Good precision S/H T/H capacitor options for 1kHz, 10kHz, 100kHz, 1MHz? - Page 1
Forum section: electronics / projects

author: evb149
author_rank: Regular Contributor
timestamp: March 15, 2015, 06:12:12 am
content: What're your favorite sample and hold or track and hold capacitor types / series / models for the 1Hz to say 1MHz (though only 1kHz, 10kHz or 100kHz would be interesting as well) bandwidth applications?Good features would include very low leakage currents, very linear dielectric capacitance vs. voltage, non hysteretic (low soakage) dielectric, low dielectric loss, low inductance, high SRF, good temperature stability, good age stability, low microphonics characteristics, low thermoelectric characteristics, and relatively easily solderable.Obviously some materials like teflon, polystyrene or polypropylene historically got good praises for various of those characteristics but some of those are increasingly commercially uncommon (at least with the former levels of quality & performance) these days.C0G/NP0 isn't all that bad and is cheap and ubiquitous and is good for high frequencies but is not necessarily ideal for lower frequencies in comparison, particularly for larger than a couple nF of capacitance.Beyond chosing the particular dielectric there's also the packaging and geometry and such that will affect the leakage and temperature stability and mechanical stability etc.So what are good "mainstream" relatively inexpensive commonly available (digikey, mouser, newark, ...) good performing manufacturer / model series choices for audio band or 100 kHz or 1MHz types of ranges?My particular interest is precision low level DC or AC sensor signals, sometimes 100s of nV, sometimes uV, sometimes mV, sometimes 1V range depending on the application. Mostly uV / uA or nA DAQ front end stuff.Capacitance requirements between 10s of pF minimum to 100s of nF I suppose though if there are larger values of nice units available inexpensively that would be interesting as well.

author: Marco
author_rank: Super Contributor
timestamp: March 15, 2015, 09:29:32 am
content: I have no clue, but since when is Polypropylene rare? It is the second most common material for film capacitors after polyester.PS. have you tried silicon capacitors? Much easier to get than glass, miniaturization is pushing it's adoption, and the specs are pretty much all better than C0G (hysteresis is obviously not specced cause nearly no one cares, you'd have to experiment to find that out).

author: tggzzz
author_rank: Super Contributor
timestamp: March 15, 2015, 09:39:56 am
content: I would start by looking at the data sheets and application notes from ADC manufacturers. LT and Analog Devices would be good starting points.I would also look at anything by Bob Pease or Jim Williams.Where have you looked?

author: tszaboo
author_rank: Super Contributor
timestamp: March 15, 2015, 10:03:01 am
content: Panasonic ECH series SMD film capacitors, altough my application was slightly different.

author: poorchava
author_rank: Super Contributor
timestamp: March 15, 2015, 10:26:04 am
content: There was a similar thread a while ago in context of building a high-accuracy diy multimeter. I think some part numbers came up as good candidates for the integrating capacitor.Silicon and silver-mica generally seem to be th e best, but they are also rarest and most expensive.

author: Howardlong
author_rank: Super Contributor
timestamp: March 15, 2015, 10:31:14 am
content: I was looking into this subject a few months ago, there is a note from AD that I used as the basis for mine herehttp://www.analog.com/media/cn/training-seminars/tutorials/MT-090.pdfThe biggest problem I faced was op amp stability, or output ringing, but I was doing 20MHz sample and holds, but at the lower frequencies you're talking about I would think this is somewhat easier.In the end I used a different method to achieve what I wanted to do by using phase shifting instead.

author: tggzzz
author_rank: Super Contributor
timestamp: March 15, 2015, 11:36:02 am
content: Quote from: evb149 on March 15, 2015, 10:17:43 amThe fundamentals haven't changed.QuoteThere are no useful search facilities for this topicIt does also appear that the variery oftypesof capacitor stocked by the major distributors has reduced over the decades. A bit like in a supermarket you can get 10 brands of baked beans, but only on type of dry bean, whereas before you could get 10 types of dry bean and only one brand of baked beans

author: TiN
author_rank: Super Contributor
timestamp: March 15, 2015, 11:39:07 am
content: Try WIMA MKP?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2015, 11:40:45 am
content: Quote from: evb149 on March 15, 2015, 10:17:43 amWell, yeah? It's just a capacitor... behavior defined by the dielectric. There will be some impact from construction method (such as: single versus double metallization, tightness of wind or stack, resin bonding or other fillers, etc.), but for the most part, PP is PP. If you need a very particular value, search by tolerance. I don't see how the value and tolerance are at all important though.PPS caps (which are available in SMTs) are apparently very good, to the extent that PS and the like are no longer needed.According to a recent thread on sci.electronics.design, apparently the bare film stack type caps absorb moisture and flux, and are impossible to clean. So, they should probably be avoided, or handled specially, where low leakage is also required (high precision in the < 1Hz range, perhaps?).If you don't mind a non-production-grade solution, there is plenty of old stock ex-Soviet parts: PTFE, PS, etc. Hermetically sealed mil spec, and cheap.Tim

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 15, 2015, 06:39:30 pm
content: It seems the best dielectrics are air, Teflon, polystyrene, polypropylene and C0G/NP0 ceramic. Teflon is too hard to get and I heard the supply of cap grade polystyrene dried up years ago. SMT and digital technologies have killed off a huge amount of the demand for film caps and through-hole parts, thus the dwindling supplies at the distributor level. Allied still carries a large number of decent film caps. They also have some good deals now and then in their close-out area.IMO, polypropylene is the one to look at if you need larger values. Probably special order, but Illinois Capacitor has some high voltage axial polypropylenes that are excellent (PPA, PPS series?). They're high voltage, but that just means lower volts/mil at low voltages, something that will improve performance.C0G/NP0 can be near perfect- look at some of the old Corning "Spinguard" caps if you can find them. Glass caps are surprisingly so-so unless you need radiation resistance. Silver-mica has huge dielectric absorption. One thought might be to scale your impedances so you can use an air or vacuum cap- can't get much better than that! BTW, polyester/Mylar has some weird non-linearities, so I'd avoid that.

author: evb149
author_rank: Regular Contributor
timestamp: March 15, 2015, 11:10:34 pm
content: Thanks, I'll check out the thread and the PPS ones and old stock types and avoid the bare stack ones.Quote from: T3sl4co1l on March 15, 2015, 11:40:45 am