Weathston Bridge shortcomings - Page 1
Forum section: electronics / general

author: jorgemef
author_rank: Regular Contributor
timestamp: January 13, 2023, 04:07:27 pm
content: Hello,I built myself a weathston bridge to try to assess a couple of resistors for a Kevin-Varley I want to build.My problem is what I can measure fluctuates in the same measure of what should be the tolerance limits for the resistors of the first decade.I am using an BM869s for measuring the WeathstonBridge differences in mV range I believe the error should be on the order of 5-10 microvolts from it. I am also using 3x9V battery or 10V from AD584-M as source.I see fluctuations (from temperature drift?) of 0,15mV without leads and 0,8mV if with 50cm leads. When I came nearby if fluctuates a lot.Any suggestion?On top I started with 10Kohms SMD 0,1% which I built with selected resistors with a more crude version of the weathston bridge (on a test board with 5% resistors), and now need to measure with leads and leads are adding noise.The first decade is matching until the 4th digit when powered with 10V (hundred of microvolts), but I understand should have better accuracy.For the first decade I should stay within 0,0037% which represents 0,25mV if using 27V. With 10V I see variations of 0,15mv without leads. This puts my error above my target definition.I am under an air conditioner always cycling. Not sure if would be one of the sources of variation, but it is cold. Also the long leads seems to be another source of noise.I need to find a way to clip SMD on short leads but also need to find a way to reduce the noise.Cheers,Jorge

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2023, 04:28:53 pm
content: As long as the output voltage to measure is relatively small and thus the bridge close to balanced the reference voltage should not be important / critical.I see one possible problem with the SMD resistors directly mounted at the multi step switch: there can be quite some mechanical stress to the resistors and this can effect the resistance.For the THT resistors as DUT, there is no correction for the contact resistance, so the 4 wire contacts of the kelvin clips are not really used as intendet. For this it would need a kelvin type bridge as a somewhat improved version to get at least approximate serate current and voltage contacts on the resistors, e.g. make full use of the kelvin-clips.A possible failure point is also the isolation of the leads - not all the isolators are that good and some may have leakage resistance in the Gohm range that can disturb a precision measurement in with some 10 K.Some resistors have intrinsic noise, as some 1/f noise even without temperature variations. So there is a limit in how stable the result can be. The resistor type / series can make a difference here. At least for the reference arm one could choose low noise (wire wound or good thin film type) resistors.

author: jorgemef
author_rank: Regular Contributor
timestamp: January 13, 2023, 04:50:57 pm
content: These are my current results. Voltage measured with 10v in the first decade.Close, but not good enough. I think I will buy a batch of leaded metallic resistors for the two first decade and start again.It is not so easy to solder, unsolder and sometimes the soldering iron stays on too much time and there will be also the stress resistance with temperature variations.I will purchase 100 10K and 100 2K 0,1% as they cost as much as one Vishay resistor of bether precision.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2023, 05:31:03 pm
content: Finding 2 or 3 equal values works usually well, finding 11 equal ones would need a rather large number to start with.Especially if one starts with a larger number of resistors from a batch and looks for equal pairs one can consider using 2 resistors combined (series or parallel) for each final resistor. This way one could use more than just equal resistors, but combine a large and small one each to get many of the same value. 2 resistors combined would also reduce the resistor excess noise.In theory the TC matching would also improve a little, at least on average.AFAIK the KV divider needed the resistor values spaces 1:5 and not 1:10. So it would be more 50 K and 10 K, not 100 K and 10 K. With an additional resistor one can have the 2nd decade resistors larger if wanted, so one would not need to have the resistors matched all the way. So a resistor ratio even a little less than 1:5 (e.g. 1:4.7) may be more practical to allow for a trim between the dacades.In the extreme one could use even the same resistors for both decades: 2 in series for the first and 2 in parallel for the 2nd. One would than need an extra resistor in parallel to the whole 2nd decade of about 10 x the resistor (crude calculation in the heat - check the value).

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 13, 2023, 06:01:53 pm
content: I did the original article with common 1% metal film resistors that I could buy from Digikey in bags of 200 for a few dollars. I'd think using SMT might be trouble because you solder right at the body. With leaded parts you can put a clip on the lead to protect the body from heat. Selecting for the first decade isn't easy, but I was able to do it without spending a fortune on resistors. Not sure why you have a noise problem. The bridge should cancel out much of the noise. I usually work on a large metal ground plane and that helps a lot.

author: mzzj
author_rank: Super Contributor
timestamp: January 15, 2023, 03:51:20 pm
content: Potentiometer in your picture looks like garden variety standard carbon track potentiometer. These are notoriously unstable and noisy

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 15, 2023, 05:39:58 pm
content: Quote from: mzzj on January 15, 2023, 03:51:20 pmHelloI agree potentiometer is a nice source of problems and the first is the noise , on my strain gage system is use only high quality potentiometer and I check before what level of noise I reach from some samples but also stability both with time , repeatability and temperature , Bourns and other as Sfernice make nice quality pot which can be available as surplus parts on Ebay at nice pricesOther concern is the SMD resistor soldered between pads , SMD under strain ( so not fixed on a rigid / stiff plate as a PCB ) act as a strain gage , massive soldering is also not the best for resistance valueIt s far better when SMD and mechanical switch is used to make a back plate pcb for hold the SMD soldered flat and without strain and link with wires to the rotating switch , quick DIY a PCB with a dremel is a difficult job , not pretty but enought for testing without concerns .Question : Your bridge excitation voltage 27 V came from what kind of supply ? battery or voltage regulator ?? as noise in bridge voltage is also a worth thing .RegardsOS

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 16, 2023, 01:21:14 am
content: I think I used 27 volts because it was as high as I could crank my adjustable Kepco bench supplies. I have a pair of them for +/- 15 volt stuff and when I need more voltage I just connect across the pair and ignore ground.

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 16, 2023, 06:23:37 am
content: HelloOk for voltage level origin , but for any voltage level used to feed a measuring bridge , noise and drift shall be minimal and far under the common lab psu performance .I feed my 120 and 350 bridge with voltage from a voltage ref IC as AD586 or REF102 , in 15V I use a Wenzel circuit .I have never feed 27V in a bridge so I have no solution but for accurate resistor sorting the best is to built first a high quality low noise PSU .That could be a efficient control point to check the noise of the bridge voltage first .RegardsOS

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 16, 2023, 05:19:05 pm
content: The whole idea of using a Wheatstone bridge is that it's insensitive to the excitation voltage level and not even very fussy about noise, if the detector has any filtering. Obviously, the higher the voltage the better the sensitivity, until you hit thermal problems.

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 16, 2023, 05:32:43 pm
content: Quote from: Conrad Hoffman on January 16, 2023, 05:19:05 pmWhen the bridge is balanced the output is zero so noise or not that the same but when you try to measure the output of an unbalanced bridge the noise is included in the output signal ? I think , if I am not right on this statement please explain in detail .- on sensitivity vs voltage level I agree but usually 15V generate far enough for a bridge excitation except perhaps on special applicaions , but OP AMP gain can solve the problem too and there is a lot of OP AMP with impressive performances available at acceptable costs in easy to use SOIC or DIP .- on voltage supply quality if your voltage is unstable ( noise is just one of the parameters ) your output bridge measurement will be ?- on OP AMP that the same , low noise voltage supply is also mandatory point , PSU built quick and dirty with 7815/7915 is just crap .All strain gage bridge systems I own ( Pacific Instruments , Vishay , HBM , NI .. ) have high quality bridge voltage supply , a lot of Wheatstone bridge are powered by battery , so noise is not a real concern compare to AC/DC PSUIf the purpose is to select / match resistor in the aim to built a accurate KV divider I suppose that reach a final accuracy is the goal .RegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2023, 07:40:13 pm
content: Most OP-amps are quite good in the power supply rejection. So the supply to OP-amps is not that critical and a conventional transformer and 7815/7915 are usually good enough. An SMPS can add EMI problems.With strain gages there is often a relatively long cable from the amplifier / supply to the strain gage. This ideally requires extra care for the drive with 2 more sense wires, so usually a 6 wire cable to the bridge. For a strain gage the bridge may also not be perfctly balanced, especially one does not adjust the balance in response to the load. One could still get away with a drifty reference if the same ref. is used for the ADC to look at the signal. With the SG amplifiers the drive and recorder/ DMM may be separate instruments and thus use a separate ref.Strain gages also often have a significant self heating part - for high sensitivtiy they are often run with quite some power due to the relatively low resistance (e.g. 350 ohm). For the self heating part to stay constand a stable supply help. Ideally the bridge drive is also AC to avoid DC drift.For the KV divider the resistors are usually larger (more like 5-10 K). This implies the possiblility to use more voltage.Especially with even higher resistors more voltage helps, as the resistors itself introduce noise and bias current cause drift / offfsets. OP_amps have very high gain, but the precision is still limited. For highest resolution changing the polarity of the bridge from time to time as a kind of AC drive is a real option to reduce DC errors and possible 1/ f noise. There are however limits to this for large resistors.

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 17, 2023, 12:55:34 pm
content: HelloWhen I design some thing I use always two stepsQuick and dirty at a reasonable level to check if there is no major errors or design flawsandLess for less and more for moreas in this case less uncertainties for less errors on the result and more design reviews and improvements to get more accuracy / efficiency without to spend millions of course .An interesting article on resistor noise test , they use top of the range Stanford ultra noise PSU and Bias setup on bridge powerhttps://arxiv.org/pdf/2109.02448.pdfOn my setup I use low noise PSU DC , Low noise pulsed ( square waveform ) and square waveform symmetrical voltage to reduce / cancel thermal effectRegardsOS

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 17, 2023, 05:59:55 pm
content: Resistor noise testing is a pretty specialized thing. It's too bad Geller Labs closed, but you can still find their info herehttps://www.nutsvolts.com/magazine/article/build_the_jcan_to_measure_resistor_noiseand probably on the Wayback Machine. My take is thick film can be terrible and isn't useful for metrology projects. Thin film (metal) is usually very good and wire wound a bit better, if you can stand the inductance. IMO, the inductance problem is usually overstated but "non-inductive" winds aren't as good as one might hope. That's still all far away from the original bridge issues that started the thread.

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 17, 2023, 07:16:35 pm
content: HelloJCAN Article is not available from Nuts and Volt but I have located a copy in PDFhttp://physicsopenlab.org/wp-content/uploads/2016/10/JCan-NV-article.pdfThat a classic high gain ultra low noise amplifier for low signal imput , the PCB itself is interesting as that a key component to reach high performance , I have downloaded the pcb file I will open it this week .For metrology purpose thick film can be avoided I agree and as for small project in quantity unit price of components is nor really a problem except for capacitor where high performance capacitor are terribly costly , after the second factor is DC / low frequency or AC / high frequency and after high voltage which another domain .For accuracy quest through the hole resistor foil or wirewound are the solution , a lot of impressive performance equipment as ASL , Keithley , HP and other as Stanford use them , thin film SMD can delivered high performance IF they are soldered properlyRegardsOS

author: jorgemef
author_rank: Regular Contributor
timestamp: March 01, 2023, 10:03:09 pm
content: I had two problems on the wheatstone bridge tests I was doing. One was the TCR of the bridge resistors. I was using cheap thin film resistors with leads, and I am working under an air conditioner always cycling. The other was the voltage source used. Any battery will always continuously drop in voltage in the level of what the bridge measures. Since I wanted to grade the resistors with the bridge and not only find the ones with zero volt in the detector I needed stable voltage.After measuring the TCR of the thin film resistors I opted for replacing the bridge with 1% SMD which had much better TCR.The voltage was supplied by a source with 21v based on a LM399 as discussed here:https://www.eevblog.com/forum/metrology/driver-transistor-for-variable-voltage-reference-stage-with-lm399/25/With this new setup I was able to quantify the resistors in the rotary and replace 5 of them to bether values.I now have the 10K set, Will move on to the next ranges.With the wheatstone bridge I also selected new resistors for the LM399 module, bringing it closer to 2.5;5;7.5 and 10V (R2 is 20K//16K//(130K+5KTrim)). The blue ones which came with the module have worst TCR that the 1% I had. In the chart attached the 10k data point is from the chinese LM399 blue SMD resistors. Not sure though if this is a very good layout as the resistors are not all at same distance from the oven and the oven reaches near 58ºC as measured with a thermal camera.Appreciated for all the support and learnings.