Author Topic: Keithley 2000 input bias current (306.2, 306.4 failures) [Repaired]  (Read 8473 times)

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Offline Krampmeier

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Re: Keithley 2000 input bias current (306.2, 306.4 failures) [Repaired]
« Reply #25 on: October 25, 2017, 07:14:48 pm »
I had a nice talk to an engineer from Keithley Germany today after their distributor forwarded my questions to the Keithley tech support.

My interlocutor obviously had very deep knowledge of the circuit in (not only) the Keithley 2000 multimeter and took time with me to go through my questions and do some tech nerd chat - a quite pleasurable experience and a very positive surprise, given the negative comments here about Keithley support after they were taken over by Tek!  :-+

In short, as neither leakage current nor bias current of the Keithley 2000 are specified, the failure of the self-test steps 306.2 and 306.4 are not considered a defect. This is true especially if the meter was upgraded from an older firmware which did not contain these tests. That applies to many (if not all) meters which show the error at my employer.

If the "bias current" steps fail, this usually does not mean there a problem with real-world measurements - as long as the meter passes the calibration. The user should be aware that these universal DMMs are not electrometers, and low-voltage measurements at high-impedance sources can be influenced by these currents. 100 pA bias current should be considered a "typical" value, even though it may be higher at some older meters and much lower at some new units.
When we try to measure the bias current, we should be aware that the current depends on the voltage applied to the terminals. When we just connect a 10 MOhm resistor to the inputs, we get the pure bias current. When we add a voltage source in series to that resistor (like the self-test does), we get a leakage current in addition to the bias current. Both of these should not be confused with the input resistance, which is usually somewhere around 100 GOhm.

I was also told that newer revisions of the meters contain some optimizations which lower the leakage current, and the leakage current should be constant over time and should not change much when today's new meters get older. The correlation of multimeter age and bias / leakage current is very likely not only because of component ageing, but also improvements in parts (and maybe PCB layout, but he did not say that).

The internal self-test was rather intended for service at the factory than for the end user, and it sounded a bit like there are regrets that the test was made available on the front panel. As a consequence, that test was "hidden" when they later created the 2010, which is quite similar.

I hope this helps!

I might still do some experiments with the NXP JFETs instead of those from On someday, even though I am aware of the scattering in parameters. However, repair of my Adret 103A calibrator is given priority over that now, as the self-test errors on my Keithley are gone and the remaining leakage and bias current are not critical.

 
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Online Kleinstein

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Re: Keithley 2000 input bias current (306.2, 306.4 failures) [Repaired]
« Reply #26 on: October 25, 2017, 07:46:55 pm »
A possible way to test the bias current at a different voltage is to measure the drift rate with a low leakage low DA (e.g. polystyrene) capacitor in the 10 nF range at the input. With 10 nF a 100 pA current would result in a drift rate of 10 mV / second, thus easy to measure.

With those meters that show failure on the bias tests it might be worth checking the bias current in a voltage range, to see how much it actually is. The worst about a high bias is not knowing about it. If you one knows one can be careful with the very few cases where it can matter.
 

Offline voltsandjolts

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Re: Keithley 2000 input bias current (306.2, 306.4 failures) [Repaired]
« Reply #27 on: January 12, 2020, 04:33:46 pm »
K2000 failing 306.2 here also.
I'm fairly sure this error can be safely ignored but I also replaced U114 OP282 with a ADA4622 (although that is at the limit of its supply voltage spec).
Definite improvement, leakage starts at 30pA but over a few hours leakage creeps up and over 100pA which causes a self test fail again.
Guess it must be a JFET, hmmm.

Built-in self test
Select test (shift+'filter'), TEST:BUILT-IN, BIT:MANUAL, MANUAL:306, MODE:STEP
When test pauses press shift to view reading.
Bias current in pA = (306.2 reading - 306.1 reading) / 9.9E-6              (as described in attached Tek forum post)
Failure threshold seems to be around 100pA.
« Last Edit: March 26, 2024, 03:48:42 pm by voltsandjolts »
 

Offline voltsandjolts

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Re: Keithley 2000 input bias current (306.2, 306.4 failures) [Repaired]
« Reply #28 on: March 17, 2020, 12:16:11 pm »
Replacing Q120 has fixed it. Leakage is stable and less than 20pA now.
So, in summary, I replaced U114 with ADA4622 and Q120 with Fairchild MMBF4393.
Thanks to all previous posters for the guidance.


Leakage measurement on my Keithley 2000 which is now passing self test:

Warmed up meter 4+ hours

Built-in self test
Bias current in pA = (306.2 reading - 306.1 reading) / 9.9E-6
On my unit:
(6.950444 - 6.949998) /9.9E-6 = 45pA


Drift with floating inputs
Nothing connected to the inputs
Switch to 100VDC range (10MOhm input impedance gives 0.000V reading)
Switch to 10VDC range (>10GOhm input impedance) and start a stopwatch
Voltage reading shown will likely be moving away from 0.000V
On this unit:
23secs to +5.00 VDC
54secs to +10.00 VDC
(for comparison 34470A took 15mins to get to -3.37 VDC)
« Last Edit: March 26, 2024, 03:53:40 pm by voltsandjolts »
 

Offline doktor pyta

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Re: Keithley 2000 input bias current (306.2, 306.4 failures) [Repaired]
« Reply #29 on: February 23, 2022, 06:27:58 am »
Replacing Q120 has fixed it. Leakage is stable and less than 20pA now.
So, in summary, I replaced U114 with ADA4622 and Q120 with Fairchild MMBF4393.
Thanks to all previous posters for the guidance.

My K2000 also encountered this problem.
I replaced Q120 with MMBF4393 but without a success, so I went back to the original JFET.
Next I replaced U114 with ADA4622-2ARZ (at the moment of writing this they are available in Mouser).
After good washing of the soldering area the problem is gone- confirmed after few days of constant operation.

And finally I can get stable readings on high resistance ranges.

Cheers!
« Last Edit: February 23, 2022, 08:10:19 am by doktor pyta »
 

Offline jeffjmr

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Re: Keithley 2000 input bias current (306.2, 306.4 failures) [Repaired]
« Reply #30 on: February 24, 2022, 03:31:33 pm »
Is there any reason you chose the ADA4622-2ARZ vs. the ADA4622-2BRZ?

Thanks,
Jeff
« Last Edit: February 24, 2022, 09:20:31 pm by jeffjmr »
 

Online Kleinstein

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Re: Keithley 2000 input bias current (306.2, 306.4 failures) [Repaired]
« Reply #31 on: February 24, 2022, 03:58:22 pm »
Is there any reason you chose the ADA4666-2ARZ vs. the ADA4622-2BRZ?

Thanks,
Jeff
The A version can be cheaper and better available than the lower offset B version.
There is no need for a really low offset in this application. The original AD822 is higher offset than the A version and a 10 x high offset would likely still be no problem.
 
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Offline GnomeZA

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Re: Keithley 2000 input bias current (306.2, 306.4 failures) [Repaired]
« Reply #32 on: January 25, 2024, 10:41:20 pm »
I'm replying here because this thread is a wealth of information.

I bought a K2000 on eBay.  I swapped out the capacitors as always advised.  I also noticed a SMD capacitor was broken off under the board, but I suspect it was that way from factory, so I replaced a pair.

Anyway, after upgrading the firmware to 19 I also got the classic 306.2 and 306.4 after the meter had been on for roughly 15 minutes.
You can tell it will fail because in DCV mode with nothing connected it will start cycling from ±~0.4-1.1vDC very quickly, it gets faster as the meter gets hotter.

I bought some MMBF4393LT1G and replaced most of the JFETs.  I couldn't replace about 4 of them because a precision resistor board featuring caddock resistors was sitting over those JFETs.

Anyway after powering up 306.2 and 306.4 failed immediately and the cycling I talked about earlier was very fast and actually erratic (jumping around)
It also started to fail on 4 wire short.
When I looked into the code, the JFETs related to that error code was situation under the caddock board.

I didn't want to touch that precision resistor board with an iron but I did, I removed it and replaced the rest of the JFETs.
Went crazy with a toothbrush and some 99% iso to clean everything as well as a I could and I was wearing gloves (basically just all over)

Anyway after this, the meter only failed 06.2 and 306.4 consistently.
Next day same thing.

It is now a week later and I've had the meter on all day, and noticed the cycling I'm talking about above is now very slow.
Did the self test and everything passes now.
Even on cold start it now passes.

I suspect me cleaning the board introduced moisture that had to dry out.
I also had a voltage reference that I'd measured multiple times since I got this K2000 and it is still giving the same reading after all this crazy work.

So at least as far as the K2000 goes, it seems like you can get away with making quite a number of "repairs" and your calibration will remain relatively unchanged (whether it is in spec or not)
« Last Edit: January 25, 2024, 10:43:45 pm by GnomeZA »
 

Online Kleinstein

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Re: Keithley 2000 input bias current (306.2, 306.4 failures) [Repaired]
« Reply #33 on: January 26, 2024, 07:53:20 am »
Changing the JFETs should not effect the 10 V range very much. If at all the expected change is more like a small offset from thermal EMF. This is easy to check in the 200 mV range with a short. The other part that could be effected is the high value resistance (due to the input impedance in parallel) and the high voltage ranges that use the divider Q136 and Q114 are at the low end of the divider and there resistance does matter, especially if too high.

For the calibration there are 2 parts that get confused sometimes. One is getting the right scale factors and the other is to have the meter checked in a more legal sense that it performs as specified to have confidence in it to meat the specs for the next time. A repair always has a chance to cause some accidential changes also to part that should normally not be effected (e.g. via ESD or dirt or mechanical stress to the PCB). So the calibration in the more legal sense is void after a repair or just opening the meter.  For private use one could get away with a few checks like the ref. reading before and after.
 

Offline GnomeZA

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Re: Keithley 2000 input bias current (306.2, 306.4 failures) [Repaired]
« Reply #34 on: January 26, 2024, 09:04:54 am »
The other part that could be effected is the high value resistance (due to the input impedance in parallel) and the high voltage ranges that use the divider Q136 and Q114 are at the low end of the divider and there resistance does matter, especially if too high.

Good to know, I'll do some more tests around there.  I don't have precision resistors so I compare the K2000 against my Tonghui TH2830 in resistance mode.  It is new and calibrated in December.  Unfortunately, at the moment it is the closest thing I have to a comparable "standard".  Thankfully in 4 wire mode I get very much the same readings from both meters.  After the decimal things start diverging pretty quickly, but if that did not happen I'd be mightily impressed.
 

Offline voltsandjolts

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Re: Keithley 2000 input bias current (306.2, 306.4 failures) [Repaired]
« Reply #35 on: February 09, 2024, 09:04:13 am »
Just adding a little more info...

I have seen three K2000s that needed U114 replaced, seems quite common failure. As mentioned it's a OP282 but I replace it using ADA4622 with good results.

I have also replaced one U113. On older units this is an LTC1050, but on newer units it is a Nat Semi LMP2021 which has a reduced power supply voltage range of 2.2-5.5V. Zeners for the bootstrapped power supply VR107 VR108 have changed accordingly and are marked W50 = NXP BZX84-A2V4  2.4V Zener.
 


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