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

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MiDi:
Ok, that is a different error, nothing to do with offset, it is an intermittent fault.
The switched relays are suspect, preferrably the yellow one.

Hydron:
Just got one of these, was wondering what sort of voltage source improvement I should expect to get from a pricey LTC7541AKN in place of the original MP7623JN (itself meant to be a better equivalent of an AD7541). I'm currently seeing about 15mV offset (will look at opamps to try and fix that), and about +/- 0.3 LSB of INL (so about 16mV). This is already within the max spec of the LTC part, let alone the original - do the LTC DACs often wildly exceed worst case spec, at least at room temp?

Smokey:
Firstly, this is such a good thread... Thanks for everyone that has contributed here...
I don't think I'm to the point of doing surgery on my 617 yet, but the Q308 JFET to LMC662 opamp mod got me thinking...

If Keithley was to have changed the design to use the LMC662 in production, would they still have to hand-air-wire the chip like this on the Teflon standoffs or is there some leakage minimizing way to use the SO8 on the board to make production easier and more consistent?  Could a little raised PCB for the LMC662 that is then air wired in be low enough leakage maybe?

Alex Nikitin:

--- Quote from: Smokey on December 12, 2024, 11:11:12 pm ---Could a little raised PCB for the LMC662 that is then air wired in be low enough leakage maybe?

--- End quote ---

Yes, if the PCB is made from Rogers 4350B laminate and thoroughly cleaned. No chance if you use the standard FR4 material.

Cheers

Alex

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