How to deal with cap piezoelectric effect  - Page 1
Forum section: electronics / projects

author: thereza
author_rank: Contributor
timestamp: April 18, 2013, 03:04:24 am
content: I'm working on a board that needs to measure small (<1uV) voltages. I just learned, the hard way, that caps exhibit a piezoelectric effect and flexion of the PCB causes these caps to generate a voltage which messes a lot of stuff up (the pcb is now an accelerometer). I read that NPO and CG0 caps are not piezoelectric, but their values do not go very high. I was wondering if anyone had any thoughts/suggestions.Thanks,Rezap.s. how do you make all the animated icons stop? they are making me dizzy as I write this post.

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 03:07:22 am
content: You can easily get C0G up to 10nF - what are you doing with <1uV voltages that requires a higher capacitance than that in the signal path?Quote from: thereza on April 18, 2013, 03:04:24 amAll I can say is to scroll up or take Dramamine before visiting EEVblog! Some browsers will freeze animated GIFs if you hit "stop" (Esc key). Google Chrome appears not to.

author: thereza
author_rank: Contributor
timestamp: April 18, 2013, 03:51:57 am
content: There are a couple places. I have a midrail voltage that's generated off of a resistive divider (2x100k - will try 10k) connected to a 10uF cap then an amp. This midrail voltage is used all over. Also, a precision 3v reference has a pin that lets you connect a cap to the internal bandgap - this is injecting noise. My output stages have a 100hz lpf made with 1uf caps. So I use them all over.Those are the main ones I've identified so far. I've found a .47uF NP0 cap (http://octopart.com/partsearch#search/requestData&q=06034W475KAT2A) but not sure where I can buy it.Any other suggestions?Reza

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 03:55:55 am
content: 470nF C0G 50V. Expensive as all hell though. I'm not finding any bigger than that in stock anywhere.

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 04:05:18 am
content: Though with voltages under 1uV, I don't think you're ever going to fully eliminate effects like that. Note that many of the really high-end voltmeters use a separate board with mounts designed not to transmit flexion (Goddammit Chrome, how do you not know that's a word??) to the voltage reference. You're going to have to make special considerations to dampen vibrations on the board. You'll have a lot of other nasty effects playing with your measurements, too.

author: thereza
author_rank: Contributor
timestamp: April 18, 2013, 04:19:29 am
content: It's not so bad when the board is in the enclosure, but the connector transmits movement to the board which is annoying. I'm going to see if there is anything I can do about it.Would I expect the noise to be any less with smaller components (sized) or smaller values?This is a long shot, but what about mounting the cap (0805) on the end (tombstone wise) and having the board shop do rework by adding a wire from the top to the other pad? Or how effective would it be to make a daughterboard to mount the cap and then just mount that to the main pcb (via the 1/2 via edge connectors)?Reza

author: thereza
author_rank: Contributor
timestamp: April 18, 2013, 06:11:08 am
content: Found this -http://www.analog.com/static/imported-files/application_notes/AN-1099.pdf- seems that tant caps might be my solution. -r

author: Unknown
author_rank: Unknown
timestamp: April 18, 2013, 12:40:23 pm
content: Make sure you keep below there voltage rating, otherwise the tendency is failure, and going leaky.

author: EEVblog
author_rank: Unknown
timestamp: April 18, 2013, 12:47:31 pm
content: Quote from: c4757p on April 18, 2013, 03:55:55 amI can remember designing precision stuff stuff when 10nF NPO was rare as hens teeth!

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 12:52:15 pm
content: Quote from: EEVblog on April 18, 2013, 12:47:31 pmHell, I'm still a bit amazed every time I buy 1uF 0805 caps, despite doing it all the time...

author: marshallh
author_rank: Unknown
timestamp: April 18, 2013, 03:30:00 pm
content: Also consider milling slots along the length of the cap to relieve torsion

author: PChi
author_rank: Frequent Contributor
timestamp: April 18, 2013, 06:27:15 pm
content: My suggestion is to use a platic film capacitor which are much better.

author: SeanB
author_rank: Super Contributor
timestamp: April 18, 2013, 06:36:14 pm
content: Also consider vibration damping, using a flexible encapsulant, along with a set of rubber mounts like those used on CDROM drives to mount the laser board. Along with piezoelectric effects you get extra fun if one side is warmer than the other. With the caps consider tantalum, and if you want low noise use glass sealed ones, though you may need to sit down when you find the cost of them.

author: thereza
author_rank: Contributor
timestamp: April 18, 2013, 06:45:04 pm
content: Quote from: c4757p on April 18, 2013, 12:52:15 pmI bought some 10uF 0402 caps for this project. And not outrageously expensive. That blew my mind.

author: thereza
author_rank: Contributor
timestamp: April 18, 2013, 06:46:40 pm
content: Quote from: PChi on April 18, 2013, 06:27:15 pmI don't think these are available as SMT components, are they? I can get drop in replacements for most of the components using tant caps. Just 100x more expensive :/

author: KJDS
author_rank: Super Contributor
timestamp: April 18, 2013, 07:10:08 pm
content: Plastic film caps are available in SMT, just not that common.

author: free_electron
author_rank: Super Contributor
timestamp: April 18, 2013, 07:57:31 pm
content: a couple of alternatives : oscon , tantalumsamsung and tdk have specially stacked ceramics to combat microphony ( which is what you are experiencing ) the elctrodes in the cap are stacked asymmetrically in the body. the caveat is that you need to know how to install the cap on the board. manual assembly takes great care. lifting the cap out of the carrier tape you need to maintain the position. if you drop it or flip it the electrode position will be wrong !i have that pizeoproblem continuously in harddisks. it messes up the position loop. filmcaps / c0g are the solutionThere are other ceramic caps that are NOT bariumtitanate based. BaTi is what causes the problem . this is a piezoelectric material. some capacitor makers (tdk , murata) have high value ceramics that are not baTi based but they come in larger packages. BaTi can be deposited very thin , hence the large capacity density.

author: thereza
author_rank: Contributor
timestamp: April 18, 2013, 08:14:26 pm
content: Quote from: free_electron on April 18, 2013, 07:57:31 pmSeems OSCON caps have large packages, I would like to avoid respinning the pcb if possible (need some 10uF in 0805 and 1uF in 0402). I think tantalum is the direction I will take, though they have higher ESR. Typically, I read that you want low ESR in the output stage of LDOs, switchers, etc.. I need one on connected to a voltage divider before an op amp, One connects to the bandgap of a voltage reference (TRIM pin of REF5030), and I use a couple in low pass filters on the output of an opamp. Do I need low ESR for these applications?I also would like to look into the special ceramics you mentioned, but not sure what to look for - do they have a special name?Thanks,Reza

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 08:57:54 pm
content: Quote from: thereza on April 18, 2013, 08:14:26 pmSwitchers, yes. I've seen LDOs destabilized by the use of atoolow ESR. Many of them don't just want "low" ESR, they want ESR within a certain range.QuoteThis will just slightly alter the frequency response of the filter. Easiest way to solve that is to just run an AC analysis of the filter in LTspice or similar, including the cap ESR, and see if the response plot is acceptable.

author: PChi
author_rank: Frequent Contributor
timestamp: April 18, 2013, 09:10:47 pm
content: I tried some ceramic capacitors that the supplier claimed to have reduced microphony but I couldn't see much difference from a crude tap the PCB and listen to the audio test but that was a long time ago. Perhaps they are better now.The TDK web site has some good data on how the capacitance varies with voltage. The 10 uF in 0805 might not be 10 uF on the output of the LDO anyway.

author: kt315
author_rank: Regular Contributor
timestamp: April 20, 2013, 07:43:20 pm
content: Quote from: thereza on April 18, 2013, 03:51:57 amYou do not think you can use 4000 of them?From:http://cds.linear.com/docs/en/datasheet/6655fb.pdfFor very low noise applications where every nanovoltcounts, film capacitors should be considered for theirlow noise and lack of piezoelectric effects. Film capacitors such as polyester, polystyrene, polycarbonate, andpolypropylene have good temperature stability. Additionalcare must be taken as polystyrene and polypropylene havean upper temperature limit of 85°C to 105°C. Above thesetemperatures, the working voltages need to be deratedaccording to manufacturer’s specifications. Another typeof film capacitor is polyphenylene sulfide (PPS). Thesedevices work over a wide temperature range, are stable,and have large capacitance values beyond 1?F. In general,film capacitors are found in surface mount and leadedpackages. Table 1 is a partial list of capacitor companiesand some of their available products.