best type of big capacitor to use for a low pass filter? - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 12, 2014, 09:34:17 pm
content: If I need a fairly big capacitor to use in a low pass filter in a precision circuit what kind would you recommend?Lets say like 100uF. Ceramic? aluminum polymer? tantalum?

author: Niklas
author_rank: Frequent Contributor
timestamp: April 13, 2014, 12:32:26 am
content: Metallized popypropylene

author: wraper
author_rank: Unknown
timestamp: April 13, 2014, 12:50:22 am
content: Quote from: SArepairman on April 12, 2014, 09:34:17 pmDefinitely none of the listed.

author: EEVblog
author_rank: Unknown
timestamp: April 13, 2014, 12:56:37 am
content: You're probably going to have to define what you mean by "precision".Ideally that means a zero tempco, zero voltage dependency, and zero dielectric absorption etc.A low pass filter is not generally going to be as critical as say a reference capacitor used in say a charge amplifier or ADC.Generally you are going to use one of the film types like Polypropylene.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 01:18:48 am
content: So where do I get a film capacitor that is 100uF but not rated for 600V and not the size of a golf ball?http://www.digikey.com/product-search/en?pv13=67&FV=fff40002%2Cfff80010&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25These seem very inappropriate for my op-amp.these capacitors seem to be intended for running heavy duty things.Capacitance multiplier inside of a salen key low pass filter?typically when I see a big capacitor used in a app note by a major company they seem to specify a bi-polar type capacitor when the size is +100uF, even in op-amp circuits.even if you dial down to 47uF for a film capacitor:http://www.digikey.com/product-search/en?pv1292=43&pv1292=47&pv1292=57&pv1293=37&pv1293=42&FV=fff40002%2Cfff80010%2C3400ba%2C34028b&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=2510$ and 1 inch by 0.5 inches. I could live with that I suppose if its the best option. and thats polyester...

author: c4757p
author_rank: Super Contributor
timestamp: April 13, 2014, 01:24:02 am
content: You're going to have a fun time trying to find a good 100uF film cap for this purpose. Ten 10uF in parallel? Four 22uF (= 88uF) and change something else to make it work? Change the rest of the circuit by a factor of ten and use 10uF?Quote from: SArepairman on April 13, 2014, 01:18:48 amYes, for instance I've seen a Jim Williams note on measuring noise that used some magical, selected (IIRC) wet tantalum. You might want to sit down for the price.

author: wraper
author_rank: Unknown
timestamp: April 13, 2014, 01:27:23 am
content: QuoteI think those questions are about the same schematic. Do you really need capacitor that big? Seems that you are trying to solve a problem in some obscure way.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 01:29:06 am
content: Quote from: c4757p on April 13, 2014, 01:24:02 amwell I have seen things like 47uF tantalum used in op-amp circuits around precision op amps, I know the nanovolt level noise thing you are speaking about, with the tantalum slug.even paralleling them up, a 10uF metal foil is fairly expensive. I cannot spend 40$ on 10 parallel capacitors in my circuit, and it is still a huuge amount of area.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 01:29:40 am
content: Quote from: wraper on April 13, 2014, 01:27:23 amnope completely different application

author: c4757p
author_rank: Super Contributor
timestamp: April 13, 2014, 01:31:16 am
content: What are you actually trying to do? Can you digitize the signal? It'd be much easier than fairy-dust-and-unicorn-fart capacitors.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 01:32:25 am
content: Quote from: c4757p on April 13, 2014, 01:31:16 amnot interested in digitizing anything. I want to filter the ever living crap out of a DAC that adjusts the offset voltage in a circuit after range change is done.i guess I want to know what is the least evil of the polarized capacitors. I only need one so I can splurge on some kind of weird polymer thing, if its any better.What about capacitance multipliers for a small film cap to turn it into a big film cap? I would rather do a RC filter -> buffer if I had a really large C, I think.

author: wraper
author_rank: Unknown
timestamp: April 13, 2014, 01:40:32 am
content: Quote from: SArepairman on April 13, 2014, 01:32:25 amwow, why do you need 100uf mentioning precision circuit for that.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 01:43:15 am
content: Quote from: wraper on April 13, 2014, 01:40:32 ambecause I recall seeing some app note some where that used a rather large C for a very similar purpose (nulling)I wanna say they used a 47uF for a low pass filter,.

author: wraper
author_rank: Unknown
timestamp: April 13, 2014, 01:47:52 am
content: Aren't you going to make a single pole RC low pass filter with a huuuge cap?

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 01:50:46 am
content: Quote from: wraper on April 13, 2014, 01:47:52 amyea, a RC filter followed by a buffer. If the capacitor is big enough that should be all it ever needs.oooo I just realized the voltage can be positive or negative. So I need a non polar capacitor.so that either means electrolytic BP, film, ceramic or two unipolar capacitors...

author: codeboy2k
author_rank: Super Contributor
timestamp: April 13, 2014, 02:16:51 am
content: Go for a larger resistor and smaller capacitor and buffer the output of the rc filter before you send it anywhere else. This is the proper way.

author: c4757p
author_rank: Super Contributor
timestamp: April 13, 2014, 02:19:28 am
content: Why does the DAC output need to be filtered so much? It's just a fixed offset, right? Go with a DC DAC like an R-2R rather than a delta-sigma or something, and you shouldn't need so much filtering.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2014, 02:33:29 am
content: Your error due to dielectric absorption, even on the best possible capacitors, will completely and utterly swamp your attempt at measurement. Find a different way.Tim

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 02:37:24 am
content: Quote from: T3sl4co1l on April 13, 2014, 02:33:29 amhmm? please elaborate.I intend to adjust the output of the DAC while measuring the offset that I want to null (after the RC and buffer, which is connected to a summer). what problem will dielectric absorption cause?The offset is only intended to change once per range switch, I don't care if it takes a few seconds to auto calibrate itself.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2014, 05:14:32 am
content: Dielectric absorption looks like an R+C across your ideal capacitor, with the R relatively large (giving a long time constant, minutes to weeks) and C some small fraction of the bulk capacitance (usually 1% or less).Consider what happens if the capacitor sits on a shelf. Over time, the two capacitances equalize in voltage. Then you pick it up and apply a voltage: you get two time constants, one dominant, another extremely slow and weak. The gain difference (between, say, 1Hz and 0.001Hz -- at least, one frequency well above and one well below the absorption's cutoff frequency) between these is very small, but since you're measuring the difference between signal and a reference, this is an error in your measurement. And even from the best capacitors, 0.01% is a whole lot of LSBs at 20 bits or more.Tim

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 05:44:56 am
content: Quote from: T3sl4co1l on April 13, 2014, 05:14:32 amall I could think then is to use a smaller capacitor in a gyrator? no other solution...?can you use a capacitance multiplier connected to a foil capacitor maybe?how about this?In this circuit, the capacitance of capacitor C1 is multiplied by the ratio of resistances, i.e. the circuit gain and the effective capacitance C = C1 * R1 / R2.When using this circuit, care must be taken not to exceed the current capability for the op-amp. This should not be an issue with low current applications, but where higher currents are required, this must be taken into account.

author: EEVblog
author_rank: Unknown
timestamp: April 13, 2014, 07:05:51 am
content: Quote from: SArepairman on April 13, 2014, 01:32:25 amYou generally don't need a precision cap for something like that. Because even if you get a horrible that that varies by +/-50%, you design your system so it doesn't matter.Are you able to chose a different DAC? I assume oyu are talking about a PWM based DAC here?

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 07:18:59 am
content: Quote from: EEVblog on April 13, 2014, 07:05:51 amit is a presumably dirty DAC connected to a micro controller. It is not PWM. I thought the same as you did until I read the postDielectric absorption looks like an R+C across your ideal capacitor, with the R relatively large (giving a long time constant, minutes to weeks) and C some small fraction of the bulk capacitance (usually 1% or less).Consider what happens if the capacitor sits on a shelf. Over time, the two capacitances equalize in voltage. Then you pick it up and apply a voltage: you get two time constants, one dominant, another extremely slow and weak. The gain difference (between, say, 1Hz and 0.001Hz -- at least, one frequency well above and one well below the absorption's cutoff frequency) between these is very small, but since you're measuring the difference between signal and a reference, this is an error in your measurement. And even from the best capacitors, 0.01% is a whole lot of LSBs at 20 bits or more.TimI initially thought that a capacitor capacitance changing would just effect the Fo of the low pass filter, meaning the vRMS noise would change only, not the DC level, however I think that Tim is saying that the DC level will drift like an RC circuit because of the dielectric absorption, or at least that dielectric absorption is the main detail to worry about while designing at this level..I suspected there would be problems like this, because there always are with precision , which is why I asked.***From my personal experiences with op amp RC filters I found that they were pretty damn stable. I am fairly sure a regular old one as I have used in the past would be used acceptably in this application however I have never attempted to build anything this precise (or have been this interested in stability) and I would like to know the fine details behind the operation, so I can write nicer specifications on a piece of paper...*tim, are you essentially saying that the dielectric absorption leads to a particularly nasty source of low frequency noise that would often be lumped in with 1/f noise? I am a bit unclear.

author: EEVblog
author_rank: Unknown
timestamp: April 13, 2014, 07:35:59 am
content: Quote from: SArepairman on April 13, 2014, 07:18:59 amWithout knowing exactly what type of DAC you are using, it's pointless looking into all this.

author: TerminalJack505
author_rank: Super Contributor
timestamp: April 13, 2014, 02:33:05 pm
content: Quote from: SArepairman on April 13, 2014, 05:44:56 amBy introducing an op amp into the mix you are essentially creating an active filter. You might investigate that line of thought and see if any of the active filter topologies meet your requirements.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2014, 06:09:41 pm
content: This applications seems like "precision" in the sense of low leakage because of the high source impedance caused by the series resistor making up the filter. Noise filters for precision voltage references share this problem and the solutions include a low leakage capacitor followed by a low noise high input impedance buffer or two shunt electrolytic capacitors in series so the filter capacitor is bootstrapped to have zero volts across it lowering its leakage.Jim Williams' circuit which was mentioned had the problem of dealing with dielectric absorption, leakage, *and* noise from the capacitor itself and he said that some aluminum electrolytic capacitors could be graded to achieve acceptable performance but that assumed long warm up and settling times, minutes to hours, are acceptable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2014, 06:16:45 pm
content: Consider that, for each and every DAC code you want to verify, you have to wait several absorption time constants for it to settle. Potentiallyweeks per point.The capacitance value is probably not very important as Dave says, but few capacitors exhibit absorption that low. This is one of many reasons why you're better off finding a better solution to your measurement.Tim

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 07:04:14 pm
content: Quote from: T3sl4co1l on April 13, 2014, 06:16:45 pmAh I did not think weeks.Also, one reason why I thought electrolytics/etc were OK is because I saw tantalums used to filter a thermocouple, but again thats not 20 bits.It seems that the only solution is to use a foil capacitor (lets say up to 1uF) and then use a capacitance multiplier, which will exadurate the temperature drift, causing capacitance change, which will not be that important.I played around with numbers on a RC-> buffer, higher order active filters and a few gyrator circuits. Using 1uF (I don't want to foil higher then this I guess) in a d-element filter with a 150k in combination with some other values will give a 2nd order filter with a low pass frequency of 1Hz. A 100uF capacitor with a 100k in a RC gives a cutoff of 0.16Hz, which is nicer..however the capacitance multiplier will introduce noise*gain, so if I want 100uF out of 1uF I will get at least 30uV rms noise if I use a low noise op amp, which is not that great...damn cleaning up DC.how about simulating a capacitor using an inductor in a gyrator circuit? inductors are ugly but I don't think a synthetic capacitor made out of an inductor will experience any phenomena like dielectric leakage. what demons would a simulated capacitor have (i know how crazy this sounds). IDK how the units transfer in the gyrator equation but it should be easy enough to get a big tightly wound inductor and turn it into a big capacitor, I think. If the inductor is big, the resistors are small, then the synthetic capacitor should also be big, right? I need to look into this.Since the ultimate purpose is to simulate a high value noise killing capacitor we don't really care about the odd-ball properties of the inductor so long the resulting capacitor has low DA, right? It seems the be the most critical aspect of this.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2014, 07:27:23 pm
content: These two articles show some alternatives:http://www.edn.com/design/analog/4327729/Filter-your-voltage-reference-for-low-noise-performancehttp://electronicdesign.com/energy/filter-trims-ultra-precision-voltage-reference

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 07:36:26 pm
content: Quote from: David Hess on April 13, 2014, 07:27:23 pmthat looks good but 10uF is still a pretty big fat capacitor. that looks like a good solution that I should probobly use.. but I can't help myself, I am going to try to simulate a capacitor out of a small ballast that i have, I wonder what will happen..I mean, the only thing preventing us from using garbage electrolytic is pretty much the DA.

author: lewis
author_rank: Frequent Contributor
timestamp: April 13, 2014, 07:56:32 pm
content: Why do you need to filter the output of the DAC? Are you experiencing noise issues? If it's just noise and the low frequency drift is nil, you might get away with a precision 100nF - 1uF polypropylene cap depending on the series R value. If you start introducing op-amps into the circuit, you have to take their noise contribution into account, and depending on your application you might need a hard R-C filter at the op-amp output anyway. Bigger caps might not filter out all the noise you expect too, their impedance often rises at higher frequencies because of equivalent series inductance. A smaller cap might workbetterfor you. You could even use an LC filter if the output impedance needs to be low.If it's quantization noise, oversample digitally to reduce the DAC output filer requirements. I doubt this is a problem for an offset trimming application where the value is likely to change infrequently and it's only a stable DC level you're after.If you're trying to filter out low frequency drift caused by your DAC's reference, get a better DAC (or reference). You'll never do that properly with a massive cap without significantly impacting settling time.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2014, 07:59:10 pm
content: Quote from: SArepairman on April 13, 2014, 07:36:26 pmThe leakage would clobber the precision anyway before the dielectric absorption.The other reference filter design I have seen uses two large electrolytic capacitors in series to bootstrap the filter capacitor so it has no leakage.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 08:28:35 pm
content: So you think the leakage in a GIC will be highly variable and render the circuit unusable?http://www.millertechinc.com/pdf_files/MTI%20TN094%20Zero%20DC%20Offset%20LPF%20and%20the%20D%20Element_files/image002.gifI thought the leakage through it would be pretty constant, making it acceptable.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2014, 08:42:14 pm
content: Quote from: SArepairman on April 13, 2014, 08:28:35 pmThat is going to depend on the details. Input bias current and input offset voltage of the operational amplifiers is going to be an issue. I have never studied using a GIC in this application before.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 08:47:27 pm
content: Quote from: David Hess on April 13, 2014, 08:42:14 pmHmm, well so long those effects stay relatively constant it will be OK because it is just nulling (my null range has quite a large span), so that even if there is a big offset voltage, I could just trim it down (-1.25 -> 1.25V).I am going to build one up using a half decent dual op amp on a solder board and see how it performs. But I know all those bias currents and stuff have a large temperature dependance...at this point I am just curious, i think the assurance that the 1/f noise will not be so bad, so long the reference is good, worked. 1uF foil capacitor filters with no modification will probably be enough. I am glad that I did not specify a tantalum there.I was re-reading some of the app notes that I thought had large capacitors in them and I found that the larger capacitors were only used on the AC side of things, all of the DC signal filters do not really go above 2uF as far as I can see (at least in linear technology app notes).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2014, 09:08:21 pm
content: Inductor won't work. The component has to be reactive at the cutoff frequency to be useful. Capacitors work because the leakage resistance is very high (an open-circuit time constant in the months being typical of films), so although the impedance is high, it's still largely capacitive. You then only need the amplifier (for C mult. or gyration) to amplify the reactive current to a useful level.An inductor's DCR plays the respective role, so you need one with low enough DCR or high enough L to remain inductive at the cutoff frequency. The impedance will be very small, so the amplifier is only required for voltage gain.I might have some monster iron-cored chokes with an RL cutoff frequency in the 10Hz range, maybe a little lower, but you'll have one hell of a hard time doing fractional Hz.Tim

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 13, 2014, 10:03:12 pm
content: I need a old battleship and a few miles of wire...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2014, 11:57:42 pm
content: Instead of dielectric absorption, you'll have to worry about it magnetizing and aligning with the Earth's magnetic poles!

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 14, 2014, 08:23:39 am
content: So how about PPS vs PP capacitors? Polyphenylene SulfideI am having trouble finding data to compare these guys with.On wikipedia and a few random websites it seems that they are pretty close, maybe there is a slight leaning to the PP capacitor depending on where you look.On the data sheets there is no leaning, because the datasheets are pretty barren, however the PP datasheet reassures me by having fancy words like "stability class 1", the PPS capacitors have nothing so comforting.I like the PPS capacitors better because they are surface mount. Up to 100nF I could use a PPS, which would be nice especially if I shunted the larger 1uF capacitor in the filter as was suggested by someone else.It makes me want to lean to PPS capacitors for anything less then 1uF requiring precision based on the small SMD form factor alone.I even saw a data sheet advertizing PPS capacitors as analog memory.But I don't know how i am supposed to compare this stuff when one data sheet says > 100Mohms and the other data sheet says > 50Gohms.........Some one needs to put all these data sheet manufacturers into a mud pit so a standard could be finally made...Testing this out for myself will make my prototype very ugly if I have to put space for PP square capacitors when I thought I could get away with PPS... This thing might end up looking like war of the worlds (tripods = film capacitors)how does dissipation factor change across frequency effect my circuits? the PPS capacitors have a much more pronounced one.

author: David Hess
author_rank: Super Contributor
timestamp: April 15, 2014, 09:16:08 am
content: I have always ended up qualifying capacitors myself at this level of performance because in general the manufacturers do not provide this level of specification. In the past polystyrene was my first choice but now I would use polypropylene.

author: TMM
author_rank: Frequent Contributor
timestamp: April 15, 2014, 09:36:46 am
content: It's still unclear to me why you need to filter the DAC. Are you trying to filter out the 'steps' in a non-oversampling DAC?

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 19, 2014, 07:51:15 am
content: Ah, this is what I was remembering (offical data sheet using a 1uF tantalum for decoupling a reference)http://cds.linear.com/docs/en/datasheet/1019fd.pdfpage 8Thats why I thought it was a good idea for my DAC....Now, I under stand that the settling might take long, but apparently its good enough for a reference... maybe not for a DAC. I'm surprised that they did not specify a capacitor with a lower dielectric absorption there.that is a high quality no nonsense reference too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2014, 09:31:01 am
content: Well, yeah... a reference is an ideal voltage source. It can drive microamperes of leakage and drift and absorption all the live long day. The cap isn't in a signal path.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 19, 2014, 06:56:39 pm
content: Quote from: T3sl4co1l on April 19, 2014, 09:31:01 amwhat makes it different though? if you look at the block diagram it is a "battery" connected to a op amp connected to a RC filter (5 ohm 1-10uF according to the datasheet).. (not shown in picture)isent the output of a DAC similar to a op amp output, so I would be in fact connecting an RC filter to the output of another op amp connected to a op amp connected to a "battery"is the op amp in the voltage reference a peculiar op amp?

author: SeanB
author_rank: Super Contributor
timestamp: April 19, 2014, 07:01:52 pm
content: It is a buffer that ideally has absolutely no signal applied, just a pure DC voltage. Thus leakage, DA and such have very little to no influence on the source, it is there merely as a store of charge to reduce noise on the reference.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 19, 2014, 07:05:43 pm
content: Quote from: SeanB on April 19, 2014, 07:01:52 pmso if I were to cut into the op amp and splice a long loop of wire connecting the reference to the buffer then a tantalum would no longer be appropriate?*drill chip, spot weld in a long wire that extends out side of the chip and goes back into the chip (longer path that travels through a noisy world)or the less exciting "insert a voltage noise source between "battery" and "triangle"