Most accurate way to use a microcontroller to measure capacitance? - Page 1
Forum section: electronics / projects

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 08, 2018, 01:12:39 pm
content: I been Twin-T'ing around and I find I am having a bit of tweek burnout to get my Twin-T's to tune.I don't have any fancy equipment, except for a good o'scope and a 3-digit DVM, but the better I can determine the precise value of a sub-uF cap, the more time I can spend at the beach. I got precision 1% resistors and lotsa common opamps, many other ics on hand, a big pile of PCB's to steal parts from, if needed.What are you ideas of the best way to accurately measure the value of a poly-film or npo/cog or other low delta C/deltaV, low temp coeff. quality capacitor?My goal is to capture the C value to .1%but I could settle for a little less, if I get more time on the beach.I need to measure values from 'bout .2uf down to maybe 50pf or so.I don't wanna buy any equipment! I need the money to pay for the gas to get to the beach and I could use a new bikini.

author: elecman14
author_rank: Unknown
timestamp: May 08, 2018, 01:25:49 pm
content: You could use an RC circuit and measure the rise time. The R value would have to be fairly large for a small value cap. If you want to implement this with a microcontroller you could start injecting the signal, start a timer and then use a comparator to figure out when you reached the desired charge level and shut off the timer. You also could do this on your scope if you did not want to build anything.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 08, 2018, 01:28:16 pm
content: What kind of syringe do I need to inject a signal? What accuracy could I expect? Turns out that making accurate measurements isn't so easy if the effect/precision of measuring equipment adds uncertainty/error to the result. If the values of the resistors are large, then the effect of probing this test setup would also be large, not to mention cleaning syringes, noise, etc.For instance, a CMOS 555 timer can vary 1% in freq (that's the spec.) with supply voltages changing from 5 to 15V.I would need to calibrate this kinda thing.

author: oldway
author_rank: Super Contributor
timestamp: May 08, 2018, 02:20:13 pm
content: Lower cost without buying any equipment ?Start with the simplest oscillator using a Cmos version of 555.http://ramon-electronica.blogspot.be/p/555.htmlC1 is the capacitor you want to measure.R1 is a high stability resistor choosen so the 555 max frequency of oscillation is about 200Khz with the lowest value of C1 you want measure.You use your µc to measure the frequency of oscillation and you calculate de value of C1.You make a calibration with a high precision capacitor of known value.It's very important that the square voltage applied to R1 goes very near of Vdd and of GND.You can't use output pin 3 of 555 directly....use the 6 buffers of a 4050B in parallel.Power supply does not need to be a very high stability one.Preferred Vdd voltage for 555 and 4050B : 12V

author: ogden
author_rank: Super Contributor
timestamp: May 08, 2018, 03:08:44 pm
content: 555 timers? LOLNote that quite decent LCR meters like 1700 series of Keysight have 0.2% capacitance accuracy and low frequency (< 10 KHz) models goes down to only 200pF.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 08, 2018, 03:14:36 pm
content: There are some projects around to use the sound card interface of a PC to use it as a kind of low frequency component tester to measure LCR. It does not need much hardware (mainly a few resistors, diodes for protection), but quite a bit of software. So if one can find a free software version of this, that works with an existing PC, this would be a cheap and surprisingly accurate way. However not all sound-cards are compatible with all such software.In principle one could build a similar way system around an µC: create a sine or triangular wave and measure the response of resistor and DUT in series with the usual µC internal ADC. Not as high resolution as the soundcard, but possible. Still quite some software to write.The timer/ oscillator version usually need a capacitor for calibration. The resolution can be very good, but accuracy may be limited, as the usual design equations are often of limited accuracy and parasitic effects can change with a variable frequency.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 08, 2018, 03:32:48 pm
content: Ogden, you gotta realize that sometimes even the supposedly best in the biz don't do their best! No big money ins selling cap testers.Kleinstein, I donno, seems kinda iffy to work at all.I have a bunch of 2200pF +/- 2.5% (quan 6) 3300pf 2.5% (quan 2) and 20000pf 2% (quan35+)Even when I search Mouser, precision caps are mostly in the 0-20pF range and intended to work in the GHz range, larger valued precision caps are few and far between, with 2% the best precision available. Besides, would take some days to get them.Calibration is everything. I have not a capacitor that is without some large blemish of precision compared to the .1% target goal.What Ifs:I have many different values 1% resistors and a CMOS 555 running with a tightly regulated 5V supply, would I then be able to measure the 555 output astable freq. by counting 62.5nS timer ticks on a MCU to determine the freq to calc value of the DUT cap to 1%???What would Mike Faraday do. He developed his knowledge about capacitance without little more than a Lyden Jar, a tall stack of Volta voltage cells, and a galvanometer to work with.Common guys..we should be able to do better than this!

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 08, 2018, 04:08:57 pm
content: At least somebody can do it!Can do .01%

author: ogden
author_rank: Super Contributor
timestamp: May 08, 2018, 04:16:17 pm
content: Quote from: SuzyC on May 08, 2018, 03:32:48 pmOh come on. There's huge money in industrial equipment. How do you think capacitors are manufactured?You can check yourself. No 555 timer here:

author: phenol
author_rank: Regular Contributor
timestamp: May 08, 2018, 05:03:35 pm
content: http://www.rfcandy.biz/communication/imp_lc.html

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 08, 2018, 05:07:21 pm
content: Ouch, measuring such a wide range of capacitance with an error of only 0.1%, and this with just a few cheap parts and no specialized equipment? You seem to like the beach a bit too much.A simple RC oscillator structure will not cut it. Unless maybe you can calibrate it properly AND very frequently (like between each measurement), because it will drift over time and temperature.You could be a little better off with a very good signal generator (meaning very low distortion), generating a sine and make a simple RC filter with your unknown cap and a reference resistor, and then calculate the capacitance based on the attenuation between input and output, this with a precision multimeter in AC (don't use a scope for precise amplitude measurements, it's not the right tool for this at all).A good sound card for the signal generation would work fairly well with frequencies in the audio range. Still, with a low or middle-end multimeter, you probably won't get enough precision in AC mode to get the precision you require on the capacitance evaluation.One approach that may get you something in the 1% error range (maybe a bit lower if you can measure precisely the reference current source and the reference voltages) could be with a current source as attached. You'd measure the charging time between two known reference thresholds (which allows you not to worry about amplitude measurement precision, again a scope would not cut it for that). The time elapsed between the rising edges of the 2 comparators outputs (Tr2- Tr1) will allow you to compute the capacitance, and can be measured with a scope (which is an accurate instrument for time measurements) or a microcontroller. You need two fast and low offset comparators, the LT1720 would work. For discharging the cap, a manual switch with very low parasitic capacitance will work. A MOSFET would add parasitic capacitance and should be avoided. The TI REF200 current source ref. is great and not too expensive.May look convoluted, but it's still simple and would be cheapish to build, need only a few parts and IMO be much more accurate than oscillator-based setups.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 08, 2018, 05:24:10 pm
content: All you kids need a uP to accomplish anything! If you can get one capacitor measured, a traditional bridge will do the trick. People are scared of them, but it's just a handful of parts and can be built in a couple hours. A couple minutes if you cobble things together with jumpers and alligator clips. See-http://www.conradhoffman.com/cap_bridge.pdfSince you're doing films, dissipation factor will be too low to bother with, so you can eliminate that pot if you want.

author: BravoV
author_rank: Super Contributor
timestamp: May 08, 2018, 06:10:24 pm
content: Quote from: SuzyC on May 08, 2018, 01:12:39 pmAll your very demanding requirements need less bikini & beach and more time on brain.

author: BravoV
author_rank: Super Contributor
timestamp: May 08, 2018, 06:12:09 pm
content: Quote from: SuzyC on May 08, 2018, 03:32:48 pmHow do you know that your 82.5ns there is accurate down to 0.1% ?May be its only 75ns or even 95ns ?

author: BravoV
author_rank: Super Contributor
timestamp: May 08, 2018, 06:14:43 pm
content: Quote from: SuzyC on May 08, 2018, 04:08:57 pmFor sure they are not done using few cheap 555 ic and few bucks of component in hand to able to make that kind of capacitor.Probably they sacrificed lots of their beach time and bikini time too to able to make that kind of stuff, have you ever thought of that ?

author: kripton2035
author_rank: Super Contributor
timestamp: May 08, 2018, 07:14:31 pm
content: here on my repository you can get some schematics of various C meters.http://kripton2035.free.fr/lcr-repository.html

author: David Hess
author_rank: Super Contributor
timestamp: May 08, 2018, 08:02:00 pm
content: For accuracy, I would avoid any design which involves a fixed reset or commutation period within the frequency control loop which includes all common 555 designs. This fixed period produces a non-linearity in the measurement due to its finite delay and inconsistent capacitor reset. Well designed analog function generators which use compensation to ameliorate this problem do not achieve the kind of open loop linearity required for this level of accuracy.Charge balancing voltage-to-frequency converters can get down to the 0.0005% linearity or better so they are a candidate but measuring up to 0.2uF could be a problem; they typically use a 1000pF for their reference which implies measurements up to 0.01uF.What I would probably try given the constraints is using the capacitor in an integrating circuit where the microcontroller either makes time measurements between two voltage thresholds or sampled voltage measurements. Either will allow calculation of the slope and then dI/C = dV/dT and multiple measurements can be used to average out noise. With some care, I think 1 part in 20,000 is possible so 50pF parts could be measured to 0.1% using a 1000pF range. This duplicates how theCTMU (charge/time measurement unit) on some Microchip PIC microcontrollersworks.

author: Brutte
author_rank: Frequent Contributor
timestamp: May 08, 2018, 09:20:25 pm
content: A cap is an RLC really. And if you dig even deeper then this could be a nonlinear device where C depends on the frequency. So it is a matter what is the purpose of that measurement.If your design needs 0.1% tolerance cap then it is most likely a poor design. Redesign it so that there is no need for such tight tolerance.

author: hugo
author_rank: Regular Contributor
timestamp: May 09, 2018, 01:32:03 am
content: Not exactly what you've asked but close enough ...https://www.romanblack.com/onesec/CapMeter.htmThis is a high resolution (0.01pF), PIC based, cheap capacitance meter (0pF to 50uF range).

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 09, 2018, 03:26:11 am
content: With small cap values you need to understand the difference between two and three terminal measurements. A GR standards bridge will measure small fractions of a pF, but to get any sensible results you need the capacitor mounted in a shield and set up for a three terminal measurement. Up to 100 pF or so you have to worry about that stuff. There are lots of references, but this one seems as good as any-http://nla.org.za/webfiles/conferences/2015/T+M%202015%20Presentations/Tuesday,%2013%20October/T205%20Capacitance%20and%20inductance%20measurements%20using%20an%20LCR%20meter.pdf

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 11:48:31 am
content: Oldway: I don't quite understand why all the extra buffers. Are you suggesting to buffer the output of the simple astable 555 circuit that feeds the squarewave output back to the the capacitor connected directly to the 555?Seems like a good idea to remove the load from the timer's output to prevent this influence on the internal comparators that determine the frequency of astable oscillation.But in every case, dealing with cap vals from .2 to 50pF, using thelargest resistorwould lower the output frequency to give a longest period to measure with the most resolution when using a MCU internal timer peripheral counting 62.5nSec internal timer ticks.So, in this case, having to buffer the output of the 555 seems to be useless.I am assuming here using a MCU that operates its HFINTOSC of 64MHz which corresponds to 16mSec instruction clock, although the internal peripheral timer clock can be the 64MHz clock.Using any MCU means there will be jitter in measurements due to inherent IRQ and HFINTOSC jitter, but it is certainly possible for this jitter be averaged out to minimize this source of error. The final measurement value would be obtained by averaging many measuring results.With a 16-bit timer, the resolution of a raw max period measurement of 555 osc. could be as good a very tolerable one count ambiguity of 62.5nsec. Assuming the 555 oscillates using a precision resistor and the DUT for timer, it means the largest valued resistor would be chosen to max out the counter resolution of the internal MCU timer.Why wouldn't this idea work?Why wouldn't the CMOS 555 timer, with a tightly regulated supply voltage, well bypassed, be well suited to do a very good job here?What is the problem the using a CMOS 555? What would be compromising in its design to be capable of introducing error?As with any precision capacitance measuring device, whether its made by Gen Radio, Agilent or by using a 555, calibration and comparison with a known standard would be a must.The only problem I have would be to make a few of my own "calibration standard" capacitors values using the timing resistor's precision itself to determine its value to better than .1%. I can fairly easily find .02% or better timing resistors.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 12:08:11 pm
content: BravoV: I know the exact freq. because it is derived from the very precise crystal connected to external clock input of the MCU and this precise system clock is used by the MCU for its timer peripherals.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 12:13:08 pm
content: Brutte:In a Twn-T circuit, stresses on a capacitor are minimal. The parasitic properties of the film capacitors to be measured and use would not be significant in sub-kHZ notch filter design.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 12:16:10 pm
content: ogden: I am sure Signalent is not doing its best work here, .2% is almost laughable as a measurement limit. Look what Gen Radio and others can do.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 12:22:42 pm
content: SiliconWizard: Thank you for your help, but in MHO, this approach seems very inappropriate to solve this measurement problem since it involves so much complexity that can introduce measurement errors and this method requires very expensive lab equipment to precisely measure attenuation.Adding cost and complexity doesn't necessarily ensure the outcome of best results or that a solution to the problem cannot be achieved in a very much simpler, less costly manner.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 12:38:20 pm
content: ConradHoffman Thanks for your input, but your idea of using a bridge seems to be a brute force method to measure the value of small-valued capacitors used in a Twin-T, but looks like it might work well with resistor measurements and large electrolytic caps.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 12:45:12 pm
content: Phenol, a very fine device you've made! Measures inductance as well. Very nice to use to measure inductors.But in your improved final design you show:Cap: 0 pF to 1 uF, resolution 0.1 pFInd: 0 nH to 1 mH, resolution 2 nHAccuracy: 1% of reading typicalAutomatically calibration built in16 character by 2 line high contrast LCDAutomatic prefix pF, nF, uF, nH, uH, mHSoftware controlled LCD contrast setting3 pushbuttons for user programmingTesting program built inSmall dimension 3.2" x 2.4" (80 mm x 62 mm)Easy to build (one evening project)

author: Gyro
author_rank: Super Contributor
timestamp: May 09, 2018, 12:53:26 pm
content: Quote from: edavid on May 08, 2018, 05:56:35 pmWhat he said ^Selecting matched capacitor pairs can be done very accurately by combining with two matched resistors in an AC bridge circuit. All you need is a Sig gen and a sensitive AC detector.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 12:56:31 pm
content: EDavid: Perhaps you are right, but trimming resistors with matched capacitorsstill means I need to match capacitors.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 09, 2018, 01:02:10 pm
content: The bridge I showed works fine for pF sized caps- just use a smaller reference cap, easily purchased at .5% or so. Accuracy and resolution are very different. The digital designs published can give you the resolution, but will have poor (by my standards) accuracy at low pF values. No matter, for matching all you need is resolution. Two more ways to do this- first, just resonate the cap with a known inductor. The frequency will give you the value. You can make a self-oscillating circuit and measure the frequency with a uP or similar. Or, wind a small center-tapped toroid transformer and make a transformer bridge. Run it at a higher frequency, say 1 MHz or so. That's how most of the best ones have done it in the past. Quite simple to do and lends itself to 3-terminal measurements and small values. You can use both sides of the winding for your two caps and look for null when they match. Amplitude (lack of null) will tell you the degree of mismatch. Download a manual for the GR 1605 to see how it's done.

author: ogden
author_rank: Super Contributor
timestamp: May 09, 2018, 01:07:17 pm
content: Quote from: SuzyC on May 09, 2018, 12:16:10 pm0.2% capacitance measurement is laughable? Mkey. Let's see your 0.1% capacitance measurement device, .2uf down to maybe 50pf or so.Gen Radio? Who? Which others? Please provide link to "Gen Radio" and instruments of "others" you are talking about.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 01:11:22 pm
content: ogden:I have yet to build mine, but lookee here:https://www.romanblack.com/onesec/CapMeter.htmHe sez:"Real" accuracy!"The "real" accuracy depends on the internal calibration, with my PIC, 10.00 kohm resistor and 16MHz xtal the calbration is accurate tobetter than about 0.2% of the capacitor value, based on some test caps.You should be able to improve this further..".This simple circuit looks the most promising, after carefully considering the various suggestions and comments!Again, thanks to everyone for their comments!I have tried to reply to them all without sometimes seemingly being troll-like as in always being critical, but my idea to work with knowledge presented to me is to carefully weigh the pro's and con's of all ideas presented here.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 01:26:53 pm
content: ConradHoffman, your idea still seems a bridge over troubled waters. Not so easy in time and money to build and calibrate andusethis bridge, compared to some other suggestions here.

author: ogden
author_rank: Super Contributor
timestamp: May 09, 2018, 01:29:24 pm
content: Quote from: SuzyC on May 09, 2018, 01:11:22 pmRight. Just claim. Not even test results. Nothing. All we know thathe saysso.Everybody can make some circuit, publish in the blog and claim virtually anything. Internet does not mind at allThis is all you have to laugh about Keysight LCR meters?

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 01:31:23 pm
content: Gyro: I don't have a sig generator and or a sensitive AC detector. I would have to carefully design, fabricate, test, calibrate and learn to use this piece of specialized equipment. It doesn't seem so easy as you say to achieve .1% accuracy.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 01:36:05 pm
content: Ogden, it would certainly be no easier nor harder to test the results of the device mentioned than to do a qualifying acceptance test of the Signalent super-expensive device.Although probably not yet tried out in a NASA grade cal lab, I can see no inherent reason why this simple instrument shouldn't meet the spec desired.People lie on the internet..that is a fact, but results do not.If you can verify the performance of this device to be less than touted, I would certainly like to see your results, especially for the intended purpose I have presented.

author: ogden
author_rank: Super Contributor
timestamp: May 09, 2018, 01:48:38 pm
content: Quote from: SuzyC on May 09, 2018, 01:36:05 pmDon't you see? It's obvious why that particular cheaper instrument cannot meet claimed performance (capacitance meter measures in 0.01pF digits. Total range 0pF to 50uF):QuoteExcuse me? I do not have to prove anything. It's you who shall show results. I do not need nor want to build useless circuit, measure it's performance just to prove I am right. Come on

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 09, 2018, 01:50:39 pm
content: Formatching, as long as you do the measurement the same way every time, meaning same component position, same lead lengths, etc., you can get almost any degree of match you want. It's comparative accuracy, and will be good. As far as absolute accuracy, claims of sub-percent accuracy on small pF value caps are all BS without proper shielding and 3-terminal measurements. As an example, let's say you have the typical 2-terminal system with a grounded panel (or just the internal circuit board). The capacitance between the body of the component and the panel adds to the measured value. With a small value cap you could be off by a factor of two! But, if you're just matching parts, and do the measurements the same way every time, you'll be dead on.

author: David Hess
author_rank: Super Contributor
timestamp: May 09, 2018, 01:56:03 pm
content: Quote from: SuzyC on May 09, 2018, 11:48:31 amThis is what is called an integrating measurement meaning that the measurement can be made for as long a time period as desired to remove noise and increase resolution so the jitter in the MCU can be reduced as needed.QuoteUsing an astable oscillator does work however the fixed delay between the comparators and output adds an error term which shows up as non-linearity and this error term changes at different frequencies. Other methods like bridge, sine oscillators, and I/C=dV/dT do not suffer from this so can achieve greater accuracy. I suggested the last one because it only requires some combination of time, DC current, DC voltage, and resistance for absolute calibration although now you probably need a 0.05% voltage and 0.05% resistance or better.QuoteIf you only need to match capacitors then stability becomes more important than absolute accuracy. High resolution multimeters have better resolution than their stability supports so they can make relative measurements which are more accurate than their absolute measurements.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 01:59:48 pm
content: Ogden: if I may quote the tag line of another eevblog poster:The difference between theory and practice is less in theory thanthe difference between theory and practice in practice.I can see why you have greater respect for Signalent and you don't have to prove anything.But the truth remains, is it necessary for someone to test the device to prove it can work just as well or not before anyone can show its performance. It is not enough to just say so. This is especially true if one fabricates this kind of device without taking shortcuts in layout, parts and construction.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 09, 2018, 02:08:21 pm
content: There's a thing called analysis. People make mistakes and it can't prove anything beyond all doubt, but we do quite a lot of it in this field with surprising success. There's also a thing called experience. Combine those two things and we can tell you quite a lot about a given instrument, without ever having to hold one or test it.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 02:10:04 pm
content: David Hess sez, "Using an astable oscillator does work however the fixed delay between the comparators and output adds an error term which shows up as non-linearity and this error term changes at different frequencies."As in all things, cannot calibration eliminate this error, especially with the power of software?All instruments require testing/calibrations before their intended use.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 02:14:36 pm
content: Conrad HoffmandYou have stated the usefulness of an experienced judgement to create a generalization, but as in everything, this is somewhat a statistical approach to judgement, but this rush to judgement also means there can exist less-common outiers that slight a generalization.The proof of a pudding is in its tasting.Taking the care to improve the performance of what might seem by visual inspection to be just a mediocre performing device might just mean performance can very much be improved by making small improvements in parts used and other choices of layout and fabrication, and in this case also software.

author: BravoV
author_rank: Super Contributor
timestamp: May 09, 2018, 02:30:44 pm
content: Quote from: SuzyC on May 08, 2018, 01:12:39 pmQuote from: SuzyC on May 09, 2018, 11:48:31 amYour requirement is clear there.Quote from: SuzyC on May 09, 2018, 12:08:11 pmWhat kind of "very precise" that crystal capable of ? Number please ?Another question, you said you can get easily 0.02% resistor, and coupled with frequency, say your ideal circuit behaves as expected, how accurate the frequency needed in order to get the capacitance measurement accuracy down to 0.1% as you required ? Enlighten us (or at least me) please.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 09, 2018, 02:37:37 pm
content: Quote from: SuzyC on May 09, 2018, 12:22:42 pmWhich method are you talking about? The filter attenuation method some of us suggested needs accurate voltmeters and a low distortion signal gen as I said so yes it's not an option for your as I reckon and this is just what I said.The constant current charging method that I favored doesn't need any expensive lab equipment nor any attenuation measurement, and is quite simple, so I'm not sure what you're talking about. And if you need to get 1% absolute accuracy or better, I still think it's probably the best low-cost compromise. I don't see what is complex about it, and it's just a handful of parts costing a few bucks. Of course if you're aiming at zero cost, good luck.The bridge approach some people have suggested is good, but it still needs things you don't have: a reference capacitor, a reasonable sinewave voltage source and a reasonably accurate AC voltmeter as I guess? If you're just trying to match caps without caring about the absolute value, you won't need the first, but the two others you will. Getting accurate results with a distorted AC source and properly measuring the closest you can to 0V with a low-end multimeter in AC mode sounds like a lost cause to me even to get just 1% of accuracy.Also as I mentioned, if you think you can get an accurate evaluation of a capacitance over such a wide range with a NE555-based oscillator and this without calibration? There are too many factors that will get you inaccuracies.You may also keep things much simpler if you don't want to mess with caps measurements. With properly chosen capacitors and resistors values for the twin-t filter (again, with not too high a Q-factor), you can probably get away with 1% capacitors. There are off-the-shelf C0G caps with 1% tolerance for under 1$. Those are significantly more expensive than regular ceramic caps, but we're talking about under 1$ here. I'm really not sure trying to save a few cents on those parts would be worth all the trouble.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 02:44:08 pm
content: BravoVCheap choices for a crystal oscillator for a MCU can be easily found with specs of 10ppm or better. That's good enough.Example: $3 10ppm Xtal attached.The point here is that the precise freq. oscillation is stable and calibration and software can cancel out any small freq. errors.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 02:48:34 pm
content: Thanks agianSiliconWizard, for so generously penning a detailed reply!Also as I mentioned, if you think you can get an accurate evaluation of a capacitance over such a wide range with a NE555-based oscillator and this without calibration?As with any instrument,with calibration.You have a source of <$1 cog/npo caps in larger than 1000pf values in min quan <1000?

author: BravoV
author_rank: Super Contributor
timestamp: May 09, 2018, 02:58:16 pm
content: Quote from: SuzyC on May 09, 2018, 02:44:08 pmOk, so are you saying, with 0.02% resistor + 10ppm crystal , is enough that you can measure capacitance down to 0.1% accuracy ?Is there any formula for that ? So I can play with "what if" scenario if I have different accuracies on resistor or crystal.

author: IDEngineer
author_rank: Super Contributor
timestamp: May 09, 2018, 02:59:47 pm
content: All the 555, etc. based solutions are nice, but the question was "use a microcontroller to measure capacitance".The answer is to use a Microchip PIC microcontroller that has their CTMU module. This module isspecifically designedto measure capacitance, with remarkable repeatability. There is some die-to-die variance but the solution is to calibrate using a high accuracy resistor. And it's built in to a wide range of their microcontrollers, thereby satisfying your other requirement.We regularly use the CTMU to measure values down in the 10's of picofarads. It works beautifully. We originally prototyped with it, figuring we'd design up our own discrete circuit later, but there's no way we can achieve the same performance at anywhere near the same price - and being on the same die as the MCU and its A/D means we cannot improve on the integration either.As a starting point, check out the PIC18F23K22 or the PIC18F66K80 series. Even better, use their parametric selector tool and dial in the features you need. These two are 5V capable parts so you get more analog dynamic range, but you might only need lower voltage or not need the on-chip EEPROM or whatever.Unless you're building a lab-grade instrument, there's no reason to use any other approach. A sub-$3 part gives you everything you asked for in your subject line.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 03:04:42 pm
content: Thanks a million Puff, IDEngineer!We regularly use the CTMU to measure values down in the 10's of picofarads. It works beautifully.This might be the best answer to this problem with the least cost and time involved!!I will check this out.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 03:12:08 pm
content: BravoVIt is not just a question of a "formula for that", it is a question of calibration and software, but, of course, it must be based on stable and precise actors involved in the actual measurements and calibration. I think I have that available.In some raw circuit, .1% means I must ace 1000ppm in freq. accuracy, and the calib. resistor is already good to .02% or better.Can something measure .1% with even 1% components. The answer is a big Yes, with calibration, software.

author: Cerebus
author_rank: Super Contributor
timestamp: May 09, 2018, 03:50:30 pm
content: Quote from: SuzyC on May 08, 2018, 04:08:57 pmI don't think you quite understand the territory.The Genrad capacitor you've referenced from IET is just that, a single variable capacitor. A secondhand one of these, without calibration, will set you back $300 USD. A new one, with tracable calibration certificate? If you need to ask you can't afford one - I'd guess in the region of $3000 USD. Their target market is national and primary calibration labs. This is the device you use to check the accuracy of an LCR meter, it's a step down from a primary calibration standard.So that 0.01% specification for measuring its capacitance has been achieved in the temperature and humidity controlled calibration lab of a world class standards company, one that itself supplies national standards labs. If they can only economically achieve only 0.01% (and trust me, if theycoulddo better they would) is it realistic to think that one would be able to achieve a result only 10 times worse on ashoestringbikinistring budget? For comparison, Aglient's $16,000 E4982A LCR meter has a basic measurement uncertainty of 0.45%, a result 45 times worse.To put the whole absolute accuracy of capacitor measurements into perspective, the best achievable uncertainty in a primary standards lab environment is ±5 ppm (0.0005%) for a precision built primary reference capacitor. To put that measurement itself into perspective, that kind of capacitor can reversibly change its value by 20 ppm just by changing its orientation.Being realistic, you're not going to see an absolute accuracy better than 1%, on a good day, with the wind in the right direction, and your tongue at the right angle. However, what you're really interested in is notabsoluteaccuracy, it's the relative matching of a bunch of capacitors. That you can achieve with very high accuracy using a very old fashioned bridge circuit (sine wave source, two matched resistors and a half decent AC voltage meter). I'm not going to put any figures on that, Conrad's your man for that sort of thing (as in I suspect he can quote figures off the top of his head).You can scrabble back yourabsoluteaccuracy in the frequency domain, where measuring to 1 ppm is trivial, or the resistance domain where 0.1% accuracy is the domain of relatively cheap handheld DVMs.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 09, 2018, 04:16:43 pm
content: Wait! I'm a hardware-centric dinosaur! I'm an idiot! Go download a free copy of Visual Analyser here-http://www.sillanumsoft.org/With the sound card in any computer and a couple opamps, it has a high resolution ZRLC meter that you can calibrate to about any degree of accuracy you have a standard for.http://www.sillanumsoft.org/ZRLC.htmAfter you finish your notch filter it will generate a signal and let you measure the response or do an FFT.Unless you're using a Mac. Or can't wire a couple opamps.I think there may be other programs that can do this to some degree as well.

author: Gyro
author_rank: Super Contributor
timestamp: May 09, 2018, 04:17:08 pm
content: Quote from: SuzyC on May 09, 2018, 01:31:23 pmAn AC source is simple, an 555 timer astable circuit will do the job. Set the frequency to match what you're interested in, you could RC filter the output to round it off a bit, but leaving it as squarewave will do (it match parasitics too). For the AC detector, don't you have some form of audio amplifier and headphones? Ears are very sensitive for such things, match for minimum sound.For the resistors, you can use the close tolerance ones that you are presumably buying for the Twin-T.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 09, 2018, 04:22:34 pm
content: Quote from: SuzyC on May 09, 2018, 02:48:34 pmHow are you going to calibrate without reference capacitors? How are you going to get ahold of those?Quote from: SuzyC on May 09, 2018, 02:48:34 pmMouser, Digikey... (but Mouser seems to have more corresponding parts.)For instance: Kemet C0402C102F5GACTU (1 nF / 50 V / C0G / 1%).Just as a last word, the approach (and schematic) I suggested IS a CTMU. Only with fair precision parts (that are still not expensive), with the added benefit that it should get you reasonable accuracy even without any calibration.Yes you can do this with integrated µC CTMUs, but you'll have to calibrate (again, with what?) and I doubt you can get 1% precision or better over this wide range of capacitance. Now if you can get ahold of a couple precision reference capacitors, why not.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 04:27:06 pm
content: Cerebus:AnAbsoluteaccuracy is only approachable withing standard calibration labs, but I find it hard to believe that even a goal of 1% is very difficult and so it follows that .1% of absolute capacitance value is not reasonably achievable..1% resolution of capacitance value seems a cinch.I can't help but reason that the limit of capacitor measurement is must be somehow limited to 1%.This 1% arbitrary limit might is likely a pessimistic guess that cannot be glued to a capacitance measuring device that employs analytic circuit design combined with software and hardware calibration. I really think this could be easily achievable(if not immediately verifiable) when calibrated with and when known valued caps(of 1% or better precision/absolute accuracy) are used for comparison.As my mama said once while reading me bedtime stories, "Even the best atomic clock in the universe is not as accurate as a broken clock, and this is so, twice each day!"

author: IDEngineer
author_rank: Super Contributor
timestamp: May 09, 2018, 04:40:45 pm
content: Quote from: SiliconWizard on May 09, 2018, 04:22:34 pmThe beauty of the CTMU approach is that you calibrate using a RESISTOR, which can be had in very tight tolerances. The reason is that what you're actually characterizing is a constant current source. Once you know the current, the standard equations for charging a capacitance apply. You do not need a bunch of reference capacitors.Our most recent CTMU design is using a 0.1% 1206 resistor for calibration. Cost is 21 cents - sure, more than a popcorn SMD resistor, but still quite affordable compared to high precision caps.A primer on the resistor-based calibration technique is in every PIC spec sheet that has a CTMU. A few minutes of reading will bring you up to speed. There are tricks that can be played beyond the basics they describe, but the primer will get you well down the road.EDIT: Note too that you can calibrate once and store the calibration data in the on-chip EEPROM (if you use a PIC that has EEPROM), or simply recal on every powerup. Or, heck, on every measurement. Lots of options depending upon your application. The latter choice is obviously the most accurate as it would take into account drift, temperature effects, etc. The calibration process isn't lengthy nor complex.We went down this road of "how do we measure capacitance" a long time ago. Everything in this thread sounds very familiar. And the end result of that journey was the CTMU. YMMV and do your own research, but I suspect you'll be hard pressed to do as good, or as inexpensively, unless you're designing a lab quality reference standard.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 04:46:29 pm
content: SiliconWizard Thank you very much for your comprehensive and well-thought out replies.I must apologize for speed reading your reply and not seeing the point you were making about using a cc source, which seems to be the basis for a simple and precise way to accomplish my goal.C0402C102F5GACTU USD 1.37 approx ea found on mouser. Just the right capacitor to use as an affordable "standard".Strange, some hours ago I first searched for similar and Mouser's search engine found just a few cog caps vals sold only in 1000+ quan.The most important thing in getting a good answer is to know what question to ask.I only have a pile of 2% 20000pf film available at the moment.Can I safely assume assume that since I have 35+ of them that most of them would lie well within 1 std deviation of an absolute accuracy of 20000pf just due to the likely tight control of the process of manufacturing?So, those that compare best with each others are going to bethe best approximation of their absolute value(if I got the right batch!).

author: oldway
author_rank: Super Contributor
timestamp: May 09, 2018, 04:47:43 pm
content: Quote from: SuzyC on May 09, 2018, 11:48:31 amSome explanations : This astable oscillator will oscillate at : F=0.722/R1C1.F is then well inversely proportional to the value of C1.300Khz is more or less the max frequency you can use the Cmos version of 555. (TLC555 for example.)Than R1 must be more or less 47K. For stability, It must be a resistor with nul temperature coeficient.You wrote:QuoteThat's right and it varies with temperature also.The frequency formula shows that the frequency is not dependant from power supply voltage, nor temperature.....Then, how to explain that it varies when in theoria, it should not ?This is because the output stage of the Cmos 555 is far from being a perfect switch...It does not go to 0V (Vss), nor to the power supply voltage (Vdd).Indeed, with a power supply of 5V for exemple, with a 1mA current, output voltage (Voh) can vary between 4.1 and 4.8V...this value varies with temperature and output current.Low level voltage (Vol) with 3.2 mA sinking current varies between 0.13 and 0.4V...Using an inverting buffers Cmos 4049B (one inverting buffer and 5 inverting buffers in parallel) or a non inverting buffers Cmos 4050B, whe are eliminating the problems of instability with variations of temperature and with variations of power supply.Indeed, first whe divide the current in 5 ou 6;Second: the buffers are far better switches then the output stage from the 555.With 5V power supply (Vdd)Voh : min: 4.95VVol : max: 0.05V

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 09, 2018, 05:00:34 pm
content: Quote from: SuzyC on May 09, 2018, 04:46:29 pmQuite right. A word of wisdom.Quote from: SuzyC on May 09, 2018, 04:46:29 pmWell, on just a few tens of parts, I'm not sure you could assume this, and still, that sounds like a lot of pairs to compare!But if you can trim the resistors, maybe 2% tolerance would be a decent fit for your twin-t filter?

author: IDEngineer
author_rank: Super Contributor
timestamp: May 09, 2018, 05:00:41 pm
content: Quote from: SiliconWizard on May 09, 2018, 04:22:34 pmYour constant current source uses a REF200. Have you priced one of those lately? And then you still haven't paid for the MCU that was requested in the title.A CTMU-equipped PIC already HAS a constant current source, without all of the standalone discretes, and it's controllable by many of the other integrated peripherals such as hardware timers which yield extremely high resolution temporal control of the constant current source without being dependent upon firmware instruction cycles. It also contains a S/H which further isolates you from the (potentially) non-deterministic behavior of a firmware-driven approach due to various leakage (read: discharge) currents in any design, whether integrated or discrete.All for under three dollars.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 05:06:16 pm
content: Thanks Oldway!I don't quite wrap my head around why you focus on the max freq of osc. of a CMOS 555, the best frequency for this measurement is always going to be with the lowest freq, that is having the longest period to maximize the available resolution of the MCU counter peripheral.So, I get it that the CMOS buffers are going to buffer the output of the 555 and connect then to the timing resistor.But again why the buffers, when of course using a very tightly regulated Vdd supply! Temperature variations are probably not too important as this device would be used with the range of comfortable room temperature, not in Siberia one day and Syria the next.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 09, 2018, 05:13:51 pm
content: Quote from: IDEngineer on May 09, 2018, 04:40:45 pmYou're making a good point. I remember using a CTMU (don't quite remember on which MCU, a Microchip I think, that was years ago), and the way it worked, it was not possible to directly do this as I reckon. I think it relied on a voltage change to work properly.Anyway if your MCU has a CTMU that allows this, that's a nice solution. But still, two things I'm wondering. You have the current source characterized, but does the CTMU not rely on an internal voltage reference as well? How can you characterize it? Is it accurate enough? Also, the internal current source is probably not as good as a REF200 or similar, in terms of output impedance for instance.So I'm still not 100% convinced that you can get a real accuracy of under 1%, but I must admit it's elegant and cheap.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 05:15:56 pm
content: SiliconWizard, Thanks again!The idea is that once I choose the capacitor that best fits the median value of the batch, I can call that my "standard cap 20000pf."Now I also have 2000pf and 3500pf 2.5% capacitors. Shouldn't the best agreement with the non-20000pF "standard" verify that the 20000pf standard value well approaches the absolute value of its capacitiance?

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 09, 2018, 05:40:04 pm
content: SiliconWizard: Thanks, you've made a very good point.The built in cap meas pin of a MCU couldn't possibly be as easy to achieve accuracy with considering the inaccuracies introduced by the MCU internally considering the limited precision/stability/noise content/offset/bias currents/reference voltage stability for the current source etc. that would also likely vary with temperature and VDD.Your idea to build a similar working meas. device using a quality CC in an carefully fabricated external meas. environment has to be the better!Somehow the CTMU idea seems too good to be true to obtain the precision I want.

author: oldway
author_rank: Super Contributor
timestamp: May 09, 2018, 05:45:18 pm
content: Quote from: SuzyC on May 09, 2018, 05:06:16 pmUsing the max frequency is important to minimize the influence of leakage and input impedance of THR and TRIGGER inputs from TLC555 on value of the frequency.R1 should be as low as possible....Here, 47K.No need to be a precision resistor.No need of a tightly regulated Vdd supply...With a buffer, frequency don't vary with Vdd.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 09, 2018, 05:54:18 pm
content: Quote from: IDEngineer on May 09, 2018, 05:00:41 pmYes, they can be had for around $8-$9 for one piece (6 € at Mouser). The overall BOM would probably be less than $20-$25. We've probably all have consumed a lot more than this just talking about it.Quote from: IDEngineer on May 09, 2018, 05:00:41 pmI admit it's unbeatable cost-wise, but I'd still be interested in comparing the two approaches.

author: David Hess
author_rank: Super Contributor
timestamp: May 09, 2018, 05:56:24 pm
content: Quote from: SuzyC on May 09, 2018, 02:10:04 pmSure it can, but it represents another source of error. Even worse, the delay varies depending on other factors like supply voltage and temperature.The error due to the delay limits the accuracy of analog function generators at high frequencies so they often include circuits to compensate for it and sometimes a calibration adjustment which causes the comparators to switch earlier at higher frequencies.QuoteSure, but reducing uncertainty makes testing and calibration easier. Bob Pease once pointed out that it is easier to compensate a 20ppm/C circuit to 0ppm/C than a 200ppm/C circuit.I was also considering your original post from the perspective of best accuracy with a minimum of calibration using reference capacitors and a simple design with more common parts. Of course you still need high accuracy resistances and voltages and time measurement.Quote from: SuzyC on May 09, 2018, 05:06:16 pmLow frequency operation with a low value of capacitance requires higher impedances which leads to a whole new set of problems like leakage and electrostatic coupling from other sources.Consider your accuracy requirements in relation to old test instruments and their complexity. I have a pair of ancient ESI 250DA impedance bridges which supposedly can achieve 0.2% + 1 digit on low dissipation capacitors using a built in 0.1% reference capacitor. Decades later I have no reference capacitor or capacitance tester good enough to verify this level of performance. An instrument like this would have no trouble however grading capacitors into matched sets of better than 0.1% and many of the discussed designs can also do that despite lower absolute accuracy.Note that I do not really recommend picking up an old impedance bridge; they are a pain to use although perhaps not so difficult for grading parts of one value.

author: IDEngineer
author_rank: Super Contributor
timestamp: May 09, 2018, 06:03:04 pm
content: Quote from: SuzyC on May 09, 2018, 05:40:04 pmAll I can say is, try it before you scoff.I personally participated in some very long, very detailed conference calls with rooms full of Microchip Silicon Engineers and Applications Engineers. They put a LOT more thought and Engineering into the CTMU than we expected. The linearity of the current source, the leakage of the switches, etc. were all carefully considered. And the results speak for themselves. Using a 0.1% reference resistor for calibration, we easily get sub-1% repeatable results on *production* devices built in the hundreds per month.A VERY carefully designed and laid-out discrete approach might equal that, but with much greater effort and cost. If you actually NEED the highest possible accuracy, like if you're building a lab-quality measurement device, then sure - a CTMU (probably) won't get there (but honestly, it's been so amazing I'd characterize it before I dismissed it). But for anything short of a lab reference, I'd start with the CTMU and prove it WON'T work in the given application because I strongly suspect it would do just fine, and the surrounding infrastructure would be far easier to implement in the bargain.SiliconWizard, you're correct that an accurate voltage reference is needed. But those are cheap and easy. If 1% accuracy is acceptable, use a 1% voltage regulator and just use the supply rail. Tighter voltage references, to almost any spec you desire, are off-the-shelf, inexpensive, and readily available (check DigiKey or Mouser to start). A very accurate voltage reference is still cheaper than the REF200, and honestly... after discussions with Microchip and our own in-house experience, I suspect the CTMU current source is more accurate.The REF200 is only spec'd to 0.5%. We're seeing better repeatability and absolute accuracy than that. And if REALLY high accuracy is desired, the CTMU has three separate current ranges each of which can be adjusted under firmware control over a +/-60% range. Slap in a single resistor of the desired precision, measure the voltage that develops across that resistor due to the current source, trim in firmware in real time, and bingo - you can get much, MUCH more accurate than the REF200's 0.5%. In reality, the CTMU's accuracy is only limited by the resistor you use to calibrate it... and tight-tolerance resistors are cheap and readily available. DigiKey lists 0.01% resistors for $1.03/100's. That's 50X better than the REF200. Back in the real world, the 0.1% resistor we use is ~50 cents and that's still 5X better than the REF200.Disclaimer: I don't work for Microchip. But I've been mightily impressed by their CTMU. I have a LOT of experience with it now, over a period of ~2 years, and you'll be hard pressed to duplicate its accuracy and repeatability in discretes for any realistic cost. And if you want to do so in a production environment, you'll burn a lot of labor hours duplicating what the firmware self-calibrating nature of the CTMU gives you for free.

author: nctnico
author_rank: Super Contributor
timestamp: May 09, 2018, 06:14:53 pm
content: An easier approach is to use a synchronous rectifier as they often use in LCR meters. Apply a known AC current and measure the AC voltage across the device. The synchronous rectifier will cancel the noise. If the measurement range is narrow then you can built this with really cheap hardware without losing accuracy. There are some examples on this forum.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 09, 2018, 06:46:03 pm
content: Quote from: IDEngineer on May 09, 2018, 06:03:04 pmYes, although I've measured a few with an expensive SMU and have seen much better initial accuracy than 0.5% (very impressive), but granted you can't rely on that.Thing is, you can measure the actual current with a decent multimeter that has 0.1% error or better in DC current mode. You can find this kind of accuracy in middle-end multimeters. The point here is that you only need DC measurements. This kind of accuracy for AC measurements in middle-end DMMs is hard to impossible to find, but for DC, it's relatively common. Same for measuring the voltage references.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 09, 2018, 07:20:43 pm
content: Measure some of your 20000 pF caps and choose one in the middle. Call that your lab standard. Even if it's slightly off, how would you ever know or care?BTW, I've built a few notch filters and I've always sidestepped the whole problem by including a trimmer cap and/or a trimpot. Tune by result.

author: PA4TIM
author_rank: Super Contributor
timestamp: May 09, 2018, 07:24:54 pm
content: Measuring capacitance is hard because a lot of other things play a roll. For instance frequency behavior, parasitics, ESR, leakage, dielectric absorbtion, tempco, voltco, piezo electric effects, and then things like shielding and guarding. You need a very, very, very good cap and meter to measure the same value for a certain bandwidth.Like Conrad Hofman I think a bridge circuit is the best option unless you can use some factory calibrated IC. But only if you build a correct circuit and measure ideal caps in an ideal fixture.My best capacitance meter is a 0,01% GR-1620 bridge that is still in production (you really do not want to know the price but it is one of the best, if not the best, on the market. Measuring a cap to full accuracy takes something like 10 minutes and that a few times because it is very hard to do it right. Everything must be shielded and the zero detector used to the fullest sensitivity of 1uV for a full scale. I also have some capacitance standards.I have a GR-1608 back then stated as 0,1% but it turned out to be much better and is later re-spec-ed, I had to replace a resistor, that was almost 40 euro. Not only the initial value accuracy is important but also tempco and things like sensitivity to humidity. This was the cheapest choice to get the right performance (it was advised to me by the original designer of the bridge who helped me with advice and documentation)My two daily meters are 0,1%. To measure with 0,1% is a whole different story. Both uses a 5 terminal measurement. If I use them 2 terminal I will not even come close to 0,1%.But one thing I missed in the discussion,Allmeters that are capable of getting this kind of accuracy measure the complex impedance or admittance. You can not measure only capacitance if you want 0,1%. You need to measure the reactance (and calculate C from it or scale it in capacitance) and real resistance part too. Or only measure calibration grade caps with an ultra low loss angle. For pF you can use aircaps or vaccuum caps, for 100 pF and up Mica is a good choise but you need to calibrate them and select them for lowest DF, 100 nF and up I think polystyreen as dielectric.I tried to make my own caps by calculating them but I not even came close thanks to things as the fringing effect. The dielectric is the biggest problem, If you only use air and two guarded rond plates it is doable but still hard, besides that it becomes huge (Marconi builded a big barn for them to hang from the ceiling) I made a coaxial one that came close (only 50% off) but because the core would not float in air alone it needed some support and that made the fault over 100%.I tried oscillators, F.E. Terman writes about a methode in his book about measurements. I tried that but it was hard to get it right. The oscillator was very hard to get stable enough (and you need to measure that with a very good counter) The biggest problem I could never figure out. As soon as I connected the CUT all went well and I could very precise measure the frequency shift but if I removed it, the oscillator had a higher frequency as before and would not go down unless I switched if off a while.But without calibration grade gear you will start a very frustrating journey. For one because you will never know if you reach 0.1%The easy way is matching them. Use a sinewave and measure voltage and current, oh wait, you only have a 3 digit meter (and low BW ?)Then use two caps in series. Use an ac signal and measure the voltage over both. You can calculate the reactance of the unknown cap from the ratio and then recaculate the unknown capacitance. Or you build a bridge....(and even then forget the 0,1%)Just use the best caps you have/buy or maybe there is someone here nearby who can measure them for you (although I think only a few of us really can measure a tracable 0,1% and then for a limited frequency range. Do an impedance measurement from your best cap from 100Hz to 1 MHz and you will not get a straight line.

author: ogden
author_rank: Super Contributor
timestamp: May 09, 2018, 10:40:09 pm
content: Quote from: SuzyC on May 09, 2018, 01:59:48 pmCannot agree more.Youdefinitely need more of both - learning and practice.You (hopefully) could find "Impedance Measurement Handbook" interesting:http://literature.cdn.keysight.com/litweb/pdf/5950-3000.pdf

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 10, 2018, 01:07:00 am
content: I perused the appnote you recommended for me to read and I have these first impressions:(1) This document has no revision date and so the the characterizations of components(but certainly not the theory) may be well out of date. In my opinion, an scientific document that fails to give a publishing/revision date is highly suspect of expressing knowledge that can likely be obsolete.The idea of a scientific/tech paper published without any publication/revision dates is very bad form!For instance, on a page(Table 5.1) describing the various dielectric materials used in making capacitors:Ceramic:Bypassing, coupling, and filtering to high frequencyHigh capacitance to volume ratioChip style availableLow D (low k type)Low costExtensive test data, reliablePoor temperature coefficients and time stabilityLarge voltage dependencyand susceptible to pressure (high k type)It seems obvious this article was composed when chip capacitors were a rare item ("Chip style available")Where does an almost ideal ceramic capacitor, the COG chip capacitor, find itself mentioned?---------------------------------------------------------------------------------------

author: ogden
author_rank: Super Contributor
timestamp: May 10, 2018, 01:31:15 am
content: Quote from: SuzyC on May 10, 2018, 01:07:00 amLast page:© Keysight Technologies, 2009 - 2016Published in USA, November 2, 2016QuoteAre you kidding? - This book is not catalog of capacitors. That table is least significant part of that book. Just read chapters *after* it

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 10, 2018, 01:43:56 am
content: Ogden, I disagree strongly. For an engineer or even a hobbiest the idea of studying theory about components is to find parts to use in practice and any publication that fails by omission to show the latest tech available and without a clear revision/history/published date on its front page is highly suspect of disseminating questionable and incomplete knowledge.This is comparable to reading an a very scientific/scholarly article on transistors that only characterizes and mentions point-contact transistors as those devices being most common and available.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 10, 2018, 03:12:42 am
content: Quote from: SuzyC on May 10, 2018, 01:07:00 amChip capacitors were around in the 1970s, although they were not in the fairly high capacitance values common today.I have memories of shunting chip caps around with a toothpick on RF power Amplifiers to obtain the required frequency response required.The reference to "chip style available" just sets out the different chip packages which were available.There is nothing in that to indicate they were rare.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 10, 2018, 03:25:20 am
content: The perfect app note has never been written. That Keysight one is quite complete and has a lot of good theory and practice. You only get somewhat complete knowledge by gathering and digesting data from a multitude of sources, so don't stop there if something isn't presented the way you like it. People have been measuring capacitors for something like 140 years or more, so there's no shortage of accumulated knowledge on the matter. IMO, a lot of the good stuff is not on the 'net or is very difficult to find. But, I think the original goal is being lost- just matching some caps well enough to make a decent notch filter.

author: Cerebus
author_rank: Super Contributor
timestamp: May 10, 2018, 03:29:22 am
content: Quote from: SuzyC on May 10, 2018, 01:07:00 amYou highlight that as if you believe there is something wrong or outdated in that little snippet. If so, I suggest you read a few datasheets for high K ceramic capacitors or watch Dave's video on the subject - thread herehttps://www.eevblog.com/forum/blog/eevblog-626-ceramic-capacitor-voltage-dependency-download/. In that video you can watch a 10uF X5R capacitor lose 50% of its capacitance just because it has a 5V DC bias across it.I have a suspicion that you're suffering a bit from the Dunning–Kruger effect here. Your continued insistence that measuring capacitance to 0.1% should be a "cinch" when experienced hands say it isn't, making rookie mistakes such as referring to COG capacitors when the term is actually C0G, finding fault with a highly reliable reference on impedance measurement from Keysight. I'd advise that you slow down and listen more, but I have a suspicion that you'll just ignore me and charge on headlong.

author: phenol
author_rank: Regular Contributor
timestamp: May 10, 2018, 05:14:27 am
content: Quote from: SuzyC on May 09, 2018, 12:45:12 pmThe design is not mine...so i didn't make it, nor did i try to reproduce it. I personally use U1733c.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 10, 2018, 09:27:04 am
content: Cerebus: You misquote me. I said ".1%resolutionof capacitance value seems a cinch."Don't make me look like a bigger fool than I know i am.

author: SuzyC
author_rank: Frequent Contributor
timestamp: May 10, 2018, 09:32:32 am
content: Conrad Hoffman: Thanks for your most recent and apropos comment."I think the original goal is being lost- just matching some caps well enough to make a decent notch filter."Phenol: My apologies!Cerebus: I had already long before I've made any comments here been aware of the severe poor characteristics of some type of cheap ceramic capacitor materials and I have several months ago watched Dave's video on this subject. What bothered me enough to make my comments about the appnote was an obvious omission of C0G ceramic caps. If someone was not already familiar that this capacitor option was available, one could easily think that their idea of design was impractical, too costly, too custom to even begin a design..there just wasn't any capacitor practically suitable for their design.vk6zgo: Thanks for giving me a better understanding of the history of the use of chip caps. I am at fault in judgement here, because the migration from through-hole to SMD devices marks a giant change in the way electronic devices are built and SMD is long now been the norm.The appnote seemed to me to be outdated because it would seem unbelievable to even imagine that anyone who has examined or designed anything built today would need to be reminded that SMD components are "available".If I look beyond the shortsightedness of C0G" omission, this article seems to me to be an excellent technical article that can give an engineer a thorough theoretical understanding of the parasitic properties of capacitors and inductors.IMHO capacitors don't exist to be exotic parts whose only use is in sitting in complicated and sterile test fixtures. Instead, they most certainly must be understood in proper terms describing their absolute accuracy and performance andmatchingwhile sitting on some PCB or breadboard.I don't hold sacred or"believe" that measuring a capacitor to .1% or so is practical outside a standards lab, that would violate my sciolist license, but I remain mostinterestedin that it might be possible that you guys with MS, pHD's behind your name, or even some hobby-hacker might come up with a clever and simple circuit design would come as closer to breaking what is widely-believed to be possible as the limit.It might be possible, for someone to find a way to find a simple way to gain ground on this somewhat arbitrary limit of measurement, and this idea was worth playing with. I assure you all that my original intent was not badmouthing the Most Sacred god of Measurement, Signalent for their high-priced equipment and poorly date-exposed appnotes when starting this post, but in the end, I've failed to avoid this faux pas.What I've learned about capacitance and its measurement:I have succeeded in discovering how to use the power of LTSpice at some level of competence.I thank everyone for their help in furthering my understanding the complexities and limitations of capacitance measurement and I can now can fiddle around in my little lab with the ideas you've so generously given me.What seemed to be an innocent request to learn how to match capacitors to more easily build Twin-T's has turned into a troll-like fight that now has reached the point of resorting to attack by misquote, focusing on my small typos, and finally by character assassination.I surrender and willingly withdraw from making any further comments on this topic.I am off to the beach!

author: Cerebus
author_rank: Super Contributor
timestamp: May 10, 2018, 01:49:19 pm
content: Quote from: SuzyC on May 10, 2018, 09:27:04 amMy apologies, you did indeed in that one instance say exactly that and I missed it. What blinded me to it was (1) that messages was a little hard to read your intent (go back and check, the grammar is all over the place), and (2) you repeated that 0.1% accuracy (or more loosely an unqualified 'measurement') must be doable.

author: Cerebus
author_rank: Super Contributor
timestamp: May 10, 2018, 02:06:44 pm
content: Quote from: SuzyC on May 10, 2018, 09:32:32 amCharacterising a little polite constructive criticism as "character assassination" is grossly unfair. Taking that attitude will rapidly lead to no-one trying to help you if they know that the response to any criticism is that you'll flounce off instead of trying to move forward. Certainly you've convinced me that it would be foolish of me to attempt to offer you help in the future lest even the slightest correction provokes the same unreasonable, ungrateful, unpleasant response.

author: David Hess
author_rank: Super Contributor
timestamp: May 10, 2018, 02:49:51 pm
content: Quote from: vk6zgo on May 10, 2018, 03:12:42 amAxial leaded capacitors packaged in glass and later in epoxy use what we would recognize as multilayer surface mount chip capacitors inside the packaging. There were a lot of these produced in the 1970s to 1980s and they could use the same assembly equipment as 1/4 watt resistors and axial leaded diodes. Capacitor values in this physical size up to 0.1uF were very common but higher values did exist.The capacitors shown below (they are all the exact same series) were originally in glass packages as shown which date from the 1970s. In the 1980s and later, they used epoxy moldings.

author: mikerj
author_rank: Super Contributor
timestamp: May 10, 2018, 03:14:49 pm
content: Quote from: SuzyC on May 10, 2018, 09:32:32 amYou are missing the point entirely here. X5R/X7R dielectrics are not used for cheapness, they are used when you need high capacitance in a smallest physical size.

author: oldway
author_rank: Super Contributor
timestamp: May 12, 2018, 10:48:34 am
content: Quote from: SuzyC on May 10, 2018, 09:32:32 amI think you should analyze your behavior on this forum because if there is indeed a troll in this topic, it is yourself.You wanted to play the team manager but you have no skill for this function ...You talk to engineers, business managers, researchers, all people who put their skills on a free and voluntary basis to help you.So, if you want to play the team manager, you have to have the right skill, which you do not have.1) the goals to be achieved must be clearly stated: there are two types of measures: absolute measures and relative measures .... what kind of measure is it? There is a huge difference between the technical difficulties of each type of measurement, for absolute measurements, precision and stability are required, as well as calibration, for relative accuracy only resolution is required.This is not clear in your presentation of the problem ....2) you should never impose impossible goals: measuring the value of a capacitor with an accuracy of 0.1% is a highly specialized laboratory work that uses high-precision devices worth tens, if not hundreds of thousands of dollars. ... So, asking to do the same thing with a simple and inexpensive circuit is to prove that you do not know anything about it.So just ask for the best possible precision, without specifying values.3) You must demonstrate that you are involved in this project, while remaining modest with regard to technical considerations, as you have demonstrated that you do not know anything about it.Someone who wants to save money to go to the beach is certainly not involved in this project.4) since it is shown that you do not know anything about it, why impose restrictions?What is the microcontroller doing here?It is only after several pages that we come to know that the goal is simply to match capacitors and not to measure their absolute value ... it is quite different from what was presented, not surprising that several participants responded with other considerations that you call "troll-like"Review your position if you do not want to mess up your reputation on this forum.

author: oldway
author_rank: Super Contributor
timestamp: May 12, 2018, 11:00:30 am
content: Something important when you measure your capacitors: