charge integrator , very low value feedback capacitance - Page 1
Forum section: electronics / beginners

author: sdouble
author_rank: Frequent Contributor
timestamp: March 22, 2015, 02:11:34 pm
content: Hi everybody,I have a question, which is, in principle, very basic.I need a "pure" capacitor of 0.3 pF with a tight tolerance of 0.03pF.To make is simple, I have a charge integrator.see the last scheme of :http://en.wikipedia.org/wiki/Charge_amplifierThe "gain" (sensitivity) is governed by the cap Cf.I need it to be 0.3pf.I went for 3 0402 capacitors of 1.2 pF (0.05pF tolerance) in serie.Those 3 caps are discharged via a set of 3x 10 MegOhms (0603) resistors (Rf).It's working quite well but I have parasitics caps which are producing other poles in the decay of the charged caps.What is unclear to me is :whether I should reduce the number of caps (go to 2 x 0.8pF caps ) or increase that number to reduce the parasiticsAnother related question. Shoud I go to 0201 capacitors instead of 0402 ?The simplest thing would be just to try. The point is that every production cost about $1,500, I try to limit the number of batches.I already went to Rogers 4003c instead of FR4 and that helped very significantly. I also went from lead free HASL to immersion silver.Any suggestion is more than welcome.

author: mathias
author_rank: Regular Contributor
timestamp: March 22, 2015, 03:13:35 pm
content: This is probably not going to work. Stray capacitances will dominate your circuit. What is that you want to measure exactly?Rogers/Duroid/Whatever isn't going to help, you'll still have moisture, grease or dirt on your board, creating leakage currents. For this kind of stuff, you work with air connections and teflon standoffs. E.g. look at the LMC6001 datasheet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 22, 2015, 03:18:30 pm
content: Is that really very different from the amp's dominant pole? If fT is not guaranteed by design, it will dominate your error.I wouldn't even use a component. Use the PCB itself for such small capacitances. Mind that it will drift (RF board materials would help in that regard). But since the above will likely be true anyway, it should be calibrated out.10s of megs against single pF isn't a very long time constant. I take it that's your sampling window? Is this something like a photodiode pulse counter?Tim

author: Marco
author_rank: Super Contributor
timestamp: March 22, 2015, 03:33:27 pm
content: Wouldn't the frequency dependence of most PCB substrates bite him in the ass if he tried a PCB capacitor?

author: mathias
author_rank: Regular Contributor
timestamp: March 22, 2015, 03:36:59 pm
content: Not if it's Rogers material, that is controlled impedance/epsilon_R up to several GHz.If this is going to be a photon counter as T3sl4co1l hints, I would recommend using an APD instead of an ordinary photodiode. Or even a PMT, but sub-pF is just plain silly.

author: TimFox
author_rank: Super Contributor
timestamp: March 22, 2015, 04:10:01 pm
content: For controlled low-value feedback capacitor, you may want to use a "tee" network, shown in the attachment, to achieve a small feedback capacitor with the disadvantage of an extra shunt capacitor across the amplifier input. This allows "sensible" sized capacitors (1 to 10 pF) to achieve < 1 pF feedback.

author: c4757p
author_rank: Super Contributor
timestamp: March 22, 2015, 04:32:47 pm
content: Replacing C1 and C2 with resistors gives almost the same effect. I wonder what practical difference that would have.

author: Marco
author_rank: Super Contributor
timestamp: March 22, 2015, 04:48:47 pm
content: Quote from: mathias on March 22, 2015, 03:36:59 pmThat's nice, but it's behaviour below 100 kHz seems relevant here (hook, dielectric absorbtion, etc). Too low for detail to even be visible on the datasheet (not that that stuff ever seems to be specced).

author: Marco
author_rank: Super Contributor
timestamp: March 22, 2015, 05:10:22 pm
content: Quote from: sdouble on March 22, 2015, 02:11:34 pmOkay .... don't see why, but okay.QuoteThat contradicts your first statement.QuoteEven more so.It's almost like you are using the term tolerance in a way quite different from what it usually means.

author: mathias
author_rank: Regular Contributor
timestamp: March 22, 2015, 05:48:29 pm
content: Quote from: Marco on March 22, 2015, 04:48:47 pmI would think that the behaviour of the material remains quite constant. Something as dielectric absorption typically goes up when the frequency increases, and limits to a constant value for low frequencies. 1% variation in epsilon_r, dielectric absorption, ... (what the topic starter was talking about) is surely out of the question for only 100kHz. For example, if you transmit a fast pulse through a coplanar waveguide, all the low frequency stuff gets transported nicely through. It's only the high frequency edges that get mutilated.Still, I'm wondering why you would need such a small capacitance for an integrator.

author: Marco
author_rank: Super Contributor
timestamp: March 22, 2015, 06:04:43 pm
content: Quote from: mathias on March 22, 2015, 05:48:29 pmYou are thinking of dielectric loss. Dielectric absorbtion is an effect more pronounced at low frequencies, see the last page ofAN-348.Really, asingle good capacitoron a Teflon standoff seems the best bet to me (there is nothing to be gained by using multiple capacitors as far as I can see, it's almost like sdouble confused tolerance with precision over temperature).PS. the resistor(s) need to be on a standoff as well of course ... as far as I can see everything mounted on the PCB becomes a PCB capacitor to a relevant degree at these capacitances.PPS. is Phill Hobbs still active on sci.electronics.design? This really seems up his alley, although I assume he'll want more detail.

author: mathias
author_rank: Regular Contributor
timestamp: March 22, 2015, 06:22:24 pm
content: Quote from: Marco on March 22, 2015, 06:04:43 pmThanks, I learned something! Really interesting app note.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 22, 2015, 07:05:39 pm
content: Quote from: Marco on March 22, 2015, 03:33:27 pmSure, and temperature, and humidity, and manufacturing... it's all a vile mess, something best avoided by smart design.The tolerance might not be as bad as you'd think; scopes are practical on FR4, needing stability in the 1% (amplitude) range. They do have the advantage of low impedances, outside of the attenuator section. This circuit apparently has to be extremely high impedance, even at high frequencies (but how high hasn't been mentioned), which makes it more sensitive to those things.If it's a pulse counting system, hopefully it can be calibrated for amplitude and delay... and calibrated frequently at that.Quote from: c4757p on March 22, 2015, 04:32:47 pmNoise? The resistors can be made arbitrarily small if the amplifier is capable, though.Quote from: Marco on March 22, 2015, 06:04:43 pmHe is, in fact; I wouldn't be surprised if he (or another poster) might have exactly what the OP needs... given suitable negotiation of course.Tim

author: sdouble
author_rank: Frequent Contributor
timestamp: March 22, 2015, 10:08:09 pm
content: Dear All,thanks for your contributions.Sorry if I was kind of unclear.I use 3 caps in serie to get a lower dispersion in my bunch of integrators.Here is the idea : i find caps of say 1.2pF with 0.05pF tolerance.Putting 3 units in serie permits me to get a lower dispersion for the equivalent cap.The info that I forgot to give you is that I built already 3 batches with slight differences. the dispersion within those batch is a good 2% RMS.I played with the spacing between the upper copper plane and the ground plane (layer 2 over 4).I etched the copper below the resistors to reduce the capitance induced by those resistors.However I need a ground plane pretty tight to the upper (signal) layer for noise reduction purpose.I need a design which could be produced by companies producing standard PCB. THe assembly cost should be reasonnable. I 'm gonna build hundreds of units.Maybe 1,000. I'm not really sure that the teflon stand could be an option. However, that's just a guess, I may be wrong.THanks again guys for your contribution

author: sdouble
author_rank: Frequent Contributor
timestamp: March 22, 2015, 10:15:34 pm
content: Quote from: T3sl4co1l on March 22, 2015, 03:18:30 pmTHat's an option. 0.3pF of parasitics is quite big....Quote from: T3sl4co1l on March 22, 2015, 03:18:30 pmThe time constant is about 10 micro second. The integrator output goes into a shaping amplifier with time constant of about 100 ns. 4 integration stages and 1 diff stage. It include a pole zero correction. THat's where I'm suffering a bit. Due to the parasites, I have more that 1 pole/zero in my decay (discharge of the 0.3pf equivalent cap). THerefore, I have trouble to correct for the pole/zeroes and my shaped output suffers from overshoot/undershoot.The integrator reads out an ionisation chamber. The signal level is pretty low : about 1,000 to 5,000 electrons.Another info which was missing is that I'm using 3 x 10 MegOhms resistors because 10 MegOhms is the max I could find in the thin film category. I need for sure to use thin film resistor due to the high parasitic cap of thick fim resistors.

author: sdouble
author_rank: Frequent Contributor
timestamp: March 23, 2015, 01:28:58 pm
content: thanks evb149 for the "couple of ideas".I will consider all of them. I need some time to go through all.Just a comment. With my Rogers 4003c based PCB, the parasitic cap doesn't vary significantly with time.We had some tests, which lasted for a couple of weeks and I could notice a drift of only 1% in gain during that time period.That's pretty good. So, no need to recalibrate very frequently.As I mentioned, I'm mostly bothered by the additional poles in the decay/discharge of the cap. That's my main reason to reduce the parasitic capacitance.In my current version of the PCB, I have 3 0402 caps. I honnestly didn't see much change in the behaviour when compared to the previous design which included 0603 caps. Would you expect a more significant improvement going down to 0201 ?CheersJulien

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2015, 02:38:08 pm
content: Quote from: c4757p on March 22, 2015, 04:32:47 pmThat is true with the approximate equations shown in the figure. Using the exact equations (see Y-delta in a good textbook), we see that the equivalent capacitor values are independent of frequency for the three-capacitor circuit, but the two-resistor one-capacitor circuit values will vary with frequency.If the three capacitors are used with a reset switch in an integrator, I'm not sure how many switches are required for ideal performance.Note that the parasitic capacitor added across the amplifier input is probably comparable to the capacitance already there for practical op amps.