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

[r6502]

Hello all,

I'm now back from Holidays, and I'v just ordered the Coto 9002-05-00 relays at Digy Key, there they are avelable for 4,90€ (10 to 25), so a lot cheper than at Mouser. I'm really excited how thees relays will perform.

When I have them on stock, I will measure the isulation resistance of the relays with the old k616, and than choose what relays I put in the K617 ...

I will also communicate the results here.

Guido

[r6502]

Hello all,

meanwhile, I received the relays, and as promised, I did the measurements of the isolation resistance.

The results look really good, and the isolation resistance is much higher, than it is specified inside the data sheet.

I measured for 1 part between all contacts, for the rest I measures less points, to safe some time here.

I have used turned IC contacts with an attached wire for easier contacting the pins of the relay. I handled the relays with tweezers, so the where not contaminated.

I also took a photo of the of the original relay compared to the COTO 9002 type.

I used 200V for testing, as this is the specification for the relay, 2nd reason was that I will get a higher current, as the K616 is in the lower range not so sensitive. I checked It with the K263, and sill got a good response when I impressed 0,05pA, but at these low current levels, the meter noisy.

Have a look at the results and please leave your comments below.

Guido

[Hydron]

Excellent, happy to hear the one I got and measured isn't exceptional - in fact it looks slightly worse than most of yours! (I did give a pretty big margin for my numbers though - possibly should have done it at 1kV to get more SNR).

Eager to hear how it works in use, and whether the shield does it's job and makes for a decent settling time.

[r6502]

@Hydron:

the relay is specified for operational voltage up to 200V, so at 1000V the results will not be informative as you are wide out of specification of the relay. May be 250V will work as well but I would not go mutch higher with the test voltage, especial between the two contacts of the relay.

Did you handle the Relay really clean, no finger prints on the relay surface, no leakage in the measurement setup ...
you also wrote, that she signal drifts a few 10sec. That is also that what I saw, but I saw this also on the original relays that where not so bad - so it is normal especial at these low current levels. The current signal drifts for a while when I turned on the test voltage, so I decided to take the value after 5 min.  When I was waiting about 15min, the measured current was even lower -  how long did you wait, until you took the measurement for the current?

I will also double check my results with an electrometer we have in the company. It is used for x-ray dosimetry, and has a build in voltage source of 200V and a valid calibration. It measures also the input current in that range. If the relay than fails, I will report again here. But I hope I can validate my first results.

Guido

[Hydron]

I don't think I was waiting too long to take the measurements - the main issue I had was that it was right at the bottom of the SMU's most sensitive 1nA range - it's 5.5 digits so the least significant digit is 10fA.

As for voltage - you're right about the rating, I was more thinking when measuring from coil to shield/contacts (1.5kV rating), not across the contacts (200V switching rating). The application I got my relay for potential use in would only see 15V or so across the contacts, so that rating was irrelevant, but I'd see up to 1.1kV between shield and coil.

[The13thParish]

I purchased a 617 recently and had overload on every range.

I changed all the electrolytic caps which made a significant difference in that the ranges no longer read OL, but some non-small number instead. I also switched out all the relays to the Meder Electronic (Standex relays) MFPN: HI05-1A66 rather than the COTO relays often mentioned in this forum. I was just curious as the insulation resistance is specified to a minimum of 100 Tohm so should work quite well. One downside to these though is that the coil isn't encapsulated or potted so could accumulate surface dust etc. which would effect measurements.

I would describe the board condition as being rather good.

After changing the relays, I have a leakage current of around 17-19 fA after keeping the unit on for two hours and after cleaning all the flux off the board with lint free cleanroom swabs dipped in isopropyl alcohol. Next I want to swap out Q308 for Alex Nikitin's LMC662AIM however I'm a bit confused about the pinout for Q308. After analysing the photographs of people who have done this and posted to this forum, I have come up with the pinout scheme; so, each arrows represents where the pins of the LMC662AIM will be soldered to once Q308 has been removed. Could I ask for a spare pair of eyes on this just to make sure I have this correct?

Pointing out any glaring issues is greatly welcomed.

All the best,
Ryan

[_Wim_]

I purchased a 617 recently and had overload on every range.

I changed all the electrolytic caps which made a significant difference in that the ranges no longer read OL, but some non-small number instead. I also switched out all the relays to the Meder Electronic (Standex relays) MFPN: HI05-1A66 rather than the COTO relays often mentioned in this forum. I was just curious as the insulation resistance is specified to a minimum of 100 Tohm so should work quite well. One downside to these though is that the coil isn't encapsulated or potted so could accumulate surface dust etc. which would effect measurements.

I would describe the board condition as being rather good.

After changing the relays, I have a leakage current of around 17-19 fA after keeping the unit on for two hours and after cleaning all the flux off the board with lint free cleanroom swabs dipped in isopropyl alcohol. Next I want to swap out Q308 for Alex Nikitin's LMC662AIM however I'm a bit confused about the pinout for Q308. After analysing the photographs of people who have done this and posted to this forum, I have come up with the pinout scheme; so, each arrows represents where the pins of the LMC662AIM will be soldered to once Q308 has been removed. Could I ask for a spare pair of eyes on this just to make sure I have this correct?

Pointing out any glaring issues is greatly welcomed.

All the best,
Ryan

A bit hard to see if everything is correct. The -5V does not seem to be correct (seems connected to ground instead of -5V)

Best way to check is with a multimeter in short circuit mode to check if the below is correct:
* pin1 in the attached drawing must be connected to pin 3 of U309
* pin5 in the attached drawing must be connected to pin 2 of U309
* pin7 in the attached drawing must be connected to R351,R352 and R353
* pin3 (input)  in the attached drawing must be connected to R333
* +5V must be connected to R340,R315 & R319 and pin7 of U309
* -5V must be connected to pin 4 of U309

[The13thParish]

I'm having a little trouble correlating the pin numbers of the Q308 on the schematic to the physical form of the TO-type can as soldered on the board. The can has six legs; does pin number 1 correspond to clockwise after the little tab sticking out, or anticlockwise?

If I also understand your comment, there is a clip with wire soldered to it that clips on to the body of the can, I take that this is ground?

All the best,
Ryan

[_Wim_]

I'm having a little trouble correlating the pin numbers of the Q308 on the schematic to the physical form of the TO-type can as soldered on the board. The can has six legs; does pin number 1 correspond to clockwise after the little tab sticking out, or anticlockwise?

That is why is it easier (safer) just to test with a multimeter in short circuit mode. Connect one lead of the multimeter to pin 3 of U309, with the other lead you can test all the pins of Q308. If it beeps, you are connected to pin 1. The same can be done with the other pins you need with the list above.

If I also understand your comment, there is a clip with wire soldered to it that clips on to the body of the can, I take that this is ground?

Yes, that is ground. But you do not need ground to make this mod.

[The13thParish]

Got it, many thanks.

[r6502]

Hello all,

I'm now more or less done with the upgrade of the old K617 with the new COTO 9002 relays.

After installation the new relays and two times cleaning of the board, the offset and the noise in the lowest current  range looks pretty good. I would say about ±10 digits, but I did not record data from the device over time to get a better feeling, it was just on the digits of the internal ADC and with closed input jack.

I tested the K617 also with a calibrated current source we are using in the company, that produces currents down to 0,1fA, and I got really stable signals with the K617. I Used the build in ADC, and did not use an external voltmeter to check the output voltage of the electrometer amplifier, as I got the feeling connecting external 4mm lab cables to the external DMM introduces extra noise.

So when I have more data ready, I will share it here. I hope, i can do more test nxet weekend.

Guido

[HighVoltage]

Hello all,

I'm now more or less done with the upgrade of the old K617 with the new COTO 9002 relays.

After installation the new relays and two times cleaning of the board, the offset and the noise in the lowest current  range looks pretty good. I would say about ±10 digits, but I did not record data from the device over time to get a better feeling, it was just on the digits of the internal ADC and with closed input jack.

I tested the K617 also with a calibrated current source we are using in the company, that produces currents down to 0,1fA, and I got really stable signals with the K617. I Used the build in ADC, and did not use an external voltmeter to check the output voltage of the electrometer amplifier, as I got the feeling connecting external 4mm lab cables to the external DMM introduces extra noise.

So when I have more data ready, I will share it here. I hope, i can do more test nxet weekend.

Guido

Nice!
Please show us some pictures of the installation of the new relays.
Looking forward to your measurements

[r6502]

Hello all,

as requested, photos of my K617 with COTO 9002 Relays:




Also updated: imput amplifier, updated with ADA4530a as Midi suggested here:
https://www.eevblog.com/forum/testgear/fun-with-low-leakagebias-current-femtompere-electrometer-keithley-617/msg2501178/#msg2501178

Guido

[The13thParish]

After having replaced my own relays recently, I'm finding that the current is slowly getting towards the 1 fA range in terms of leakage current however on voltage mode the voltage increases to overload when the triaxial connector is protected by a cap.

I think someone mentioned in an earlier post that voltage creep to overload does not indicate an issue with Q308, however as far as I can see trawling this post there wasn't an indication of what DOES cause this. The voltage source is reading pretty accurately across the full output range, only a few mV off.

Any tips?

[r6502]

@The13thParish:

After maintenance on the board like repacing the relays removing the flux is very importand, as the flux generates aditional leakage current.

The input stage has an input bias current as well, this current generates an ofstet in current mode and and in voltage mode the output goes to positive or negative rail, depending the polarity of Ib - this is normal. in the original setup there is a compensation for Ib, and adjustment is schown in the manual. If you have replaced the input stage with a modern OP like ADA4530, there is no need for Ib trimmeng, as these chips have a much lower Ib as the original double FET Q308.

General hint, operation with no source connected to K617: if input is open, press "zero check", especial in voltage mode.

A 2nd thing are cables, there are really big quality differences on triax cables. I bought one cable connecting my 263 to the K616 or K617 and with this cable the instruments are very slow, when I chenge the current, takes more than >10s - with a quality cable we are using in the company, setling time is less than 1s

Hope this helps

Guido

[little.tesla]

Hello,

I'm currently working on a K617 with rev L board.
I've already replaced the input FET, Q308 with the LMC662 and the unit is already working to some extend.
What is interesting with my unit is that the current reading in the lowest current range decreases over time (~5h) until it reaches equilibrium / thermal balance. See attached figure (X-Axis in hh:mm). This effect is repeatable. To me, this suggests some semiconductor is leaking but I'm not sure where to start the hunt. Any pointers are very welcome.

Attached is a picture of the mod with the THT version of the LMC662. Not proud of the soldering, would have preferred the SMD version .
My notes with the pin numbering and my way of the output resistors selection.

Regards

[Kleinstein]

In the very low current range it is normal to get relatively slow settling. There are effects like dielectric absorbtion in some isolation material and possible surface charges than can take quite some time to settle.
Another point can be just the warm up and temperature drift of the meter.

[little.tesla]

The input offset with "Zero Check" enabled is stable over the whole time period of 5h (+/-2 counts). But for the input current I would have expected that it does not drift that much. The test was repeated with input wire disconnected with same result.
When looking to the graphs from user Alex Nikitin and his LMC662 mod, I see around ~3fA drift over a period of ~1h or ~6fA over 24h. I did as well a test where I opened the lid of the unit to allow the unit to "cool down" a bit and the input current stayed at the current value or slightly decreased. This suggest to me an effect linked to internal temperature increase.

[r6502]

Hello all,

at the weekend I got GPIB running and i'm now able, to get Data from the K617.

I just made 4 tests in the lowest current range. I measured the following situations:

- nothing connected to input and a cover cap over the external input
- input current 1fA
- input current 10fA
- input current 100fA
- Input current 1pA


I sampeled 1000 values from the K617 device with internal ADC free running. This equals to about 5min total time.

Please have a look at the results. I will also perform a measurement over night, to see how the instrument drifts. this results will be posted as soon, as possible.

I think, the pervormance is really impressive. When you also take in mid, that I did not jet perform an adjustment of the device and did not only replace the leakey relays but also the input amplifier with the ADA4530 and defective output transistors as well as defective parts of the power supply. The higher current ranges are working also really good.

The current source, I used is a calibtated source, that can deliver currents down to 1fA. Calibration is performed on a regular base 1x per year. In the company we are using this source to adjust devices able to measure currents down in the fA range.

I used Labview Community edition and an NI GPIB-ENET/100 interface to get the Data.

My 263 is a bit off in the lower ranges, so I need to replace the high insulating relays in this instruments as well I think.

[little.tesla]

Hello,

there was as well some progress with my unit. I washed the board with alcohol and did a bake-out but this changed the situation only temporary. After rechecking the power supplies (which are actually a bit too high), LT1012 and the output transistors, there was not much left to be checked other than the relays. I disconnected relay by relay, retested and with each relay the situation improved. At the moment I've removed all of the relays, hard wired the lowest current range and after warm up readings are stable. Based on this finding and the "discharge" like behavior of the settling over time it strongly suggests that the issue is DA in combination with the relays.

Based on the great summary from MiDi regarding the relays, there are currently three options on stock:
Meder Electronic/ Standex HI05-1A66 (Mouser) ~12.75$ each
Coto 9002-05-00 (Mouser) ~6.92$ each
Coto 1240-0197 Ebay (NOS) ~31$ each + shipping+handling, total >~300$ for 6relays

With 1240-0197 from eBay the risk and price is too high. The Meder has a higher specified insulation resistance but I think the missing shield is a draw back. Therefore, I'll get some Coto 9002 as r6502 did and will report back.

[MegaVolt]

The current source, I used is a calibtated source, that can deliver currents down to 1fA. Calibration is performed on a regular base 1x per year. In the company we are using this source to adjust devices able to measure currents down in the fA range.

What is the name of this device?

[r6502]

The current source, I used is a calibtated source, that can deliver currents down to 1fA. Calibration is performed on a regular base 1x per year. In the company we are using this source to adjust devices able to measure currents down in the fA range.

What is the name of this device?

@ MegaVolt:
I'm sorry, but we have a nondisclosure agreement with the company and I can not poste details here. It is only for internal use and not comercial available product.

Guido

[little.tesla]

I finalized the COTO 9002-05-00 relay conversion as previously described by other users and I can confirm that my initial issue with the DA is gone now. I do see some small thermal drift over a few hours, but the meter settles now always reliably to a low reading.




Next thing I want to do is the conversion with the ADA4530. I hope this will improve the observed thermal drift with the current LMC660.

[_Wim_]

Nice job. I wonder if this mod makes the meter settle faster than an unmodified unit.

To test this, I propose the following:
1) let the meter settle to +-2fA with nothing connected
2) switch to ohm mode for exactly 60 seconds, and switch than back to currect mode (ohm mode will show OL and will output a high test voltage)
3) record the time it takes to settle within +-10fA

When I do the above test on my unmodified meter, it takes +-8minutes to settle again to within 10fA. The longer the meter is in ohm modus, the longer it takes to settle back down (clear indication of some DA).

[r6502]

Time for settling after change of the the range from current to ohms and back takes the same time (not measured) as with old relays, as nothing in rest of the device was optimised for this function. Maybe that in my instrument it will be a little different, as I changed the type of end stage transistors.

When the input amplifier is saturated, especial in the ohms range, the max output voltage will appear on it's output if terminals are open. It takes a while until it gets back to normal working conditions. This behaviour also described in the manual of the device.

Guido