Sourcing nanoamps and below like a Samurai, or KEI 6221 DC/AC SMU teardown - Page 1
Forum section: electronics / general

author: TiN
author_rank: Super Contributor
timestamp: March 21, 2017, 04:49:00 am
content: Well, yes, I've got yet another Keithley piece. Need a cure already!This time it's 6221, DC/AC precision low-noise current source. Essentially it's an 11W 4-quadrant SMU, but without measurement functionality and much improved current source performance.To my memory, I never saw anyone posted guts of 6220/6221 anywhere online before, so enjoy!Exterior, controls and initial conditionKeithley 6221 front panelKeithley 6221 rear connectorsInternal construction, design featuresCover removed, analog board overviewAnalog board without shieldsAnalog domain FPGA, rear section and interlock circuitryADC/DAC and main reference sectionAnalog board power supplyCommon mode inductor, perhaps? I wonder what this thing does exactly.PCB cutouts to avoid leakageHigh-current range relays and resistorsMedium current range relays and resistorsLow current range high value resistorsOutput power stageHigh impedance, low leakage amplifier front-endHigh impedance low current circut designAuxilary bias?Fully shielded low-noise power suppliesMu-metal shielding and board labelTriaxial 3-lug connector and input circutryVoltage reference section and auxilary circutsRelay driversDiagnostics and secret menuBy holding PULSE+RECALL during power on we can enter secret menu section.This also unlocks self-diagnostics in TEST menu. I think they hide self-diagnostics for safety reasons, as 6221 can generate potentially lethal voltage levels on it's output.Vacuum display burn-in mode for segment brightness correctionTest with 10pA current with Keithley 6485RepairNo repair was required.CalibrationCalibration procedure in manual calls forHP 3458A 8.5-digit multimeterand Keithley 6487 picoammeter. Unlucky me, these ones I have, with little deviation by usingKeithley 6485instead of 6487. DMM is used to verify current ranges from 100mA to 100nA and picoammeter take care of lowest 10 nA and 1 nA ranges.Performance testTBDApplication example, measuring resistanceTBD. I will use this source for one of it's typical purposes, source current thru resistors to measure tempco. Before I was using Time 9823 calibrator or HP 3245A in current source mode, but these guys can't provide compliance higher than 10-something volts, which limits resistance measurements to about 50-80 KOhm. With 6221 I can extend voltages up to 100V, and that should let me measure few Meg resistances with better uncertainty. I don't have 2182A for delta measurements, but I have Keithley 2002 + EM A10 preamp which is better if I go milliohm-microohm regions

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 21, 2017, 09:26:19 am
content: I used this unit a lot at the university to characterize transimpedance amplifiers (1G and above with several kHz of bandwidth). The 6221 is capable of a few kHz but the output signal isn't very pure. Therefore, I used it only for DC in the end and a photodiode for the AC measurements.Nice unit

author: ap
author_rank: Frequent Contributor
timestamp: March 21, 2017, 10:05:31 am
content: Tin, what is your plan in measuring high-ohms resistors with it? The accuracy of these (as well as their picoamp-meters) is relatively low. Depending on the range, as bad as 0.3 to 0.4% (nanoampere ranges). The calibration with your 6485 would lead to a simliar error that adds. So you will be close to about 1% in current accuracy for the current. So will you be measuring current at a higher accuracy by using a lower value shunt and use the 6221 as a source only?Also, not sure what you paid, but these tend to not be really cheap. If you wanted to measure say 100Meg, and you had access to e.g. a Fluke 5440 (or bought it at a probably much lower price than the 6221), you would be able to use your 3458A in the 10/1uA range (depending on voltage applied), where it has a 1-year accuracy of about 30/60ppm. Combined with the 5440 uncertainty, that would still be pretty precise.

author: TiN
author_rank: Super Contributor
timestamp: March 21, 2017, 11:14:28 am
content: I've got this box for around 1.3k, shipping included. I think it's decent price for box made in 2008, also given no repairs required.Likely I'll use just as a source of stable current, as for TCR measurements absolute current does not matter, as far it stays stable and low noise. Will be interesting to see how current output compare to other sources, such as Time 9823, HP 3245A, Keithley 2400 SMU, Agilent 4142B(41424B) SMU, HP 3458A ohm source and also DIY current source using opamp, foil resistor and LTZ1000-based ref.F5440 is out of reach for me, due to it's shipping weight, even if I get one for great price, shipping cost and difficulty would kill the deal. Otherwise I wouldn't bother with Time electronics 9823 MFC. Low current ranges on 3458A are pretty noisy. Also I'll be recalibrating both 6485 and 6221 anyway.

author: ap
author_rank: Frequent Contributor
timestamp: March 21, 2017, 11:52:30 am
content: Sounds like a pretty good price, and if you are only measuring trends, then the fact that its not very precise or even not calibrated will indeed not hurt you.

author: plesa
author_rank: Frequent Contributor
timestamp: March 21, 2017, 02:56:34 pm
content: Thanks for teardown. It is pitty it is so expensive. I suppose that 26xx series killed marked for this instrument.Strange is how yellow is the wheel compare to rest of front panel. The most recent gear do not have print on wheel, they needs to save few cents

author: SeanB
author_rank: Super Contributor
timestamp: March 21, 2017, 06:06:16 pm
content: Different plastics, panel probably is polycarbonate and wheel is a PVC formulation, so they age at different rates, and have different failure modes.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 21, 2017, 07:13:08 pm
content: They have a quite impressive DAC (16 Bits 50 MSPS - likely used for waveform generation). So chances are it could produce a really pretty, low distortion sine.What is that large toroid core ? It looks a little like a transformer, but very few turns.

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 21, 2017, 07:18:00 pm
content: My experience was a quite bad AC behaviour

author: julian1
author_rank: Frequent Contributor
timestamp: March 21, 2017, 09:11:53 pm
content: Great pics. I saw that one on Ebay!That inductor/choke looks like a piece of minimalist modern at.Googling the components and there are quite a few high-speed/video bandwidth devices. Several buf634 buffers with 180MHz bandwidth, and the maxim4545 which are RF switches from DC to 300MHz with controlled impedance. The ltc1668 is a DAC with 50MSPS. The current ranging looks to be all switched with relays - perhaps to better handle the AC?

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 21, 2017, 09:26:03 pm
content: I found a picture which I measured five years ago. I'm not sure, but I think it was around 1nA at 1kHz. Measured by a Femto DLPCA-200. I tried another 6221 from another department. That unit showed the same results. Therefore, I went with photodiodes to measure low AC currents.Is it possible to measure something like that for comparison?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 21, 2017, 09:31:56 pm
content: That low current curve looks like it has quite some switching noise. I would expect the source more at one of the power supplies, not the signal generation itself.

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 21, 2017, 09:37:12 pm
content: Quote from: Kleinstein on March 21, 2017, 09:31:56 pmWherever it may come from, it is quite ugly for TIA characterization.

author: TiN
author_rank: Super Contributor
timestamp: March 21, 2017, 11:55:36 pm
content: I'd expect 1nA @ 1kHz level measurement is itself already challenging, very outside of usual approach "hook wires together, plonk buttons, enable output and bob's ur uncle".Based on previous posts and tests with Fluke 5700/Datron 4808 even microamp currents at AC are already difficult and depend a lot on cabling, shielding, isolation, etc.Anyhow, once I get to performance tests, I plan also to test AC performance.I've also bought same DAC devboard while ago for calculable AC reference project idea.So it was fun to discover same chip used in 6221.

author: VintageNut
author_rank: Frequent Contributor
timestamp: March 22, 2017, 07:25:01 pm
content: The output impedance of a current source is very high. The HI conductor is the high impedance of the two HI and LO. If this HI conductor is not shielded, electrical noise WILL be impinged on this conductor. If there is not a proper guard, leakage from HI to LO will happen.This is why electrometers measure current on the LO side and use a low impedance voltage source. Much less prone to noise problems.The Achilles heel of a current source is that you have to guard and shield the output otherwise you will have leakage and/or noise problems.Sourcing mA or uA is easy-breezy. Sourcing nA with low noise and low leakage requires some know-how and very good cables as well as proper guarding and shielding of the device being powered.

author: CalMachine
author_rank: Frequent Contributor
timestamp: March 23, 2017, 03:21:01 am
content: I would like to join the nanoamp sourcing club.My new beauty I got earlier this week is going to be a great addition to my home lab. I, first, had to take her in to get a fresh adjustmentThough, low current is one of our weak spots where I work... I know DC is not nearly as difficult to achieve the low levels you have with AC. What kind of guarding was implemented? Can you take some pictures of the cabling and test setup?

author: TiN
author_rank: Super Contributor
timestamp: March 23, 2017, 04:15:22 am
content: Whatever you do, keep those nice pics coming

author: VintageNut
author_rank: Frequent Contributor
timestamp: March 23, 2017, 01:51:01 pm
content: Figure 2-2 is an artist's rendering of guarding a low current force/measure from the 6430 instruction manual.Next is an implementation. The three alligators at the end of the cable are HI, LO and GUARD. HI and LO connect to the resistor. GUARD is connected to enclosure.Finally, the enclosure is sealed and the measurement is taken. Settling time is hours to charge all of the air in the enclosure to GUARD potential. A stable reading is achieved but after a long wait.

author: CalMachine
author_rank: Frequent Contributor
timestamp: March 23, 2017, 02:20:20 pm
content: I will do some more rigorous testing on the 237 here in the next few days, with lots of pics!Here we go! Some more hurdles for the 237.

author: CalMachine
author_rank: Frequent Contributor
timestamp: March 25, 2017, 02:06:12 am
content: Let's check out the pA performance.Both the 3458A and 237 were performing surprisingly well

author: CalMachine
author_rank: Frequent Contributor
timestamp: March 25, 2017, 02:09:43 am
content: Now for the nA hurdles!

author: Vtile
author_rank: Super Contributor
timestamp: April 01, 2017, 09:31:16 pm
content: This is severe. TiN you need to get a metrology grade thermometer to measure the illness.

author: plesa
author_rank: Frequent Contributor
timestamp: April 11, 2017, 07:56:20 pm
content: For one project I needs to compare the K6485 and 6517B @ 100pA. As a source was used K2602B.As you can see the K6485 is little bit noisier ( second graph with NPLC25) compare to K6517B ( first graph, blue line NPLC 10). Green and yellow line is burden voltage, where K6485 has 2 times higher burden voltage compare to 6517B.So the name Picoammeter for K6485 is little bit misleadingAny ideas what to compare with this setup?

author: voltsandjolts
author_rank: Unknown
timestamp: January 18, 2024, 02:59:40 pm
content: Sorry to wake this sleeping beauty...Just adding some info.Keithley 6221 Interlock ConnectorThe interlock connector within the unit is Phoenix Contact 1825129The mating part is Phoenix Contact 1825310

author: bsw_m
author_rank: Frequent Contributor
timestamp: January 18, 2024, 10:54:39 pm
content: 10aA source&measurement

author: voltsandjolts
author_rank: Unknown
timestamp: January 19, 2024, 01:04:09 pm
content: Quote from: bsw_m on January 18, 2024, 10:54:39 pmApproximately 62 electrons per second!

author: VintageNut
author_rank: Frequent Contributor
timestamp: January 25, 2024, 05:43:55 pm
content: Quote from: bsw_m on January 18, 2024, 10:54:39 pmI am scratching my head for an application that requires the measurement of 10aA.The noise of the Keithley 6430 is around 380 aA p-p. This was an actual measurement. The 6430 is arguably the best bench instrument in existence for sourcing and measuring low currents.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 25, 2024, 06:11:27 pm
content: There are not many measurements that need / care about such small currents. This a regime for speciallized instruments / labs. The Keithley 6430 may be best from Keithley and US general purpose sources. The meters shown are from Soviet time special scientific instruments build mainly for users like national metrology labs and similar. Alone the connectors with saphire isolators make them quite expensive (and also delicate) - a bit more than the teflon isolated triax found in more general purpose ones.In this area the Soviets were quite good, sparing not much expense in handcrafted systems for special use. The Keithley meters are more the afforable version for every day use in the semiconductor industry.One use for very low currents and resolving them accurate are electron turnstile systems - kind of trying to count electrons to get the ultimare current reference. This way one can provide small currents with good accuracy, but with the difficulty to transfer to more managable sizes.

author: bastl_r
author_rank: Regular Contributor
timestamp: January 25, 2024, 06:17:13 pm
content: Quote from: VintageNut on January 25, 2024, 05:43:55 pmMaybe Tunnelrastermikroskop.Sorry, i don't know the english name.Regards

author: Kleinstein
author_rank: Super Contributor
timestamp: January 25, 2024, 07:06:57 pm
content: A tunneling microscope would still use a significant higher current (still low, but more pA or even nA), as they need to be realtively fast to still get a picture with some 100000 and more pixels in a reasonable time. The very low current also implies a rather slow operation and long waiting times from parasitic capacitance and the associated dielectric absorbtion.

author: VintageNut
author_rank: Frequent Contributor
timestamp: January 25, 2024, 08:36:08 pm
content: If I understand correctly, the application is metrology. Specifically, measuring as close to zero as can happen with careful practice.I have played with similar for resistance. Being able to measure in the 10 to 20 nano ohms region requires exactly the correct instruments and very careful practice.I still have my ohms labs zero ohm standard. It is fun to try to measure.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 25, 2024, 11:12:02 pm
content: Quote from: VintageNut on January 25, 2024, 05:43:55 pmMass-spectrometry is one application which I am very familiar with. For example,these guys measure down to aA(the name of their patented amplifier is ATONA = aA to nA range ),CheersAlex

author: VintageNut
author_rank: Frequent Contributor
timestamp: January 26, 2024, 12:13:52 am
content: Quote from: Alex Nikitin on January 25, 2024, 11:12:02 pmThanks. That helps. Looks like the instrumentation may be buried in the big iron.https://www.isotopx.com/product/tims

author: MegaVolt
author_rank: Frequent Contributor
timestamp: January 30, 2024, 09:18:07 am
content: Quote from: Alex Nikitin on January 25, 2024, 11:12:02 pmHow do they do it?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 30, 2024, 10:30:42 am
content: I know of one masspectrometer front-end, that use a integrator at the input and photodiode as additional feedback. There the integrator is also directly part of an ADC.Other very sensitive mass spectrometers use channe-plates as a kind of electron multiplier, a bit similar to a photo mulipliers. The output is than counting pulses, not really measuring the size of the current.