ADC frontend/mux design for current sensing - Page 1
Forum section: electronics / projects

author: RossS
author_rank: Contributor
timestamp: December 03, 2020, 03:33:01 pm
content: I'm working on a design that feeds 8 pairs of sense lines from current shunts into an ADC. Shunt voltage is as low as ~100uV (20V common mode), and I'm thinking through the frontend options.Ideally I'd use separate differential amps right at each shunt and then mux the outputs into the ADC. Since cost is a factor, I'd prefer to mux the sense lines into a single high performance amp. I don't have a lot of experience here though, so I'm trying to understand if this is a workable approach. Is my biggest concern the added error from the mux resistance, and do I compute this by multiplying it by the input bias difference for the op amp? Should I be worried about thermal EMF? Are there other issues to watch out for? Is this problem generally solved a different way entirely?Thanks!

author: Kleinstein
author_rank: Super Contributor
timestamp: December 03, 2020, 03:58:59 pm
content: There are special high side current sense amplifiers, kind of specialized differential amplifiers that do a voltage to current conversion and than back to a current at the ADC with a resistor.Thermal EMF can be a factor, especially directly at the shunt, as some alloys (e.g Konstantan) have a relatively high Seebecke coefficient.With shunts for higher currents there is also heat and this can also effect the resistance, causing nonlinear readings.For large currents (e.g. > 10 A DC or > 1 A AC) magnetic / inductive sensors may a better alternative than a simple shunt.CMOS switches are quite good, so first switching and amplification only than is real option especially with resistors that are not very large.Are the shunts separate, or are there some in groups for different ranges ?Which current range and what accuracy is aimed for ?

author: exe
author_rank: Unknown
timestamp: December 03, 2020, 04:11:29 pm
content: I'd suggest use a floating power supply for shunt sensing amplifier. This way you can sense from ground, so any good/auto-zero/chopping/whatever opamp will work.When I had a similiar application, I did research on available options, 100uV is pushing to the limit of technology. Although, if voltage doesn't go down to zero, there are some nice current sense amps from LT (AD). One of best I found was LTC6102 , but check other parts too:https://www.analog.com/en/parametricsearch/11082#/sort=4501,asc(sorted by offset).Watchout for offset, offset drift and common-voltage rejection.Disclaimer: I've never used this parts, I only did "paper" research.PS as of MUX, idk. If you use relays and isolated ADC, then it's easy, but relays are bulky, expensive and may cause short if activated simultaneusly. Otherwise idk, I don't think there are muxes with isolated inputs.

author: fcb
author_rank: Super Contributor
timestamp: December 03, 2020, 04:13:13 pm
content: If your scanning speed is slow (few Hz to perhaps few hundred Hz) and cost is an issue, then I'd try muxing them directly into an instrumentation amplifier.Probably something like DG413's - if you use a symmetrical design then seebeck effect could be minimised. Charge injection from the mux will upset your instrumentation amplifier, so watch for settling time, pay careful attention to protecting the ADC front end.

author: lordnoxx
author_rank: Contributor
timestamp: December 03, 2020, 04:35:18 pm
content: In a project I had to sense 3 phase currents. Thus only 3 pair of sens lines. Nevertheless here is the hardware I used:- Sense resistor wsl3921-150µR (2x 300µR in parallel):https://www.vishay.com/docs/30110/wsl3921-wsl5931.pdf- Isolated current sensor/aplifier:http://www.ti.com/general/docs/suppproductinfo.tsp?distId=10&gotoUrl=http%3A%2F%2Fwww.ti.com%2Flit%2Fgpn%2Famc1302-q1- Differential to single-ended amplifier:https://www.analog.com/media/en/technical-documentation/data-sheets/AD8226.pdfAt the microcontroller I just used 3 single ended ADC-Channels.Hope that helps!

author: RossS
author_rank: Contributor
timestamp: December 03, 2020, 06:00:51 pm
content: Quote from: Kleinstein on December 03, 2020, 03:58:59 pmThanks, I’ll take a look at these.QuoteThe shunts are all separate. Current is 0-8A DC, aiming for ~50mA accuracy. Accuracy is less important than overall power loss though, which is why I’m looking at low value shunts.

author: exe
author_rank: Unknown
timestamp: December 03, 2020, 06:10:24 pm
content: What's shunt resistance?

author: David Hess
author_rank: Super Contributor
timestamp: December 03, 2020, 06:21:44 pm
content: Quote from: RossS on December 03, 2020, 03:33:01 pmThat is how to calculate it however offset current should be so low that any added voltage offset is insignificant.QuoteIf your shunt voltage is 100 microvolts, then thermal EMF will be a significant source of error.QuoteI might include two extra multiplexor inputs with one shorted and one with a reference voltage to test for and remove remove input offset errors and check calibration.

author: RossS
author_rank: Contributor
timestamp: December 03, 2020, 08:06:57 pm
content: Quote from: exe on December 03, 2020, 06:10:24 pmIdeally 1-2mOhm, and at most 5 before I have to make tradeoffs elsewhere in the design.Quote from: David Hess on December 03, 2020, 06:21:44 pmAh, great idea, thank you!