Products > Test Equipment

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

<< < (80/82) > >>

ZhuraYuk:

--- Quote from: David Hess on May 22, 2024, 11:22:33 pm ---
--- Quote from: ZhuraYuk on May 22, 2024, 10:48:52 pm ---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.
--- End quote ---

An LMC6082 would have lower offset and drift.

Are there schematics showing what you did?

--- End quote ---

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:

--- Quote from: ZhuraYuk on May 22, 2024, 11:39:37 pm ---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
--- End quote ---

Thanks, that is exactly what I needed.


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

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:

--- Quote from: ZhuraYuk on May 22, 2024, 10:48:52 pm ---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.

--- End quote ---

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:

--- Quote from: MiDi on May 23, 2024, 11:48:36 am ---Is it possible to trim to same digits in V & I mode?

--- End quote ---

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


--- Quote from: MiDi on May 23, 2024, 11:48:36 am ---Is it the same at the analog output?

--- End quote ---
yes, analog output repeats display readings.


--- Quote from: MiDi on May 23, 2024, 11:48:36 am ---I have encountered maybe the same problem, where trimming the offset on one mode doubled the offset in the other.

--- End quote ---
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



Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod