Keithley 182 leakage - Page 1
Forum section: electronics / general

author: e61_phil
author_rank: Frequent Contributor
timestamp: June 19, 2022, 11:54:00 am
content: Hi all,in another thread we discussed how well a Hamon divider (like Fluke 752A) could be self-calibrated using anything other than a proper Null-meter.In my opinion the most critical parameter is the leakage of the instrument, when floating in the middle of the bridge on half the supply voltage.My Keithley 182 showed ~100Gig at 10V. That is way better than specified, but other voltnuts measured better values. Maybe we can collect some data from different Keithley 182 here.There is no need for a fancy SMU, electrometer or something like that. A simple meter with 10Meg input impedance, low enough input current and sensitivity will do the job. I appended an example using a HP 34401A. The 10V source could be a simple lab power supply.In this setup my 34401A reads ~0.8mV.0.8mV / 10Meg = 80pA10V / 80pA = 125GigIt will take a bit of time to settle. It makes also sense to swap the polarity on the source. Keep in mind, that the GUARD/SHIELD of the K182 is tight to Mains earth.Thanks for your help!Philipp

author: HighVoltage
author_rank: Super Contributor
timestamp: June 19, 2022, 12:48:02 pm
content: Hello PhilippNo 182 here but 2 Keithley 181Your setup with an external PSU at 10V and 10 MOhm DMM in series does not reveal any usable data for me. The measurements are all over the place. Maybe I have too much noise today in my lab.So, I used a Keithley 617 Electrometer with the internal 10V output of the 617.Keithley 181 #1: 26 to 28 pAKeithley 181 #2: 5 to 6 pAThese readings are taken after about 10 min stabilization time and me being 3 m away.

author: e61_phil
author_rank: Frequent Contributor
timestamp: June 20, 2022, 10:17:14 am
content: Hi Ulf,thank you very much! It seems that my K182 is a unit with higher leakage than usual (even if it is way better than specified).Interesting, that your DMM setup was all over the place. The unshielded part of my cable was rather short (see picture) and I used 100NPLC. My currents are higher than yours, but it should work even with lower currents.Just for fun I also compared the 34401A setup against my Keithley 617 at 10pA (100Gig 1V). There is some offset in the 34401A measurement, but it is ok for a rough idea.

author: gamalot
author_rank: Super Contributor
timestamp: June 20, 2022, 11:40:10 am
content: Sorry to get off topic, how did you guys connect the shield of the cable and the plug together?

author: The Soulman
author_rank: Super Contributor
timestamp: June 20, 2022, 12:26:36 pm
content: Also, for a simpler/easier/cheaper and (arguably) better setup one could use 9V alkaline battery as a source and a handheld meterfor lower noise and truly floating measurement.Add more batteries in series for a higher test voltage if needed.

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 20, 2022, 12:55:10 pm
content: Quote from: gamalot on June 20, 2022, 11:40:10 amI have a similar assembly for my 182A. The shield is connected only at the DUTs side.The case ground of the 182A unfortunately does not serve as a GUARD connection, as would be required.Its usage might cause additional trouble, especially at common mode voltages.I measured an isolation resistance to the input of 10V/15pA~670GOhm, though.The input bias of my 182A in its 3mV range at zero Volt is about 4pA only. I used a 100k or 1M resistance to let it measure its own bias. These measurements always vary with the resistance used.In its 30V range, I encountered up to 400pA at different common mode voltages between -10.. +10V, wildly varying.I'm not sure if the 182A can really be used as a Null meter at 100mV, 1V, or 10V common mode voltage as required for the 752A setup.I prefer the Fluke 845AR, as I don't have to care for bias or isolation current characterization, being in the fA range only.Frank

author: e61_phil
author_rank: Frequent Contributor
timestamp: June 20, 2022, 01:26:39 pm
content: Quote from: gamalot on June 20, 2022, 11:40:10 amOn my cable the screen is clamped to the cable relief (see picture).Quote from: The Soulman on June 20, 2022, 12:26:36 pmThe stability of the voltage source isn't that critical. And you need a resolution of 10µV for 1pA.If your handhed meter has sufficient resolution it might be better than a bench top meter.EDIT: I tried it with two cheap handheld meters (AENG and Uni-T). 100Gig @10V. The results are completely useless.Quote from: Dr. Frank on June 20, 2022, 12:55:10 pmWas the voltage source connected to mains earth to make sure the shield of your cable was connected to the K182 shield/thread?I would say it is possible to use the K182 if you do it carefully, but the Fluke 845 is preferable.

author: MiDi
author_rank: Frequent Contributor
timestamp: June 20, 2022, 07:36:11 pm
content: My K182-M reads ~3.8mV with +-10V CM & 3mV range, so 380pA/26GΩCable disconnected from input reads ~1.1mV/110pA/91GΩ.Same with flying probes gives ~0.4mV/40pA/250GΩSeems not so easy to measure correctly in pA ranges...Quote from: gamalot on June 20, 2022, 11:40:10 amJust clamped on ring:

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 20, 2022, 10:30:46 pm
content: Quote from: e61_phil on June 20, 2022, 01:26:39 pmFor this isolation measurement, I used a battery driven F7000, so no connection to ground on the source side, only a cable from minus of the 10V source to the connector plug of the 182A. I didn't use the shield of my cable for this setup.Concerning the K182A (or any low voltage DMM) compared to the 845AR, I fully agree to you.FrankAnother remark:To mitigate fluctuating e.m.f. voltages in the connector, the space inside the connector is usually filled with some material to avoid air flow.I used cotton which I stuffed inside the plug.. a very bad idea.It attracts humidity, and then leads to additional leakage currents between each metal connections, in the nA range.So you need to use highly isolating plastic tubes around the crimp/solder joints first, or use non conducting, water repellant plastics inside the connector.

author: HighVoltage
author_rank: Super Contributor
timestamp: June 21, 2022, 11:34:43 am
content: This is how the original keithley cable is connected to the outside metal connector.

author: gamalot
author_rank: Super Contributor
timestamp: June 21, 2022, 11:45:39 am
content: Thank you everyone!Quote from: HighVoltage on June 21, 2022, 11:34:43 amThis looks interesting, I happen to have some 1mm copper sheets to make two copper washers to connect the shield.

author: HighVoltage
author_rank: Super Contributor
timestamp: June 21, 2022, 04:40:35 pm
content: Quote from: Dr. Frank on June 20, 2022, 10:30:46 pmThe original Keithley shorting plug 1488 also has some sort of foam inside.I did not do this on my self made shorting plug.And I also did not do this on my self made cables with the Amphenol connector.What foam would you recommend?BTW, I just looked at the cost of the Amphenol connector, it has doubled in the last 2 years to Euro 27.49 each from the same supplier TME.

author: Bill158
author_rank: Regular Contributor
timestamp: June 21, 2022, 08:39:11 pm
content: Doing what you show in your diagram and setting my 3458A input Z to a fixed 10 megohms. (via the FIXEDZ command) Then being careful about shielding of all cables.I get a fairly stable 0.47 uV so that is about 47 pA.So my K182 input leakage is about 213 gigohms.Bill

author: Bill158
author_rank: Regular Contributor
timestamp: June 21, 2022, 08:40:58 pm
content: Sorry I meant .47 mV in previous post.Bill

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 21, 2022, 09:17:24 pm
content: Quote from: HighVoltage on June 21, 2022, 04:40:35 pmI did not further investigate on that issue, as leakage was not interesting yet (only the low noise nV measurement).I would investigate on Styrofoam, or maybe bulk PTFE, drilled and cut into appropriate shape.As the 182A is not intended for ultra low leakage, I would only avoid very badly isolating stuff.Yep, I also got my connectors from TME, for a reasonable price.. I think the pins were made of silver .. sufficient for my needs.

author: 1audio
author_rank: Frequent Contributor
timestamp: June 22, 2022, 03:34:11 am
content: The Keithley connector pins are crimped. I don't think they are silver but more likely gold plated copper or brass. More than anyone would want to know about silver in connectors:https://advancedplatingtech.com/wp-content/uploads/2013/10/Use-of-Silver-in-Connector-Applications-M-Myers.pdfI found 12 AWG bare solid copper makes a good shorting plug to mate with the connector. And no possible issues from connections internally causing thermals. Without everything being carefully managed its really a temperature gauge.Where do you get the right connector with crimpable pins? Part number? I had to make one by dissassembling two AN connectors to get the right mix of parts.

author: HighVoltage
author_rank: Super Contributor
timestamp: June 22, 2022, 06:15:38 am
content: Quote from: 1audio on June 22, 2022, 03:34:11 amI don't think that connector with crimpable pins is available.This is how I made my own short plug with the Amphenol connector

author: MiDi
author_rank: Frequent Contributor
timestamp: June 22, 2022, 08:34:14 pm
content: Quote from: HighVoltage on June 21, 2022, 04:40:35 pmUsed 2cm XPS (PS sheet) cutted with an apple core knife:

author: gamalot
author_rank: Super Contributor
timestamp: July 23, 2022, 09:47:35 pm
content: I have tried many times and can't get a stable reading with either a benchtop or handheld multimeter.So I decided to make a meter with good shielding. Although it only has a 12-bit ADC on the MCU, it is sufficient for this application.The input impedance is 10M, and there is only one range of 1mV (or 100pA).It is headless and transmits data through infrared. The device used to receive data is a Raspberry Pi ZERO W.I have done a simple test, the leakage current was started at 30pA and down to 7pA slowly, battery voltage was 7.4V.I'm still working on some code that implements logging and data visualization.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 24, 2022, 05:25:31 am
content: That circuit for the leakage tester looks rather odd:The measurement is a combination of the DUT and the testers own drift an bias current.The AZ OPs should have a relatively stable offset and low offset. So one should get away without the extra analog offset trim, which can easily introduce drift, especially in the way it is build with reference to the supplies and the ground. So the virtual ground circuit part also enters.The other point is that the 1 M resistor at the input that sets the current sensitivity is already a bit on the high side for the AD8639 OP, so the OPs current noise tends to dominate.A nasty point with Az OPs is that they also react a little to variations in the input capacitance, so there should be some filtering to isolate the input from direct higher frequency effects of the input and sensitivity to input capacitance.The more usual way to build a pA meter is to have a much higher resistor (e.g. 100 M) and than a non AZ OP-amp with very low bias (e.g. LMC6482).

author: gamalot
author_rank: Super Contributor
timestamp: July 24, 2022, 09:37:06 am
content: Quote from: Kleinstein on July 24, 2022, 05:25:31 amI totally agree with your suggestion about low current measurements, actually I do have some LMC6482s in my hands and I did think about using them at first, but in the end I decided not to use them for this project.The goal of this design is to achieve a well-shielded, fully floating, low-cost, lightweight low-voltage meter, not a precision picoammeter. It would preferably have the same input impedance as the multimeter used by the OP and others to measure leakage current.Neither offset nor long-term stability is very important, the most important thing here is to have both voltage and current offset and noise relatively low enough (within the capability of the nulling circuit), low voltage operation, low current consumption, rail to rail output, etc. Everything is a little easier considering that 5% or even 10% error is acceptable, like all resistors are just 1% 50ppm chip resistors.Trust me, if I made a high-performance picoammeter, I wouldn't put it in this thread, I'll start a new thread just to show it off!

author: Kleinstein
author_rank: Super Contributor
timestamp: July 24, 2022, 09:58:49 am
content: Except for the zero trim part, and the divider at the input the circuit is reasonable low level voltmeter, e.g. like a null meter.The offset adjustment can be better done in software. The offset should be relatively small - like usually less than 10 µV and thus not much of the range lost.The divider is somewhat odd, as the signal is relatively small anyway. Just for protection less restance is sufficient.If analog offset adjustment is wanted it should be closer to ground, so more like the trimmer between the +- supply and than a divider to ground to reduce the range, not the other way around.If done right, there is no more need for a virtual ground with an AZ OP.

author: gamalot
author_rank: Super Contributor
timestamp: July 24, 2022, 10:28:47 am
content: Quote from: Kleinstein on July 24, 2022, 09:58:49 amThe voltage divider at the input is a compromise, an input impedance of 10M is a prerequisite, but I feel it is a bit too high for the AD8639.The zeroing circuits were added at the last minute, as you can find in their designer. In fact the offset is acceptable without it (about 100 readings out of 4095).Speaking of AD8628s, I had to use them because I bought 100 of them from a Chinese seller for a very low price, maybe fake.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 24, 2022, 10:39:51 am
content: The AD8628 is normally a pretty good AZ OP with a pretty low noise and not to much in put bias. With still close to 100 pieces it may be worth checking if they are original or maybe good chinese euqivalents.One can get 10 M input impedance even without a divider, just some resistance (e.g. some 100 K) / filtering for protection and than if needed the 10 M directly to ground, so that the open input would not drift away and float. For the lower voltage one does not want the divider and for a low level meter like the 182 they even cut down on the series resistance to keep the noise low. The voltage noise of the AD8628 is about comparable to the noise of a 30 K resistor.

author: gamalot
author_rank: Super Contributor
timestamp: July 26, 2022, 02:49:28 am
content: Quote from: Kleinstein on July 24, 2022, 10:39:51 amNoise is not that important here, after limiting the bandwidth I measured noise around 1pA peak-to-peak, which is good enough for this leakage current test.The AD8628 is my favorite low voltage jellybean op amp, I still have 97 of them in hand, maybe will do a test later, but something about $20 in total doesn't feel like it's worth spending a lot of time.

author: gamalot
author_rank: Super Contributor
timestamp: July 26, 2022, 03:08:41 am
content: It took me some time to get familiar with chart.js and Flask and finally made a simple logging application.Then I did a test. The meter was disconnected from the 182 in the first 3 minutes, and the zero offset was about 1.7pA, I compensated in the software. When the meter is connected to 182, the current is too large, in the chart I limit it to 30pA to avoid scaling problems.It can be seen that the leakage current is about 4pA after 10 minutes, and the voltage of the battery is 8.8 volts.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 26, 2022, 05:36:29 am
content: Much of what one sees as leakage can be dielectric absorbtion in the isolation material. So the current is expected to go down quite a bit. Som of the isolation materials (PCB board and PVC) have low conductivity, but still rather high dielectric losses.One should also be able to see the other side of dielectric absorbtion: first apply a voltage between ground and the 182 for quite some time and after that measure the current with no more applied voltage.The impedance between the isolated DMM part and ground is probably well decribes as a rather lossy capacitor, not so much a resistor in the TOhms. It is still a bit tricky to tell appart low levels of true leakage current and dielectric absorbtion. Mainly patience helps there, but it can take hours.

author: gamalot
author_rank: Super Contributor
timestamp: July 27, 2022, 12:00:35 am
content: Quote from: Kleinstein on July 26, 2022, 05:36:29 amIt does seem to be exactly the same as the charging process of the capacitor. I did another 1 hour test last night, and the result was completely expected, the second half of the curve is flat and can't tell the difference between it and a straight line.The difference is that the final current is 7pA instead of the previous 4pA, I'm guessing because of changes in temperature and humidity, I'm having the worst winter since moving to Melbourne 9 years ago.Before using this DIY voltmeter, I forgot to do a gain error test on it, so I did it last night. Applying a 200mVpp/10mHz sine wave voltage to its input through a 1GOhm resistor in series, the result is acceptable.

author: gamalot
author_rank: Super Contributor
timestamp: July 28, 2022, 01:59:40 pm
content: I have planned to disassemble the DIY meter into parts and reuse them, but when I went to the supermarket yesterday, I found that there are 12V batteries for sale, so let it work again!Two MN21 batteries in series is 25.3V, and the leakage current down to 21.4pA after one hour, which is consistent with the previously measured result of 7pA at 8.8V.

author: gamalot
author_rank: Super Contributor
timestamp: August 11, 2022, 04:24:11 am
content: Sorry I'm going off topic again!When I got the new Amphenol 97-3057-1008-1, I just realized that the 97-3057-1008(621) I ordered years ago was wrong, not just the color, but also missing the bushing.

author: gamalot
author_rank: Super Contributor
timestamp: August 13, 2022, 10:00:13 am
content: Finally found the Advantest R8240 in the corner of the closet in my room, the voltage source was the same battery pack (25.3V) as last time, and the leakage current was less than 11pA after an hour.---I left them powered on for over 10 hours and the temperature in the room is now about 10 degrees hotter than when I measured in the afternoon and the leakage current reading has risen to about 16pA

author: guenthert
author_rank: Frequent Contributor
timestamp: February 04, 2023, 01:39:07 am
content: Quote from: HighVoltage on June 19, 2022, 12:48:02 pmThe 617 is very slow and will average the current (not sure, how accurate it'll be with varying currents). The bias current of the 181 (like most DMM) is very peaky though. Note that the service manual wants one to adjust the pump out current compensation so that the voltage peaks across a 10MOhm resistor [of your oscilloscope's probe] on the input is less than 2mV. That'll be peak currents of 200pA.Edit:The service manual for the Keithley 182 has the procedure for verifying that the input bias current meets specification p 7-21 ff. There indeed the Keithley 617 is used, albeit as coulomb meter. If that was obvious to everyone but me, then I apology for the noise.