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

[BD1QMP]

after 2 days power on. the meter become stable.

from noon to night, room temperature changed from 25 degree c to 21 degree C, the no input, the read change from +75 to -50.
   

After put back the original BNC connector, I recorded the offset. Is it normal or not?  thanks

[ZhuraYuk]

For those looking for cheap Keithley cables/connectors. Recently a wery cheap triaxial equipment apeared on ebay, I bought few items and the quality is good. The connectors are precise and fit nicely, the cable have triple PTFE (FEP) insulation, really low leakage

https://www.ebay.com/itm/276059871116

[ZhuraYuk]

Question to everyone, how much time it takes for your Keithley 617 to settle down to at least 15fA when zero check disabled? My input  circuitry seems to be very contaminated, because input current start from 150 fA and goes to 15fA in 20 minutes. Tried to wash input connector but did not helped.

[David Hess]

Question to everyone, how much time it takes for your Keithley 617 to settle down to at least 15fA when zero check disabled? My input  circuitry seems to be very contaminated, because input current start from 150 fA and goes to 15fA in 20 minutes. Tried to wash input connector but did not helped.

If construction is on a substrate, then I think that level of leakage could be caused by circuit board hook, which is sort of like dielectric absorption.  Circuit board hook can be caused by contamination of the substrate and the solution is usually to clean and then bake out the substrate.

If it was just leakage, then I would not expect it to get better as the circuit warms up.

[Alex Nikitin]

Question to everyone, how much time it takes for your Keithley 617 to settle down to at least 15fA when zero check disabled? My input  circuitry seems to be very contaminated, because input current start from 150 fA and goes to 15fA in 20 minutes. Tried to wash input connector but did not helped.

Takes about 20 seconds to settle below 5fA. The input connector is the last thing to touch. The main sources of leakage in the 617 in my experience are the relays and the input FET. The relays can be a real problem as a good replacement is not easy to find. The only option from stock I know of is the MEDER HI05-1A66, available from Mouser. These relays are not quite as good as the original though much better than any standard relays. I've measured the leakage across the open relay contacts at 20fA for 100V. A really good low leakage relay (as some Crydom custom made relays I have in my lab) leak
less than 3fA at 100V (see the graph below, 0V for 5min, than 100V for 5min and back to 0V for 5min) and have considerably smaller DA, settling in seconds rather than in minutes.

Cheers

Alex

[Alex Nikitin]

If construction is on a substrate, then I think that level of leakage could be caused by circuit board hook, which is sort of like dielectric absorption.

In case of the 617, the high impedance part of the circuit is all wired point to point using PTFE stand-offs.

Cheers

Alex

[ZhuraYuk]

Check out what I have scored on ebay - 35617EBS Dosimeter  https://www.ebay.com/itm/186416162087 there is one more for 228USD.  Zero information in the internet but inside it is the same K617 with additional bias board.  Cant figure out yet on which output this bias is applied and how.
They symptoms are the same as in my previous K617 - high input current 250fA with zero check disabled slowly going down to zero for 20-30 min. 

[Alex Nikitin]

The way to look for the leakage source is to disconnect bits one by one, starting from the input connector (just unplug it from the board), than disconnect all the switching parts, starting with the K307 pin 4 and R355, leaving only the input JFET and bias compensation network . I've modified my 617 some years ago mostly to reduce the offset voltage drift however the input current is also very good with the mod. The Keithley is still on my bench, working mostly 24x7 for last 8+ years (though it did develop some faults and so has been repaired several times over that period).

Cheers

Alex

[HighVoltage]

Check out what I have scored on ebay - 35617EBS Dosimeter  https://www.ebay.com/itm/186416162087 there is one more for 228USD.  Zero information in the internet but inside it is the same K617 with additional bias board.  Cant figure out yet on which output this bias is applied and how.
They symptoms are the same as in my previous K617 - high input current 250fA with zero check disabled slowly going down to zero for 20-30 min.

Nice score!
Seems to be a very special 617.

[ZhuraYuk]

In my both K617 the 250fA input current was caused by C307 in my case. In one unit it was  bloated, the foil was  visible from the sides. In second unit it was replaced by someone with incorrect value of 0.01uF instead of 1000pF and poor dielectric type.  Replacing C307 with  WIMA MKP 1000pF  1250V decreased bias current to 50fA  and it was impossible to compensate. So next step would be to buy and install original polystyrene capacitor of at least 600V as in  datasheet.

[Alex Nikitin]

You can use a ceramic NP0 from TDK, for example:
this one
Cheers

Alex

[Kosmic]

Hmm looking at my pictures, I think I need to change C307 in my unit too. Completely missed that last time it was open.

[ZhuraYuk]

Does anyone had an issue when barely deforming or knocking on electromer's chassis or housing makes measurement go crazy for few seconds? I rechecked electrometer board but cant find any cold joint there. Resoldered few suspicions places but that not helped.
Maybe there is a place that fails often? My second K617 not doing such thing so it is for sure an issue.

[David Hess]

Does anyone had an issue when barely deforming or knocking on electromer's chassis or housing makes measurement go crazy for few seconds? I rechecked electrometer board but cant find any cold joint there. Resoldered few suspicions places but that not helped.

That is common with sensitive high impedance circuits.  Vibrating wires cause movement of charge.  Mechanically securing the parts and wires helps.

I would tap around with an insulated poker or something to see if any areas are more sensitive than others.

[ZhuraYuk]

That is common with sensitive high impedance circuits.  Vibrating wires cause movement of charge.  Mechanically securing the parts and wires helps.

I would tap around with an insulated poker or something to see if any areas are more sensitive than others.

I have 6 electrometers of different vendors including second K617 and non of them is doing that. There is some drift when you shake the device but just a little.
The one I have issues with, goes to higher range even from lightly knocking on the housing.

Anyway, I was able to remove old reed switches from the relays. Some are easily sliding out, some require desoldering the relay and poling the switch with tweezers.
Replaced only first 4 relays and now input current goes to 5fA in 5-10 seconds after switching to amps. Easily can be compensated to zero current.
The original look of the board was  preserved.

[snik]

Anyway, I was able to remove old reed switches from the relays. Some are easily sliding out, some require desoldering the relay and poling the switch with tweezers.
Replaced only first 4 relays and now input current goes to 5fA in 5-10 seconds after switching to amps. Easily can be compensated to zero current.
The original look of the board was  preserved.

Nice work.
Which Reed Relays do you use ?

[ZhuraYuk]

Nice work.
Which Reed Relays do you use ?

Initially I used MEDER KSK1A85-2030 because they are larger than original and rated for 1kV. However after breaking two because of too hard leads I switched to  MEDER KSK-1A66/3-1015 which are same size as original ones  and found no differences. So I installed KSK-1A66 in both K617 and all were fine.
I assume that epoxy filling is just to hold reed inside and to avoid glass breaking when forming the leads. The epoxy absorbs water with time thus leading to high leakage.
Also insulation tube around switch seems to be made of PTFE it holds 250 degree C just fine, so I reused it as well.

[ZhuraYuk]

Did you know that triaxial connectors with sapphire insulators existed? These were some experimental stuff that was supplied together with only one soviet electrometer V7E-42 (В7Є-42). And even there in documentation for this electrometer it is not explained what is their purpose, they were not used in electrometer itself but supplied as spare parts for some undocumented  switch box filled with sapphire  insulators relays.  I guess they were there for connection with western electrometers like K617.

[ZhuraYuk]

After replacing input JFET pair with LMC662 I got 20mV input offset in voltage mode which I can compensate with trimpot but then input current goes far beyond zero. I guess this can't be avoided ?
Also I see that after switching to ohms mode and pack to current DA is crazy with LMC662.

[David Hess]

After replacing input JFET pair with LMC662 I got 20mV input offset in voltage mode which I can compensate with trimpot but then input current goes far beyond zero. I guess this can't be avoided ?
Also I see that after switching to ohms mode and pack to current DA is crazy with LMC662.

An LMC6082 would have lower offset and drift.

Are there schematics showing what you did?

[ZhuraYuk]

After replacing input JFET pair with LMC662 I got 20mV input offset in voltage mode which I can compensate with trimpot but then input current goes far beyond zero. I guess this can't be avoided ?
Also I see that after switching to ohms mode and pack to current DA is crazy with LMC662.

An LMC6082 would have lower offset and drift.

Are there schematics showing what you did?

I did exactly the same as Alex Nititn advised in this post https://www.eevblog.com/forum/testgear/fun-with-low-leakagebias-current-femtompere-electrometer-keithley-617/msg759354/#msg759354

A lot of people here did the same, but I never saw mention about constant input voltage offset when zero check is enabled.

[David Hess]

I did exactly the same as Alex Nititn advised in this post https://www.eevblog.com/forum/testgear/fun-with-low-leakagebias-current-femtompere-electrometer-keithley-617/msg759354/#msg759354

Thanks, that is exactly what I needed.

A lot of people here did the same, but I never saw mention about constant input voltage offset when zero check is enabled.

I think someone would have mentioned it.  Maybe your LMC662 is bad?  What was the source for the part?

[ZhuraYuk]

LMC662  was purchased from Mouser.
I already changed input JFET pair to LMC622 in some other device which has identical input schematic, the result was similar - offset in voltage measurement mode.
I assume this replacement is not suitable for this type of devices, it breaks voltage and ohms modes as well. The offset voltage of 20mV cause resistance never going to zero even with zero check enabled, it shows 200 Ohms. 
Amps works great on other hand, stability and settling time is much faster with OP Amp mod.

[MiDi]

After replacing input JFET pair with LMC662 I got 20mV input offset in voltage mode which I can compensate with trimpot but then input current goes far beyond zero. I guess this can't be avoided ?
Also I see that after switching to ohms mode and pack to current DA is crazy with LMC662.

Is it possible to trim to same digits in V & I mode?
Is it the same at the analog output?

I have encountered maybe the same problem, where trimming the offset on one mode doubled the offset in the other.
It is an offset at the AFE output, but could not find the cause.

[ZhuraYuk]

Is it possible to trim to same digits in V & I mode?

Not sure what you mean, when I trim zero offset in voltage mode then the current mode offset will be 1000fA+

Is it the same at the analog output?

yes, analog output repeats display readings.

I have encountered maybe the same problem, where trimming the offset on one mode doubled the offset in the other.

Maybe operational amplifier offset can be the reason, just the fact that it has it. All devices with opamp input buffer have input offset compensation circuts, but K617 does not have exactly that.

I uploaded the video showing the issue.  The analog output measurement are shown on UNI-T multimeter.
The issue is not stable, so the reason might be the bad relays? Coto ones were replaced in that device but other ones in dip package and larger yellow ones were not.

https://youtu.be/L0kWBU30Xck?si=1eNb5ajwA4-_-C4r