Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617

[Kleinstein]

One can check the input current by using the amplifier / meter itself to watch a capacitor accumulate charge.
So the input bias translates to voltage drift.

With the very low currents / very high impedance part the scale factor and thus the reference is not that important. A well aged reference is often better than a new one. So unless known broken, there is no need / use to change the ref.

[snik]

Meanwhile it turned out that my five 2N5906, which i ordered specially in America and not in China, seems to be Fake. Though the Electrometer works with them, but the Input-Current was always very high 300 - 600 fA and i couln't regulate that very much. After changing the current poti and three Transistors i cracked one open and the Die doesn't look like the other FETs (See my picture). So after that i have changed the Input-FET to an 2N5909 which i ordered here in germany and now the input-current is, how it should be. So the question for selecting one of the 2N5906 was obsolete but interesting to know.

As summary : After changing the Input Fet (to often ), the LT1012, two MPS-U60, the LM337L (which was 8V instead of 5V), a poti, a broken 100 R Resistor (which was 6.5kOhm) and all Capacitors (one was bursted), my Electrometer works now as expected.  Last step is now calibrating the unit.

My original broken Input-Fet goes to Richis Lab for a nice Die Shot, so in some weeks it should be at his Homepage.

@r6502
When my MPS-U60 transistors was on order, i've used for further testing MPSA92 Transistor as a replacement. Maybe they could work also. I orderd my MPS-U60 Transistors via eBay in Italy.

[r6502]

As summary : After changing the Input Fet (to often ), the LT1012, two MPS-U60, the LM337L (which was 8V instead of 5V), a poti, a broken 100 R Resistor (which was 6.5kOhm) and all Capacitors (one was bursted), my Electrometer works now as expected.  Last step is now calibrating the unit.

My original broken Input-Fet goes to Richis Lab for a nice Die Shot, so in some weeks it should be at his Homepage.

@r6502
When my MPS-U60 transistors was on order, i've used for further testing MPSA92 Transistor as a replacement. Maybe they could work also. I orderd my MPS-U60 Transistors via eBay in Italy.

Very interesting, so the defect end stage of the input amplifier seems to be a common problem with this instrument?

Question: Where in Germany did you by the double JFET sitting in the input?

I'm not really sure, how to proceed - improving the instrument by replacing the defect part wit newer better components or putting it in the original setup and replace the defect parts with the originals.

Guido

[r6502]

For the OP conversion I am wondering why you need the super high impedance LMC662 to replace both FETs. The feedback side is not very high impedance and could use a lower noise/drift type with no problem.

For the spike like noise, this could be cosmic or radioactive background radiation, hitting something like protecting diodes. Smaller diodes (less sensitive volume) and maybe radiation shielding could help.

What OP do you have in mind? The LMC662 has an ultra low Ib, that's what you want in the input stage to get the high input impedance here. It combines also 2 amplifiers in the same chip, that's from the thermal point of view also very nice. Also power supply is needed only one time ...

If the ADA4530 would be available as a double OP, I would use this one instead of LMC662 I think.

Guido

[Kleinstein]

Yes the LMC662 and some other very low bias OPs come as dual.  Some, like the LMP7721, LMC6001 also come as singles.
I would todays consider something like an OPA202 for a simple low noise follower for the less critical path.
The second OP of the input stage could also be just skiped alltogether. The input side has a source impedance of about the required value - so no real need for an extra buffer. A slightly differenten resistance would change the BW a little, but not much and a different cap could correct this.

The 2 nd not really needed OP would add to the drift an noise. With the JFETs it makes sense to have 2 to get drift compensation.
However with most OPs the offset drift of the duals does not correlate that much. So the 2nd OP is not really compensating the drift and definitely adding to the noise.

[David Hess]

Linear Systems has some dual JFETs specified to have lower than 1 picoamp leakage.  It seems like an old style 3N series MOSFET would be better for leakage, but they only test them down to 10 picoamps.

Yes the LMC662 and some other very low bias OPs come as dual.  Some, like the LMP7721, LMC6001 also come as singles.

Before the LMC6001 became available, I used the LMC6081 which later is what National graded to get the LMC6001; they are identical parts.  If you are willing to grade the parts yourself, then you can buy like 8 LMC6081s for every LMC6001 and I never saw an LMC6081 with more than the typical input bias current specification.

Bob Pease mentioned what they went through to test LMC6001s for input bias current.

[MiDi]

Very interesting, so the defect end stage of the input amplifier seems to be a common problem with this instrument?

I'm not really sure, how to proceed - improving the instrument by replacing the defect part wit newer better components or putting it in the original setup and replace the defect parts with the originals.

As common as leaking/blown input JFET and leaking reed relays 
Imo recapping all electrolytics is mandatory, even if they seem to be fine.

One could improve a lot in this instrument, it is an old design with old parts and rather bad layout...

My original broken Input-Fet goes to Richis Lab for a nice Die Shot, so in some weeks it should be at his Homepage.

Richi has already done die shots of my leaky JFET, but it is first gen, would be interesting to see if there are differences to newer gen 

[snik]

My original broken Input-Fet goes to Richis Lab for a nice Die Shot, so in some weeks it should be at his Homepage.

Richi has already done die shots of my leaky JFET, but it is first gen, would be interesting to see if there are differences to newer gen 

That's exactly the reasons why i've contact him. 

[r6502]

Hello All,

one question, my 617 seems to be an earlier device - it does not have the input protection Q311 and the Jumper W303 for (better?) offset trimming of the preamp is missing as well.

In this older instrument the offset trimming then is done only with the trim pot R314 correct?

Does somebody have a schematic for the older version of the device?

Kind regards

Guido

[MiDi]

What revision has your EM board?

[r6502]

The board has the marking 617-162-02F

Guido

[MiDi]

one question, my 617 seems to be an earlier device - it does not have the input protection Q311 and the Jumper W303 for (better?) offset trimming of the preamp is missing as well.

In this older instrument the offset trimming then is done only with the trim pot R314 correct?

Does somebody have a schematic for the older version of the device?

Yes, the voltage offset trimming is implemented differently in the earlier revisions - seems rev. F is like rev. E in this regard (see my previous pictures), IIRC it trims the right resistor R336 of JFET Q308 instead.
Added Q311 (R355, R353) & voltage offset trimming are the major differences in the schematic, the layout had some other changes in later revisions, too.
I could not find any schematics for other revisions, but there is no need for.

[Kleinstein]

Trimming R336 effects the temperature drift. So with both the old and new style trimmers in place one could in theory trimm both the offset and TC. It is still a bit tricky and time consuming.

[r6502]

Difference between REV J and L is at least added fuse on COM, there is a screw on backplate between COM post and BNC connectors for fuse-holder - so watch out for this if you want at least REV L 
Edit: See attached pics.


Interesting: between my REV E and Dave's REV J (oldest known) there is maximum only 1 year in between according to highest datecodes on components (23/84 vs. 28/85)

Overview of known Revisions of EM-Board:
MiDi: E
r6502: F
Alex: G
Smith: G
MadTux: L, G
Dave: J
TurboTom: J
HighVoltage: J, L
_Wim_: K
math_indy: L
baltersice (Marco Reps): L

Many thanks for this overview . interesting, how many people are interested in this kind of instruments.  I just started to get into the details of the instrument. Due to the fact, that I also own a 616, where Keithley used a similar preamp, so it was easy, for me to understand how the different power supply's are working.

Guido

[r6502]

Hello All,

I took a look on the input stage of the K617, electrometer board, Rev. 0F. Please take a look and comment it.

This schematics of the input stage is different from the later versions, available as download.

Kind regards

Guido
corrections:
the 2 Resistors in the input line are 1st 10Meg an then 100k
the 2 resistors at the bottem, connected to the JFET and -5V(B) are 200k

[r6502]

Hello All,

meanwhile, i managed to get the device to do something ;-)

the following parts have been defect:

• input double JFET, replaced with 2N5909
• defect LT1012, actual replaced with OPA177
• defect trimpot R348 and R314
• defect -5V regulator LM337LZ
• defect MPS-U60 PNPs in the end stage (Q303, Q305 and Q307), replaced with MJE350

Parts that have been replaced, but have not been defect:

• all electrolytic caps
• MPS-U10 NPNs in the end stage (Q301, Q304 and Q306) have been replaced by the MJE340, so they fit to the MJE350

Now the instrument is in the status, that it shows real values and no nor "OL" in all ranges. Voltage works fine, but I'm not sure, what's with the input impedance. I have bought 10 2N5909, and selected the one, with the lowest leakage current. I measured the leakage current at 5V reverse voltage with my 616 electrometer. The measured current varies from about 2pA to 0.2pA.

I will replace the input OP with the original LT1012, when they arrive here.

With the input double JFET's I'm currently not sure, if I want to keep close to the original design with the double JFET, or switch to a modern OP like the ADA4530 or so. If I go to the ADA4530, this will also replace the LT1012 ...

So at the next weekend I will continue with the performance check first before I do further modifications - luckily I also own a 263 current calibrator with good performance, so it will also easy to check the lower current ranges.

I will update you here, how it goes on with repair / performance check.

Guido

[David Hess]

I recently added the OPA140 as an alternative to the LT1012 with lower input bias current, comparable precision, slightly lower noise, but the largest package it is available in is SO-8.

[r6502]

Hello all,

Now  I replaced the original double JFET from the Input stage wit the 2N5909. I bought 10 parts, and measured the leakage current between gate and D/S with my Keithley 616 and found 2 2N5909, that had a leakage current in the range of 200fA. one of these parts was assembled to the electrometer board. I alsoused the 2N5909 case connection and connected it with a 100k resistor to BOOTSTAP COMMON, like in the schematic Keithley published for the model 617. Then I cleaned everything (with ethanol and isopropanol) dryed it with a heat gun and let all stay over night. Next day in the morning I powered the unit on and let it warm up the whole day.

In the evening I started to adjust the offset of the preamp in lowest voltage mode and it works fine.  Now I tried to adjust the bias, with the procedure described in the manual. 1st again  the offset was adjusted in current mode then the bias that was not possible in the lowest range (2pA). In the 20pA range the lowest possible value I could adjust was about 5pA.

I have the revision F of the electrometer board that has a different setting for the bias then later models - I published a sketch of the input stage in an earlier post.

Should I have a look again on more intensive cleaning, or should I look on other things like leaky realays or the bias circuit.  Does it make sense, to modify the bias circuit here, because on the newer models trimming goes from negative to positive or should I go direct to replace the double JFET by the AD4530?

Guido





 

[r6502]

Hello All,

in the mean time, I did some testing with my instrument, 1^st I replaced the input JFETs, but due to the large imput offset current in 10^-12 A range (> 1pA at the end)  I went to replace the original JFET + LT1012 with the ADA4530 (->thanks to MIDI for this suggestion). Yes, I cleaned the PCB and let it dry after replacing the parts. This works a lot better from the beginning, no more voltage ofset in all ranges without adjustment when "Zero Check" is enabled, but still some current leakage in the lowest current range (just below 300fA), so I will now disassemble the hi insulating relays.

I have curently two opotions:

First Option:
See atached datasheet from Steinecker, costs 29,57€ for 10 parts and more + tax. Delivery time is about 8 to 12 weeks and if I have luck, they have a few on stock. I will need also update my K263 calibrator, so I need more than 10 ...

second option:
https://sanyu-usa.com/products/95d-series/
here I got also an offer from te german sales office, but need to buy minimum 20 parts at a price of 23,59€ + tax - Delivery time here will be 28 weeks after the placcement of the order.

I'm not sure, but I tend to order the  relays from Steinecker - what do you think?

guido

[analogNewbie]

Yesterday, I replaced all my relay to extremely cheap relay and jfet to lmc662 for fun. After adjusting R314 and R348, the reading of 0 current in 2pA range is about 3fA+-4fA. The only problem is 0 current reading takes too long time to recover back from ohm mode. Not sure if it's the DA problem of cheap relay.

The resistance of my cheap relay is about 30T ohm. The resistance of old relay from k617 is 9G~200Gohm. 

https://lcsc.com/product-detail/Reed-Relays_PAN-CHANG-SIP-1A05_C10496.html
https://lcsc.com/product-detail/Reed-Relays_Cosmo-Electronics-S1A050000_C150532.html

It's not recommended to use such relay for serious application.
I have B2987A and 6517A, my k617 is just for experiment only. 

[_Wim_]

The only problem is 0 current reading takes too long time to recover back from ohm mode. Not sure if it's the DA problem of cheap relay.

What do you consider too long? I always wondered what is the "normal" settling time for this...

[David Hess]

The only problem is 0 current reading takes too long time to recover back from ohm mode. Not sure if it's the DA problem of cheap relay.

It might also be a thermal effect.

[David Hess]

The only problem is 0 current reading takes too long time to recover back from ohm mode. Not sure if it's the DA problem of cheap relay.

What do you consider too long? I always wondered what is the "normal" settling time for this...

Fast settling time at low currents and voltages requires a lot of attention to design and it is easily not apparent that settling time is extended unless a detailed analysis is done to predict what it should be.  If noise is high enough, then settling time becomes never.

Dielectric absorption, in more than just capacitors, and thermal effects are particularly troublesome.

Noise, especially flicker noise, and linearity are this way also.  More than once I have tracked down a problem after noticing that the last digit of a high resolution measurement flickered more than the predicted noise.

[r6502]

Hello All,

what I saw from analogNewbie will work for tests, but in the real world the original relays do have a electro static shield arround the reed contact, that is in most relay designs also isolated from the glass of the reed contact with a plastic tube. You can see this having a look on the COTO types. This electro stactic shield is on the relay used by analogNewbie not present - may this be a source of the slow reaction?

@analogNewbie:
Did you monitor the output voltage of the preamp with an independend DMM - this is very helpfully during the repair, you do not need to rely on the K617 AD converter. I used my HP 3478a for this, because it has a good mV range.

Guido

[_Wim_]

Fast settling time at low currents and voltages requires a lot of attention to design and it is easily not apparent that settling time is extended unless a detailed analysis is done to predict what it should be.  If noise is high enough, then settling time becomes never.

Dielectric absorption, in more than just capacitors, and thermal effects are particularly troublesome.

Noise, especially flicker noise, and linearity are this way also.  More than once I have tracked down a problem after noticing that the last digit of a high resolution measurement flickered more than the predicted noise.

Thanks for your reply. I understand that prediction is not easy. I was more wondering how fast a 617 in "perfect" condition (or a 6517) settles when switching from ohms to current. I expect even a "perfect" unit takes some time, and it would be good to compare to evaluate if the DUT has a problem or not.

In the case of my unit, it takes about an hour to fully settle (results posted here: https://www.eevblog.com/forum/testgear/fun-with-low-leakagebias-current-femtompere-electrometer-keithley-617/msg2941340/#msg2941340)