0.0025Ohms Deviation of a 1k Resistor - Page 1
Forum section: electronics / general

author: luky315
author_rank: Regular Contributor
timestamp: July 12, 2022, 06:46:49 am
content: Now I have final specification on my measurement problem: It is a prototype / lab equipment for experimental use at the university, but it has to be as small as possible and portable (field use, powered with batteries):Part cost is not an issue, I need to build only 5-10 pieces.A (chemical sensitive, but that's another story) resitor of nominal 1000Ohms changes his resistance according to the presence of some substances. The maximum change is arround 10Ohms and they would like to have an accuracy of arround 0.0025Ohms. The temperature of the sensor is also changing, but it is also measured with a seperate temperature probe and there is a complex compensation model. But that is not my problem, I have to measure "only" the resistance. The measurement rate is not fixed, I told them about the theoretical limits, naturaly it would be good to measure "as fast as possible" to get dynamic processes, but "fast" in the chemical world is really slow for us, except when it explodes :-) Therefore I would target 100Hz to 5kHz bandwidth.The first problem that I have is that the resistor is not mounted in a bridge configuration and it is also not possible to add more resistors in the measurement probe head for temperature compensation directly on the head. This would be also a problem because each sensor needs its own temperature model, so the temperature errors need to be corrected in software. But fortunately we have a nice 4 wire kelvin connection with shield to the probe (2m CAT7 twisted pair cable with 8 wires, 4 for the chemical sensor and 4 for the PT1000)So my first idea would be to use an ADC which is made for this sort of measuremets like the AD7124-4 and use a really good and stable reference resistor. The problems that I can see are thermal effects and offsets, so it would great to have some sort of "ac excitation" and to swap the connections to the sensor. Or is there a better solution?

author: ap
author_rank: Frequent Contributor
timestamp: July 12, 2022, 07:13:35 am
content: So you need an accuracy of 2.5ppm. The question is is it absolute or relative, over what temp range, and so on. That has an impact on the reference resistor and test setup stability and calibration requirements. Do the math depending on your choice of resistor used (a VPG foild resistor is a good start) and environment, cal abailities and so on.You can source a constant current (stability requirements in line with your overall error budget and setup) into the series of the 1k sensor and the 1k reference and measure the voltage drop. Since you have 4-wire connections, that makes it easy. To compensate for EMF, you can reverse the current. AC would make it very complicated.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 12, 2022, 07:59:05 am
content: 2.5 ppm accuracy is very demanding. Chances are one would first build a version that comes somewhat close and only than with the experience of this make a final version.A question is if this really is 2.5 ppm accuracy or just 2.5 ppm of resolution and short time stability that is needed. I somewhat doubt that the sensor resistance would be that stable.For the measurement speed it get a lot simpler if one can average over 20 ms (1 power line cycle) and this way reduce the sensitivity to hum.An AC measurement may be a good idea, as this may allow lower noise and this way allow to use a lower excitation power. It also avoids problems from thermal EMF, that may come from the sensor or connectors. AC also allows to use a special transformer as a precision element, not available for DC.The ready made ADCs could be tricky - one has to keep an eye on the gain stability as this one enters if the ADC is direktly used with a reference direived from the ref. resistor.This a nice easy way for less demanding senstors, like PT1000, but can be tricky with really high demands.

author: luky315
author_rank: Regular Contributor
timestamp: July 12, 2022, 09:08:13 am
content: We need 2.5 ppm of resolution and short time stability, not absolute accuracy. And there is time for offset and gain calibration. The errors of the sensor are not my department - they are made, characterized and selected by a different group. So I take them as "perfect" per definition and worry only about the errors of my measurement equipment.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: July 12, 2022, 09:09:44 am
content: Vishay's way to track resistance changes.

author: luky315
author_rank: Regular Contributor
timestamp: July 12, 2022, 09:35:07 am
content: Whow. Thanks @MegaVolt. I'v never seen such a device like the ISL28617. Unfortunately it't obsolete and not available anywhere :-(

author: MegaVolt
author_rank: Frequent Contributor
timestamp: July 12, 2022, 09:45:42 am
content: Quote from: luky315 on July 12, 2022, 09:35:07 amИнструментальный усилитель, в котором усиление определяется сопротивлением, не редкость. От полно других компаний. Возможно, они будут каждый в своей отдельной упаковке. Вот первый, который я нашел:https://www.ti.com/product/INA333-Q1

author: dietert1
author_rank: Super Contributor
timestamp: July 12, 2022, 10:51:43 am
content: When looking at the two input stages of the ISL28617 i thought that "bridge" in this application may require a second sensor/PT1000 combo for reference.I remember making a resistor bridge from a SR1010 1K for measuring a 500R foil resistor about 2 years ago. Stability after weeks of caliibration work was about 0.01 ppm over a week or so. The results are somewhere in this metrology section.Regards, Dieter

author: mawyatt
author_rank: Super Contributor
timestamp: July 12, 2022, 04:43:03 pm
content: @luky315Around ~1980 some of our colleagues were working on a Chemical Sensor that used a quartz crystal as the detector. The idea was that certain aerosol chemicals would attach themselves (selective absorption) to the quartz crystal surface and change the surface mass, and thus the resonate frequency. The crystal was placed in a oscillator to measure the resonate shift.To compensate for temperature, aging, power supply and whatnot, two identical crystal sections were used, made from the same quartz blank, each was employed in an identical oscillator. Identical here meaning as close to the same as practical, but only one of the crystals was exposed to the aerosol. The two oscillators outputs were mixed together to produce a difference frequency output, as the selective chemical(s) were absorbed the magnitude of frequency difference would increase which was detected.The scheme worked, and they could coat the quartz crystals with special substances to absorb different select chemicals. However, moisture became an issue, and don't recall if this was ever resolved.The project was part of the USG Chemical Agent Detection Agenda with a goal to detect/protect against Chemical Agent attacks, like our XM21 Remote Sensor highly successful program.Anyway, hope this might help with your project since we don't know what you are attempting to do.Best,

author: Sengcid
author_rank: Regular Contributor
timestamp: July 14, 2022, 04:19:51 pm
content: Have you considered a simple 1:1 ratio transformer bridge? The ASL F25 configuration will do it (see attached).You will need to change three “jelly bean” op amps to do better than the Johnson noise limit.A mass-produced miniature transformer would probably be OK (e.g. 240V primary and 12V + 12V secondary; Not centre tapped) – better than 1ppm stability but not absolute accuracy. This will reduce your requirement to 100ppm (14 bit) for measuring current and ADC (for measuring the out-of balance). You can easily check the accuracy and stability of the transformer by subtracting one secondary voltage from the other (see fig. 2).With an extra (tapped) IVD or precision resistive divider you could get closer to a null balance and reduce your requirement to 12 bits, easily done with a low cost ADC or even a V/F converter, just like the F25. With two measurements (either side of null balance) you can eliminate any change in the measuring current.I recommend an operating frequency of around 400Hz and a capacitor across your reference resistor to balance (approximately) the quadrature component (cable capacitance). You should be able to achieve a 10Hz bandwidth.If/when you get closer to market/volume production I recommend you talk to Paul Bramley of Metrosol Ltd. There is no point in re-inventing the wheel.More theory can be found in my collection of monographs. Herewith a link and contents attached.https://drive.google.com/folderview?id=1TbomXeoBbIe-IVADaOOmyyLH9le-3wAt

author: luky315
author_rank: Regular Contributor
timestamp: July 25, 2022, 06:05:28 am
content: Thanks a lot!

author: Kleinstein
author_rank: Super Contributor
timestamp: July 25, 2022, 07:51:45 am
content: The idea with a AC driven bridge and transformer is good. I think there is something wrong in the ASF25 circuit however, and a center taped transformer would also to the job.For the detection one has the option to use an analog phase sensitive rectifier as shown for the ASF25 or use a somewhat faster ADC and do the rectification / demodulation in software.

author: tszaboo
author_rank: Super Contributor
timestamp: July 25, 2022, 07:59:53 am
content: The AD7124-4 is a good choice, it will measure the RTD and your chemical sensor nicely.You probably want to order a very nice hermetically sealed Vishay foil resistor for it (from VPG, not Vishay), depending on your temperature range, this should be 2KOhm or something like that. Don't concentrate on standard values, they will make resistors for order for you. Probably the VH series, but it's better to ask them what they recommend. And probably it's easier and cheaper to just order the best that you can, instead of trying to come up with a "good enough" resistor for it.