep sample rate could be a problem. I expect most of it is in the 24bitA/D, getting that much accuracy generally takes a fair bit of time. Although with a 50hz or less input signal change 1000 SPS seems sampling overkill. You won’t see much change between samples because the gauge won’t react that fast (at least I wouldn’t expect it to). That is why the 80 SPS limit on the Hx711 sounds about right to me (they look to be assuming about a 40hz max update rate or possibly even smaller)
. I’d expect the differential input from the bridge to be pretty much cancelling 60hz noise on the input as it should be pretty much equal on the two input wires (assuming they are running parallel to each other and perhaps twisted pair or shielded, so the induced noise is the same and gets cancelled), that’s why the inputs are usually differential (as in this case) from a bridge. If you have a high accuracy voltmeter and a lab supply you could replace the bridge with the lab supply and check the linearity of the XR18910 A/D combination to see if that is the problem or if the non linearity is coming from the sensor. If it is the Exar/ A/D that is non linear them move the power supply to the A/D input and see if it is the Exar of the A/D which has the problem (on a board this may not be easy or even possible to do though). This is why noise problems in an analog circuit are a nightmare because there are so many possible causes and measurements have to be made very carefully so as to not add new unexpected errors to mislead you.