How to read 2mV/A accuracy - Page 1
Forum section: electronics / beginners

author: samshabz13
author_rank: Newbie
timestamp: July 21, 2025, 05:57:10 pm
content: Hi all. I have got a current sensor - hall-effect based - that outputs 2mV/A sensitivity analog differential output, up to 1000A and bipolar. My ADC is powered by dual +-2.5V rails, so it can directly interface with the signal+ (+-2V) and signal_cmn. I employ an RC filter - with either 50,500,2000Hz cutoffs (depending on which jumper is shorted). The entire circuit is isolated via a separate ground plane, an isolated 5V supply, and no traces in or out the zone except at the dedicated digital isolators (SPI for the SPI, and an I2c isolator for !drdy). It uses the ADS1220 as the ADC for very high accuracy. This device is going in an electric vehicle for context, and the 12V vehicle supply is regulated into a cleaner supply with the CCG30-24-12S, before being isolated and regulated to 5V specifically for this zone. Am I missing anything? Will I achieve this accuracy (preferably around or under 1 mV accuracy).

author: jwet
author_rank: Frequent Contributor
timestamp: July 21, 2025, 10:37:01 pm
content: You might but probably not.Also, if you think about it, you don't really want to read 1 mV, you want to read .5 amp out of your hall effect device- correct? You haven't mentioned anything about the hall effect sensor. Its sure to have an accuracy spec associated with it as well as a noise spec which will dominate at these low frequencies (1/f and all that). I would imagine if you got an excellent hall effect sensor it could be .5% of FS accuracy- this would be about 5 amps or 10 mV. The noise is like on the same order and will dominate at this bitter end of scale, like another 5 amps of uncertainty.You also are just using the standard internal reference which has 30 ppm max drift, 5 typ. The converter will produce 20 bit numbers at very low data rates- this is approx 1 ppm but they won't mean much. The reference will typicallly show a 5 bit/degree C change (30 bit/degree C max). For something automotive, you'd probably want to go from -20C to +60C, this is 80C or about 400 bits of temp drift, this is like a loss of 9 bits. This isn't fatal but you're left with an 11 bit result with just this error term, which means that you'll per getting 1/2000 type readings best case.I am guessing you're looking at about 10 amps of uncertainty and a real useable minimum current around 20 amps.In order to do precision measurement like this, you have to go through a detailed error analysis and define error budgets for subsystems.( spreadsheets work nice for this). Just layout what you know, estimate what you don't and compare to your target. There are plenty of examples online, in texts and App Notes. Using a good external reference for the ADC would certainly be required. Doing an analysis for the what the current sensor can do is important too. Its likely that you'll have to have three or more current sensors and shunts with big relay bypassing to do reasonable job over this range. Do you have an expectation for what you want. Its sound like you'd like to resolve .5 amps, there is little possiblity of this with the current approach. Maybe you can relax the specs or get fancier.Good luck

author: samshabz13
author_rank: Newbie
timestamp: July 21, 2025, 11:18:07 pm
content: Hi JwetThank you for the detailed reply. Yes, the GMW clip on current sensor only has a 1% accuracy. so thats 10A on 1000A range. I am also using a fluxgate sensor that outputs CAN, giving me +/- 0.3%. This is just for scenarios where a light, rugged sensor is required as opposed to one that you mount - sacrificing some accuracy. My understanding was that their 1% would mean when the input is 1A then the device shall output 2mV +-1% of that (not 1% of 2V). Which would mean I need to at least be reading below 2mV if I want to read those low currents. I did test it out and it does seem to output decently (I had no true reference) at the lower range. I suppose I am happy to take a pay cut in terms of accuracy. I think -20 to 60 is not what I am expecting, in South Africa I'd expect 0 to 40 in most cases. And probably most of the time wed be in 20-30 degrees, so +-5 celsius. And we must consider the temperature reference is not at the endpoint, but at 25 celsius. Then its +-25ppm or 0.05 A resolution (15 bits or so). Plus some offset error. But this is really wishful thinking. And I am beginning to understand your point of estimating what I dont know, because there seems to be a lot of uncertainty. I think for reasonable accuracy this solution is okay? Or do I need the internal reference. I suppose I just need to justify what I am doing for my Masters, not necessarily have the perfect system.

author: jwet
author_rank: Frequent Contributor
timestamp: July 21, 2025, 11:33:27 pm
content: You're welcome. Good luck with your project.Accuracy is generally defined as a percent of full scale not percent of current reading. Its hard to see a lot of these errors without some careful experiments. There is a lot of confirmation bias, its hard to believe how hard measurement can be.Temperature is highly dependent on where this box will be located and a lot of mechanicals. Zero to 40 might be reasonable but under hood gets hot as does big conductors carrying big currents in enclosed spaces.Since I responded here, I went off and googled some stuff. Its a tough problem but it all comes down to what you need. No one talks about noise much. I started the Automotive group at Maxim around 2000 and we built customs for LEM, one of the big packaged automotive sensor suppliers. This was for start/stop hybrids mostly. We did battery management for Tesla too. Mostly looking at huge stacks of cells. Some of these ELM sensors directly output CAN or 485 (internal converter). Might be worth a look. Digi-Key sells some of them but they're generally over a $100 in onesies.If you have some sensors, do some tests. You can get a feel for the accuracy and noise. Lash up a board with your ADC and an Arduino banging on it and get some data. You can tell a lot with your freezer- usually pretty close to 0 degrees C. Put it in something so it won't get condensation all over on exit.Have fun.Good luck.

author: samshabz13
author_rank: Newbie
timestamp: July 21, 2025, 11:48:19 pm
content: Thanks Jwet. Yes, I believe I shall send it out, give it a test and work with what I get, I think it's roughly alright for decent accuracy as is? In fact yes, the other current sensor I am using, the Fluxgate sensor is the LEM CAB SF-006, and it outputs CAN too! That sounds super interesting, what an exciting journey.I suppose I should ask you one last question, I am keen on some entrepreneurial ventures after I finish up with my Masters, do you have any good advice - in entrepreneurship or otherwise?

author: jwet
author_rank: Frequent Contributor
timestamp: July 22, 2025, 12:10:58 am
content: I think entrepreneurship is great but not as a first job. I would go to work for medium sized, profitable, well staffed companies with good internal processes for their engineering and plenty of mentors. Bust your ass, take on challenging assignment and do your best. The goal is to learn. Avoid getting shunted off into anything but mainline design (avoid R&D, Test, Manufacturing, Sales, etc.). After 10 years of very solid, design work, you're ready to go explore small companies and entrepreneurship. Remember that you're an incredibly valuable person in this position. I worked 12 or so years for a couple of instrument developers, first for civilian use in power plants and then military stuff- both were design type jobs and fun. Designing hardware tends to turn into a lot of long hours- things never work quite the way they should. I then went to Maxim in my mid 30's and moved up the ranks- retiring as a Managing Director- two levels from the CEO. I've started some little side businesses that have made some money but have mainly stuck to companies. I had a wife, kids and a California mortgage- my wife mostly stayed home after our kids came- a decision we made together.Good luck and enjoy the ride.Use the thanks button for thanks, its my way of measuring my impact.

author: samshabz13
author_rank: Newbie
timestamp: July 22, 2025, 12:44:17 am
content: Thanks Jwet, that advice is incredible, I will keep it safe. All the best !