Test fixtures for impedance measurement in the mHz to MHz range - Page 1
Forum section: electronics / projects

author: jogri
author_rank: Frequent Contributor
timestamp: June 29, 2023, 12:52:45 pm
content: I want to measure the impedance of CR2032 coin cells (Nyquist/Bode plots, electrochemistry as part of my job) and for that i need some test fixtures to hold the cells. As i haven't really done AC measurements before i don't really know what i have to be careful about in the MHz range.Usual parameters: 100 mHz to 10 MHz range, 10 mV, usually ~100 kOhm impedance (but that can range from 100 Ohm to MOhms)My first idea was Kelvin clips: How good are they in the MHz range? For most of them the manufacturer seems to list an upper frequency limit of 100 kHz. What is the limiting factor there?Is it better to use a 2 wire or 4 wire approach for this? I've found the Keysight Impedance Measurement Handbook, and they're a bit vague about that.Also, my "problem" is that i need a rather large frequency range: high kHz-MHz to look at interfaces and 1-100 mHz for diffusion processes. So ideally the test fixture should be good and reliable at both frequencies.The main question is therefore: What kind of test fixture do i have to buy/build to get reliable data? Or if you can recommend a good book or paper that would also be great.

author: jwet
author_rank: Frequent Contributor
timestamp: June 29, 2023, 01:23:57 pm
content: I would build or buy a low current DC load simulator to DC load the battery- you'll want AC measurments at different loads- the connection to the battery should be large inductance to keep the AC out of the load. At coin cell currents, this could be an op amp and a FET or just some R's. Always use kelvin sensing if you can- it never hurts. Put the battery in a standard coin cell holder- Keystone, etc. I would then AC couple the battery positive using a cap having a very low impedance at your lowest test frequency to a Vector Network Analyzer that that has good LF span- this cap might be 1000 uF. Getting to your low frequency is a challenge- the old HP4195A VNA could go down to 10 Hz. .1 Hz is pretty low- you might have to roll your own VNA using something like an Digilent Analog Discovery. Their software has some VNA and Bode analyzer type modes.Primary lithium batteries have some odd behavior especially after prolonged storage. They have a spec called voltage delay where they kind of burn off the oxides formed during storage. These oxides are what give them the long shelf life. I would precondition batteries to get rid of this voltage delay or it will be in the way of measurements.Another couple of tips- Duracell has amazing scientists and apps people that you can get a hold of if you've got a serious application. It may take some work to get to them. The other resource that I've used is the US Army's ETDL Battery Lab at Ft. Monmouth, NJ. They know a lot about about batteries and have a lot of pubs detailing a lot of battery esoterica. Finally, there is a great big book called Handbook of Fuel Cells and Batteries that you shouldn't be without. Its about 2000 pages probably- there are likely 100 pages on coin cells.Have fun.

author: jogri
author_rank: Frequent Contributor
timestamp: June 29, 2023, 02:26:51 pm
content: Thanks for the reply, the book recommendations are great.But i probably should've written a bit more about why i'm doing impedance measurements: I'm building my own cells as i'm looking at the behaviour of the electrolyte in new kinds of batteries (mostly sodium or potassium based solid state cells), so usually it's some solid electrolyte sandwhiched between two potassium electrodes. I already have the measuring equipment needed for impedance spectroscopy, namely some Novocontrol and Solartron FRAs (instrument limitations are no concern at all for me).The goal of doing impedance spectroscopy is developing an equivalent circuit model to describe what's happening in the cell. But as i will also measure the effects of the test fixture on the signal i need to minimize that effect. I can (and pretty much always have to) develop an equivalent circuit to describe the series inductance and parallel capacitance of the test leads/fixture and subtract that from my measured data, but if that's larger than the response of the cell i just get bad data/noise.So i need to build some sort of test fixture that ideally has only a small, predictable influence on the measurement.-For coin cells, nothing really exists since most instruments aimed at people doing battery research only go to 10-100 kHz-For general research, there are holders that go to tens of MHz:https://www.novocontrol.de/php/sa_cell_overview.phpProblem: Not that great if you want to test multiple cells, so i want to avoid that if possible.And so far i haven't really found any documentation about how to build test fixtures, so i have no idea what works and what doesn't...

author: mawyatt
author_rank: Super Contributor
timestamp: June 29, 2023, 03:39:29 pm
content: Quote from: jwet on June 29, 2023, 01:23:57 pmLong ago we were giving a presentation for CERDEC at Ft Monmouth on the "doomed" JTRS radio system, our solution was a core custom chip set common to all JTRS radios available to all JTRS suppliers (long story if others are interested). Our presentation was centered on designing the core radio chip set to accommodate the battery technology rather than the other way around, since the battery technology dictated the single largest component in the handheld versions, and was the usage limiting factor. Our approach was to give the battery folks a clean slate to design the best battery based upon the best technology for the missions and we would design the core chip set to accommodate said battery technology.Some of the Ft Monmouth Battery Lab folks were in attendance and resonated with what we presented. After the presentation they came by and offered to take use for a private tour of their battery facility. Very impressed with what we saw and their research, and understood the the danger of various battery technologies as a colleague lost a friend to an early battery technology explosion.Recall they had a fire that destroyed much of the Battery Labs at Ft Monmouth, but thankfully no one was hurt.Also had an opportunity to work with some very "special batteries" (not from Ft Monmouth) that were so sensitive they had individual serial numbers, had to be locked up in certified containers, signed in/out when accessed, in certified facilities, only available to "preselected" folks, and shipped and returned by "special courier".So have the utmost appreciation for battery technology and the folks involved, altho our experience with Duracell hasn't been good, but that's another story.Anyway, to the OP a likely means to measure that covers most of the desired range in an LCR meter such as the Hioki IM3536, which covers 4Hz to 8MHz. They and others have many different type fixtures that could be employed, maybe modified to fit the desired battery configuration. One such fixture is the SMD fixture used for measuring SMD devices, a modification of the plastic SMD device holder between the plungers would allow the battery to be inserted vertically and the plungers to engage the front and back of the battery directly, thus avoiding any cabling and such. This provides a true 4 wire measurement and a simple metal disc (maybe a coin) could serve as a short for calibration.Best,

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 29, 2023, 04:47:55 pm
content: +1 on the SMD fixture. Though it's stupidly expensive for what it it, this is a good place to start-https://www.keysight.com/us/en/product/16034G/smd-test-fixture.htmlThe frequency limit is 3 MHz, but 110 MHz if compensated. In case you thought there was some exotic shielding or plumbing inside, here's a photo-

author: mawyatt
author_rank: Super Contributor
timestamp: June 29, 2023, 05:25:58 pm
content: We did a quick removal of the SMD fixture plastic holder on a cheap no name LCR fixture. Then did a plot with our IM3536 and Custom DC Bias fixture to isolate the battery DC wise. Also use our custom Python plotting routine for the LCR meter.Seems to work, here's the results.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: June 29, 2023, 05:29:10 pm
content: Additional information, plots to 8MHz. Note how the battery (maybe fixture also, but did a full Open and Short cal with everything in place), becomes inductive (phase goes positive) after ~1MHz.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: June 29, 2023, 06:07:55 pm
content: Quote from: Conrad Hoffman on June 29, 2023, 04:47:55 pmThis cheap one is similar, likely a direct copy of one of the better known OEMs. We've added the ground straps and of course there's no "spec" on the frequency range or anything else, but at least they bring the "Kelvin" leads to a point at the attachment to the plunger. This is where one needs to look if getting erratic results, as we found the attachment is my means of a brass threaded bolt into the plunger which was loose.Best,

author: jogri
author_rank: Frequent Contributor
timestamp: June 29, 2023, 06:37:39 pm
content: Oh yeah SMD fixtures sound good, have to get one or two of those.@mawyatt: Do you by chance have some Kelvin clips that you could measure the same battery with? I always wondered just how bad those are in the MHz range. (as for the wiring: if they are true Kelvin clips wire one side of each clip to high and the other to low)So i guess i'll just use SMD fixtures when i really need to go to 20 MHz and just see how good this holder is when it arrives:https://www.biologic.net/accessory/cch-8/The ultimate goal is to hook the impedance analyzers up to an 8 channel multiplexer (that goes into a temperature chamber), it would be ideal if that holder for 8 batteries is useable to 20 MHz, but i guess i can always pop a cell into the SMD fixture for a quick 20 MHz measurement.Thanks a lot for the helpful advice

author: mawyatt
author_rank: Super Contributor
timestamp: June 29, 2023, 07:25:02 pm
content: We don't have any Kelvin type clip leads that are rated beyond 100KHz, and experience has shown even at ~100KHz results are highly dependent upon lead orientation and physical stability (read move something and all bets are off!!). That's why we recommended the SMD fixtures, they are mechanically stable and thus less prone to physical induced errors. Even a cheap SMD fixture like we've shown likely will outperform an expensive set of Kelvin clips at higher frequencies.Even our DC Bias fixture shown has some uncertainty due to the additional length involved, and as such wouldn't place too much confidence in higher frequency measurements, so one option would be to acquire a cheap SMD fixture, modify it to allow isolating the battery DC voltage by inserting series caps in the Hcur and Hpot ports. Since you are measuring a battery, leakage probably isn't a big issue and you could use a quality tantalum, or polymer capacitor rather than a film type, just pay attention to the polarity.Best,

author: wilhe_jo
author_rank: Regular Contributor
timestamp: June 29, 2023, 10:16:33 pm
content: 10MHz isn't really high frequency... However, you could make something coaxial (if costs are not your issue).So you could take your coin cell diameter and figure out some coax with PTFE, PET or even air as a dielectric. That would incorporate the coin cell on the inner conductor. Then it would be easy to come up with tapers to get everything down to a size compatible with N or BNC.Now you could get the coax terminated on one side an you could add some resistor to the other side. As a load. Now add some big capacitor as a dc- block and you can measure the impedance with a VNA (possibly, the dc-block and the load would call for some numerical massaging...).Taking this route should get you in the 100MHz region with ease. To get down in the mHz range, I'd guess taking current and voltage samples from some pulse and doing some curve fitting would be the way to go.Regards

author: jwet
author_rank: Frequent Contributor
timestamp: June 30, 2023, 03:25:36 am
content: I'm sorry- I completely missed the plot.As far as fixtures go, though these may seem high to you, these are close to DC really. Real AC effects other than just parasitic C's and L's are caused by transmission line type effects and don't really become even a noticeable issue until you get to some real fraction of a wavelength- lambda/100 perhaps for the real onset. A wavelength at 10 Mhz is 30 meters, you won't see much effect until you start to span a about a foot. I would fabricate something small and fairly stable mechanically with high conductivity, gold plated electrical contacts if possible. Skin effect does comes into play at these frequencies, it would be about 20 um- < .001 ". Use find stranded wire interconnects for lowest loss. This is a concern mainly at the top of your range.The process of backing out a fixture like you're talking about in the RF test world is called "de-embedding" if you want a good google search word. Many modern network analyzers can de-embed fixtures (parastic C's and L's automatically and line extensions). If yours can't , in the olden days, it was something you might do in a spreadsheet or in the really olden days in HP Basic on your lab HP-85 instrument controller.Good luck with your testing.

author: mawyatt
author_rank: Super Contributor
timestamp: June 30, 2023, 01:59:59 pm
content: Quote from: jwet on June 30, 2023, 03:25:36 am10MHz wavelength is 30M in air, and ~20M in most common dielectrics (Er 2~2.5). Lab LCR meters are often 0.05% accurate, (IM3536 is 0.03% for phase), so 0.05% of +-180 degrees phase at 10MHz equates to ~9mm in air and ~4.5mm in common dielectrics. Some lab meters even have a cable length compensation parameter.Careful calibration is required to achieve a given instruments capability and why many of the lab LCR meter OEMs offer special fixtures (often expensive) where all the signals are contained within mechanically & electrically stable environments.General purpose cables are noted for having varying characteristics with stress induced bending and/or temperature variations, so cable handling and arrangement becomes important and why we mentioned the usual Kelvin type cables are difficult to rely on as frequency increases, our Tonghui TH26011CS Kelvin clips are specified for use below 100KHz.We've even gone to the effort of taping the Kelvin leads to the workbench in attempts to achieve better repeatability, and why we acquired Kelvin type tweezers TH26009 which are rated to 15MHz and overall better, but still not as good as a dedicated SMD fixtures mentioned.Anyway, not knowing the OP test details and what's expected, maybe +-20% measurement uncertainty/repeatability is acceptable, but without such a general statement like above can be misleading. In our case we've certainly gone well below these levels, and the lab grade LCR OEMs must think so as well since they offer the various LCR meter fixtures which are helpful in minimizing measurement uncertainly. As alway YMMV!Best,

author: jogri
author_rank: Frequent Contributor
timestamp: June 30, 2023, 03:45:53 pm
content: I'm doing chemistry, so accuracy is not that important for those setups. For a start, i build the batteries that i measure myself, so i might have something like 10% deviation between samples.The absolute value that i get is not that important (the main numbers that you publish in papers are specific conductance and activation energies, and you round them to 2 digits), but the setup has to be repeatable with a small deviation between measurements (so if i'm measuring 10 cells to demonstrate the repeatability and the values change drastically after five cells because i looked at the setup too hard it's not good).Some of the instruments that i use are quite accurate, but as far as i can tell that's more of a happy accident than a main design feature as you need extremely high phase angle and loss factor resolution-> you need good ADCs anyway.If anyone is curious, the instruments that i use arehttps://www.novocontrol.de/php/ana_alpha_n.phphttps://www.ameteksi.com/products/frequency-response-analyzers/1260a-impedance-gain-phase-analyzerand normal impedance analyzers like the HP4192.But as we're on the topic of cables/test fixtures anyway: My current plan is to connect multiple coin cells to a single analyzer via a multiplexer (i have to demonstrate repeatability with 5-10 cells and manually changing the cells every hour for a measurement gets boring really fast).My plan is to use 2 HP3499A multiplexers with four N2268A 1 to 4 multiplexers each and a N2262A card (only solution that only requires 2 instruments) as a 4 wire multiplexer for 8 cells.For space reasons i bought RG178 as all eight cells have to fit into a cooling bath (temp chamber) and that's 32 BNC cables. To hold the cells, i'm getting a coin cell holder with Kelvin connections and if that doesn't work in the MHz range i'll probably resort to buying 8 SMD fixtures.Does that sound like something that could work? My plan is to match the cable lengths for all 8 cells and use a single calibration for all of them. And i guess i'd have to fixate the cables (can i heatshrink them or put them into a tube or do you have better solutions)?And a big thanks to all of you for the help so far

author: mawyatt
author_rank: Super Contributor
timestamp: June 30, 2023, 05:32:38 pm
content: That's why we mentioned "repeatability", often folks mistake accuracy needs as crude but usually repeatability and accuracy go hand in hand in electronic measurements, so one may require an accurate/stable setup/instrument to achieve the desired repeatability, even tho the overall accuracy isn't demanding.I would worry about the cables/connectors since you are also doing temperature excursions, something like teflon based double shielded RG142 might serve better than teflon based RG178, or maybe SS402 or LL142. Not sure what the better connector types are regarding temperature, would expect BNCs aren't that good tho.You'll probably need to do an Open/Short frequency dependent calibration across the temperature range to get acceptable repeatability. The short could be a "dummy" metal plug machined in the size/shape of the CR2032.Anyway, sounds like you've got some good equipment available, done your homework, and with a carefully crafted fixture & setup should yield good repeatable results. Keep us informed of your measurements, as we were curious to find the Toshiba CR2032 battery we quickly tested to have such a high internal impedance, so already learned something useful from our simple endeavor!!BTW the SMD fixture we've shown is a cheap knock off because were/are doing some physical modifications, and didn't want to hose up a quality fixture from Hioki, Tonghui, HP or Keysight. You can find these knock offs everywhere for ~$60, so even using 10 isn't that expensive. Not sure how well they would hold up under changing environment temperature tho, something we haven't considered, nor experienced, having only localized heating of the DUT and fixture plungers with a power resistor.Best,

author: jwet
author_rank: Frequent Contributor
timestamp: July 01, 2023, 03:22:07 pm
content: The switches and mux controller are very, very good- probably overkill but fine. With the MUX, etc, you'll want to keep wire length constant if possible to keep phase related measurements meaningful. Again- at these frequencies, matching to a few inches would likely be unnoticeable. As said, I would do a VNA type cal with Shorts and Opens at least and port extension if you're not at the DUT. I think you've got it.