measuring small capacities around 1 pF - Page 1
Forum section: electronics / general

author: Physikfan
author_rank: Regular Contributor
timestamp: October 25, 2020, 09:54:29 pm
content: To all electronics freaksPlease, does anyone have experience with measuring small capacitiesaround 1 pF?I'm just trying to measure the self-capacitance of an electrostaticvoltmeter, but so far with no real success.regardsPhysikfan

author: TimFox
author_rank: Super Contributor
timestamp: October 25, 2020, 10:13:27 pm
content: The old Tektronix 130 L,C meter uses a coaxial connection to the capacitor being tested. The useful bit is that approximately 75 pF of cable plus fixture capacitance can be nulled out, before connecting the capacitor under test. This allows for a few feet of normal coax cable to the unknown. The minimum full-scale setting is 3 pF. The stated accuracy is 3% of full-scale, calibratable to 1%.

author: bob91343
author_rank: Super Contributor
timestamp: October 25, 2020, 10:27:03 pm
content: I have a few ways. My Boonton RX meter has great stability and resolution, smallest division is 0.1 pF and interpolation possible. My nanoVNA can measure down to fF (femtofarad, or 1/1000 of a picofarad). Both instruments have wide frequency ranges of measurement.I have a Tektronix 130 (not working at the moment) that is a nice instrument.My GR 1658 can measure to very small values also but has the restriction that the unknown be floating from local grounds. Resolution is about 0.01 pF but the stability isn't that good.You can cobble up a setup where you resonate an L and C and then connect the unknown, readjusting the C to accommodate the unknown and see the difference.Electrostatic voltmeters have a fairly high capacitance, maybe a couple hundred pF I am guessing. If you isolate the unit before measurement it should give good results. However, since the principle of operation is to measure the force between plates, the capacitance will change with the reading. While measurements at zero reading are easy enough, it's a different story at several hundred volts. One way would be to mechanically move the pointer and see what you get, of course with the caveat that this is a delicate mechanism.

author: tggzzz
author_rank: Super Contributor
timestamp: October 25, 2020, 10:51:55 pm
content: I haven't evaluated this, so take it with a pinch of salt...https://www.edn.com/sub-picofarad-measurement-with-cmos-inverters/

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: October 25, 2020, 11:29:41 pm
content: Quote from: bob91343 on October 25, 2020, 10:27:03 pmI have done something similar, using a crystal oscillator. The unknown shunt capacitance was connected in parallel with one of the capacitors in the oscillator.Substituting known capacitors instead of the unknown gave a calibration curve of added capacitance versus oscillator frequency. That was good enough for the application at hand.The tricky part was getting reproducible results, because the setup was very sensitive to small changes in physical arrangement.Only repeated and consistent measurements convinced my team that I was measuring what I thought I was measuring.The application was to measure the internal capacitance of an analog IC. A batch of these were not behaving as expected. The capacitance difference(between good chips and bad) was evidence that something had changed on the IC, like a mask revision or manufacturing fault. We put that evidence to the vendor,who shipped us a new working batch but otherwise said nothing.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 26, 2020, 03:03:35 am
content: The old GR 716-C can do it, but not with a huge amount of resolution. The 1615 can do it way better, for a price. You can easily build your own shielded bridge using a small trimmer for the internal cap. You can also build a simple transformer bridge- look at designs for antenna bridges. Small caps are easiest with higher measurement frequencies like 10 kHz and up to 1 MHz or so.

author: Kibabalu
author_rank: Regular Contributor
timestamp: October 26, 2020, 05:55:04 am
content: There are several ways to measure capacitances by using a Vector Network Analyzer, e.g. by measuring the S11 parameter. You´ll find a lot of papers and videos about it in the net, also for very cheap VNA like the nanoVNA (50$ ... 70$). Measuring 1 pF is still a challenge, because it is in the same range like the parasitic capacitances of any measurement setup and you have to take care about the calibration very precisely.

author: chekhov
author_rank: Regular Contributor
timestamp: October 26, 2020, 10:16:14 am
content: There is ahttps://www.analog.com/media/en/technical-documentation/data-sheets/AD7745_7746.pdf(high resolution, Σ-Δ capacitance-to-digital converter), probably applicable.

author: Physikfan
author_rank: Regular Contributor
timestamp: October 31, 2020, 02:31:10 pm
content: I want to determine the capacitance values of these electrometers:

author: TimFox
author_rank: Super Contributor
timestamp: October 31, 2020, 03:06:06 pm
content: What accuracy do you require for this measurement?

author: Physikfan
author_rank: Regular Contributor
timestamp: October 31, 2020, 03:46:23 pm
content: I would like to get the values of the capacities within 5 to 10%.By measuring the voltage of a charged electrometer as a function of time and using the equation:U(t) = U(0) * exp(- t/(R*C))I would like to find out the insulation resistance of each of these electrometers.

author: TimFox
author_rank: Super Contributor
timestamp: October 31, 2020, 03:53:35 pm
content: My suggestion of a Tektronix 130 would do that accuracy directly. You could try your voltage decay measurement with two different high-value resistors, and no added resistor, and calculate both the capacitance and leakage resistance from the decay times. The redundant resistance value would give you a consistency check. You might need to compensate the measurement for the capacitance of the resistors and the off-state capacitance of the switch that applies the initial voltage.

author: Physikfan
author_rank: Regular Contributor
timestamp: October 31, 2020, 04:07:44 pm
content: It is very difficult to find any very high resistors around 10**12 Ohms with known values.At one day I have measured about 100 minutes for a voltage decrease of the big electrometer to (1/e) * U(0).But at another day this time was only about 10 minutes.Therefore I would like to find out the reason for such a behavior of the insulation resistance, humidity etc. etc.

author: jimmc
author_rank: Frequent Contributor
timestamp: October 31, 2020, 04:08:56 pm
content: A 'Transformer ratio arm bridge' such as the ancient (1960s) Wayne-Kerr B221 can measure very low capacities at 1592Hz (w =104).Extract from the specification below:Jim

author: kripton2035
author_rank: Super Contributor
timestamp: October 31, 2020, 04:27:45 pm
content: Roman Black did one some times ago :https://www.romanblack.com/onesec/CapMeter.htm

author: TimFox
author_rank: Super Contributor
timestamp: October 31, 2020, 04:30:38 pm
content: Quote from: jimmc on October 31, 2020, 04:08:56 pmI own a B221 and it is a beautiful instrument that holds its calibrations, due to the use of hermetically-sealed standard capacitors. The two dual-eye magic eye tubes are the best null indicator I have used.However, the device under test should not be grounded.Modern instruments such as the DE-5000 also need an ungrounded capacitor, although the battery-operated meter can be floated on top of a plastic box.

author: TimFox
author_rank: Super Contributor
timestamp: October 31, 2020, 04:37:11 pm
content: Quote from: Physikfan on October 31, 2020, 04:07:44 pmYou could also try adding a known capacitor (< 10 pF) instead of the external resistor and repeating the decay measurement before the humidity, etc. can change. Of course, this must be a good capacitor and totally dry. Either a polystyrene or a good disc ceramic (not MLCC).

author: Physikfan
author_rank: Regular Contributor
timestamp: October 31, 2020, 05:08:22 pm
content: For the last proposal with the high voltage capacitor the insulation resistivity of the capacitor must be at least one order of ten higher than that of the electrometer.

author: TimFox
author_rank: Super Contributor
timestamp: October 31, 2020, 05:29:40 pm
content: Of course. The insulation resistance of the capacitor can be measured using a Keithley (or similar) electrometer and a relatively high voltage. It all depends on what instruments you have or can borrow. If you measure the capacitance directly, the only problem is fixture capacitance; the fixture shunt resistance should not be important. The reason I like the Tektronix 130 is that the cable capacitance up to and including the connector that mates with your unit can be nulled out (max ca. 75 pF) and the extra capacitance of your unit measured with 3 pF or 10 pF full scale. Per your original question, this is how I measure such small capacitance to ground. I have also used a DE 5000 (with coaxial adapter) setting it on a large plastic box, and subtracting the cable capacitance from the final reading.

author: Physikfan
author_rank: Regular Contributor
timestamp: October 31, 2020, 05:33:08 pm
content: Using such expensive test equipment you can measure resistors around 10**15 Ohms?

author: TimFox
author_rank: Super Contributor
timestamp: October 31, 2020, 06:33:29 pm
content: My elderly Keithley 610C analog electrometer has a most-sensitive range of 10-14A. A stable 10V applied carefully through 1015ohms (or higher voltage more carefully applied with a less-sensitive scale) would work. The devil is in the details of fixturing with these minuscule currents.I have used both the Tektronix 130 and the DER DE 5000 to measure the capacitance of small Teflon stand-off insulators in the low pF range.With all of these instruments, it may be handy to use the guard terminal.

author: Physikfan
author_rank: Regular Contributor
timestamp: October 31, 2020, 09:21:05 pm
content: For my resistivity measurements I need voltages around 20 kV, therefore surface effects like microscopic-discharges may also have some effects on the resistivity values of almost perfect insolating materials like teflon or polyethylene.Still I am trying to measure the intrinsic capacities of these electrometers with this LCR meter:

author: TimFox
author_rank: Super Contributor
timestamp: October 31, 2020, 09:52:53 pm
content: With that expensive LCR meter, what is your problem measuring the device’s capacitance? Is the problem due to a grounded device?

author: Physikfan
author_rank: Regular Contributor
timestamp: October 31, 2020, 10:33:17 pm
content: I will check the performance of this HP4277A more carefully.The device shows reasonable values down to 1 nF.For lower values I am not very sure to trust what is shown at the display.Also I have another LCR device available and I will compare these two instruments.

author: jfphp
author_rank: Regular Contributor
timestamp: November 14, 2020, 04:07:15 pm
content: The old HP 4332A could be a nice competitor : less sensitive range is 3pF full scale and it is the single instrument able to mesure the capatitance of a scope's input with 1 MOhm in parallel.

author: TimFox
author_rank: Super Contributor
timestamp: November 14, 2020, 05:07:57 pm
content: The 4332 is newer than the Tektronix 130, but it also measures capacitance to ground with a guard terminal.The main thing I like about the 130 is the analog nulling of cable capacitance before the 3 pF full-scale measurement. Does the 4332 offer this feature?

author: jfphp
author_rank: Regular Contributor
timestamp: November 17, 2020, 10:48:33 am
content: Cable capacitance is eliminated by the special test leads : p. 3.1 manualhttps://nscainc.com/wp-content/uploads/pdf/A_4332A.pdf

author: Physikfan
author_rank: Regular Contributor
timestamp: November 17, 2020, 07:38:13 pm
content: After the measurement with the HP4277 did not work out,I would like to measure the internal capacities of these electrometers with this LCR meter:

author: TimFox
author_rank: Super Contributor
timestamp: November 17, 2020, 07:42:33 pm
content: Does that 4-terminal LCR meter allow one side of the capacitor to be grounded?

author: bob91343
author_rank: Super Contributor
timestamp: November 18, 2020, 04:38:44 am
content: The nanoVNA does the job. You can select the frequency, and should make it high to avoid error due to computational resolution. I made a little cable with RG-58/U about two inches long with a connector to the vna and some clips. I take a reading with the clips open then put the unknown on. It measures to around 3 digits with resolution very good.If the frequency is too high, the unknown's lead inductance must be factored in. So take the open circuit measurement with the clips spaced for the unknown's size without leads.I looked at the kit from Roman's work and it's almost as much money as a brand new nanoVNA.

author: Henrik_V
author_rank: Regular Contributor
timestamp: November 18, 2020, 10:35:41 am
content: Seems like the actual application if more in the nearly static.So I question the way to measure it with HF.Assuming the capacity of the e-static voltmeters is constant and the (temp) and humidity is constant ..I would apply a voltage ramp and measure the current.(or, since the thread starter want's to measure HV-high value resistors, apply high voltages and measure currents without the e meter)Or add a known 10nF(+x) HV cap in parallel and ignore the 1 pf

author: jonpaul
author_rank: Super Contributor
timestamp: November 30, 2020, 03:05:01 pm
content: Bonjour PhysicFan, just saw this.We designed special low capacitance transformers and high voltage ignitors for arc lamps, and avainoc CRT Power supplies.CRTs. The electrometers are great as AC, DC or HF are all treated the same.We have a collection of electrometer VMs including the "milking stool" 100 kV from Sensitive Research.A few notes please:1/ Guarding and shielding need attention2/ use Higher bridge F, below 10 kHz the Zo is too high. We have bridges to 10 MHz. Wayne Kerr 3245 goes to 300 kHz3/ Network analyzers are an alternate: Our HP 4195A has a special impedance test set, 100k - 500 MHz.4/ Surprisingly the small HP RLC meter, 4332A, has lowest C range 3 pF FS!Easy zero adj and guard but freq is not adjustable. In R&D and prod test we had good and repeatable test down to 1-3 pF on 4332A,http://prc68.com/I/HP4332.shtml5/ We made a test fixture for the SMD partfor HP4195A, Setup:GEN>>HP 4195A TX>>cable>>50 Ohm term>>High of cap -[]- low of cap >> direct to HP 4195A 50 Ohm RX.We swept 100 k - 500 MHz, and plot the resulting output on db scale.Its of course a line of slope proportional to the C, due to the cap displacement current. From the slope one can infer the capacitance.We calibrated with a precision low C, perhaps a 3 or 10 pF mica capacitor.Using this technique we had repeatable measurements of 0.3 - 1.0 - 3 pF.BUT it had BOTH electrodes floating, unsure how this can work on an electrometer with one side at chassis ground.Hope this note solves your problem!Just the ramblings of an old retired EE!Bon ChanceJon

author: jonpaul
author_rank: Super Contributor
timestamp: November 30, 2020, 03:06:57 pm
content: http://mavin.com/pictures/bw/sensitive_esh_1.pdf

author: Physikfan
author_rank: Regular Contributor
timestamp: November 30, 2020, 10:36:27 pm
content: Ad TimFox"Does that 4-terminal LCR meter allow one side of the capacitor to be grounded?"I think that my two small electrostatic voltmeters can be used without grounding one side.Ad JonPaul"We have a collection of electrometer VMs including the "milking stool" We have a collection of electrometer VMs including the "milking stool" 100 kV from Sensitive Research."Did you ever measure the insulation resistance of one of your electrometers, especially the electrometers from sensitive research?And please, can you give some comments of your experience with and the performance of your collection of electrometers?

author: TimFox
author_rank: Super Contributor
timestamp: November 30, 2020, 11:04:17 pm
content: The basic circuit of the 4263 and 4277 is the same as in the DER DE-5000, but the latter is a battery-operated plastic-cased smaller unit. I have used it to measure grounded impedance by floating the DE-5000 on a polyethylene box. You could try carefully floating your electrostatic voltmeter in the same way, to minimize capacitance from the “low” end to the chassis of the LCR meter. When using a QuadTech 1920 LCR meter in my previous company’s lab (very similar to the two HP units), we had to carefully float the DUT, which was difficult for large circuit boards.

author: jonpaul
author_rank: Super Contributor
timestamp: December 01, 2020, 10:21:59 am
content: Bonjour, Tim Fox and PhysicFan: A few notes please:0/ Tim, the 4 term RLC meters generally are used with both sides floating but the low side terminals are near ground. For HP 4332A, both terminals must float.1/ The smaller EL meters indeed have two floating terminals. The large units 20-100KV and up must have a ground due to a metal case.2/ We used the Sensitive Research Electrostatic VM in applications of measuring 12 kV Avionic CRT power supply, with 100 uA current capability. The leakage was never an issue.3/ The large meters were for testing arc lamp ignitors up to 55 kV of RF bursts 100k - 10 MHz. Again leakage was not relevant, only capacitance.4/ Ultra high resistance needs a special bridges and guarded electrodes. all of these apply a high test voltage, the GenRad to 1000VWe have several methods,GenRad Megohm Bridges and meters series 1862, 1863, 1844Ahttps://www.ietlabs.com/pdf/Manuals/1644_im.pdfDanbridge (Danish) JP30A 30 kV insulation tester, up to 30 kV, 1 uA FS indications can read ~ 2-10 nA least)Lab HVPS, with Keithley elecrometer set on current range in series at return end of DUT.5/ The Sensitive Research devices are excellent BUT dust, dirt film and exposure to humidity naturally degrade the leakage.We cleaned with 99% iospropyl alcohol every insulator before every measurement, had room humidity below 40% , used Teflon or similar insulated wires and jumpers.6/ All electrometer voltmeters are VERY delicate, move with care, store upright and NEVER drop even a few inches! CAREFULLY LEVEL AND ZERO ADJUST BEFORE EVER MEASUREMENT!I hope this info and experience can be interesting and benefit your work.Bon Chance,JonPS: my background.. since 1957 built tesla coils. in TCBA since 1980.Fascinated by vacuum systems, photomultipliers, CRTs, arc lamps.At Lawrence Berkeley Labs, 1967.PPS: For the highest and craziest HV on a very professional scale, See my old friend, Greg Leyh, lod.org

author: jonpaul
author_rank: Super Contributor
timestamp: December 01, 2020, 10:24:25 am
content: Genrad 1862, 1863 1864 are Megohm METERSThere is a megohm bridge, 1644

author: jonpaul
author_rank: Super Contributor
timestamp: December 01, 2020, 11:08:05 am
content: Confusing that you have this thread and " HP 4277A" for the same topic, testing of electrometers.The moderator should combine these two topics into one eg " HV Electrometer capacitance and leakage"Jon

author: TimFox
author_rank: Super Contributor
timestamp: December 01, 2020, 02:44:36 pm
content: The "low current" terminal on four-terminal meters is usually a virtual ground (op amp inverting input), so it is near ground. I don't believe it is wise to short it to ground.I worry about the capacitance between the case of the electrometer (I have no experience with these electrostatic units) and the case of the large LCR meters.As discussed above, there are LCR meters designed to measure capacitance to ground.

author: Physikfan
author_rank: Regular Contributor
timestamp: December 02, 2020, 03:26:27 pm
content: Ad jonpaulThank you very much for your very interesting comments on your collection of sensitive research electrometers.For the two small electrostatic voltmeters I couldmeasure now finally the self-capacitances at 100 kHz,taking into account the capacities,due to the geometry of the measuring cables:Trüb and Täuber, Zurich, 0 - 500 VHP 4277A: 7.1 pFHP 4263B: 6.2 pFGB instrument, 0-300 V.HP 4277A: 10.5 pFHP 4263B: 10.0 pFWithin the experimental error there is a good agreement of the values of these two different LCR instruments.The way is now free for the measurement of theinsulation resistances due to observing the decrease of a short time applied voltage as a function of time.

author: jonpaul
author_rank: Super Contributor
timestamp: December 03, 2020, 02:22:19 pm
content: Hallo PhysikfanI must unearth my Sensitive Research meters and test the Cap and resistance.They are in a 3rd sub basement!1/ The Sensitive Research large meters are multirange, the stator can slide on a detent. different capacity per voltage range.2/ yes 100 kHz bridge measurenets should be fine3/ what is your exact application and voltage you shall measure that needs such high resistance?Jon

author: Physikfan
author_rank: Regular Contributor
timestamp: December 04, 2020, 12:44:41 pm
content: Hallo Jonpaul"I must unearth my Sensitive Research meters and test the Cap and resistance."I am very interested to compare your values of capacities and insulation resistance of your electrometers withthe corresponding data of my electrometers."The Sensitive Research large meters are multirange, the stator can slide on a detent. different capacity per voltage range."Your 100 kV electrometer is also a multirange instrument?There should be also a 150kV electrometer available."what is your exact application and voltage you shall measure that needs such high resistance?"My main goal is to measure the resistance, around 10**15 ohm, of teflon and polyethylene samples at 20 kV, rods and disks, in the lab and finally in the free air.

author: jonpaul
author_rank: Super Contributor
timestamp: December 04, 2020, 04:44:34 pm
content: Dear Physikfan!The meters are in a subbasement, and hard to access up 3 flights. Next week!From memory: One single range in square wood box, as you have pictured probably 3 , 5 or 10 kVOne multirange in the small black metal case with side window, 5-10-15 or 5-1--20 kVOne huge, beehive or milking stool, 4-5 ranges 50 cm dimensions with 20 cm insulator, 50 or 100 kV. I have never seen the 150 kV." measure the resistance, 10**15 ohm, teflon and polyethylene 20 kV, rods and disks1/ The volume resistivity of these materials is well documented.https://omnexus.specialchem.com/polymer-properties/properties/volume-resistivityNotice that volume resistivity in ohm.cm is a materials property and does not change with shape or size as resistance will.The resistance is greatly affected by surface contamination and preparation, and how the sample is prepared and fixtures.Please note that HP, Yokogawa, and other fine instrument firms make special volume resistivity fixtures for this exact purpose with the required shielding and guard rings!Normally a disc is used not a rod! The discs must be machined to exactly fit the fixture.HP 16339A, also see keysight 16008B resistivity-cellFor a DIY measurement, suggest to use the electrostatic meters just to monitor test voltage and to use this simple setup:HV supply....current limit R....(fixture and DUT)....return of fixture.....Keithley electrometer set for lowest current.Then you can test at 20 kV and see the leakage.Hope this proves of use to you!Jon

author: jonpaul
author_rank: Super Contributor
timestamp: December 06, 2020, 10:04:47 am
content: Physikfan, I found my original 1980s Sensitice Research catalog and app notes, in a 1" black binder.Full specs on ever unit, with capacitance etc.Mentions R ~ x 10 exp 15 ohms.My memory was bad, I have: Multirange 30 kV and multirange 50 kV, NO 100 kV.Each has 4-5 ranges EACH RANGE HAS DIFFERENT CAPACIANCE.So testing will take time.SR does NOT spec any exact resistance.You should be able to find the exact SR specs on web, you need the model numbers you have.I could make a scan if necessary.Best Regards,Jon

author: Physikfan
author_rank: Regular Contributor
timestamp: December 06, 2020, 02:23:48 pm
content: Hello JonIt would be absolutely great, if I could get such a scan from you.I will start today loading my small electrometers with about300 or 500 V respectively and afterwards I will observe the decrease of thevoltages.Best regardsPhysikfan

author: jonpaul
author_rank: Super Contributor
timestamp: December 07, 2020, 11:29:39 am
content: Hello again Physikfan:1/ Sensitive Research has NO exact spec on insulation R, just a magnitude ~ order of 10 exp 15 ohms.2/ Big project to move all my meters to the lab and test on 30 kV power supply.3/ Will try to scan the Sensitive Research cat, use PM or email so i can send you a link.Kind Regards,Jon

author: jonpaul
author_rank: Super Contributor
timestamp: December 07, 2020, 12:35:24 pm
content: http://mavin.com/pictures/bw/sensitive_esh_1.pdf

author: jonpaul
author_rank: Super Contributor
timestamp: December 07, 2020, 02:28:30 pm
content: PhisikFan:Scanned Sensitive Research catalog and apps 1980http://scientificonversion.com/SensitiveResearch1980.pdfImpossible to send to ancient Yahoo emal addr you should gert a modern alt like gmails.Jon