Agreed, don't risk a reed switch, it will probably ruin your accuracy. You'll probably find that the offset only needs zeroing every few months at worst (it drifts only slightly as the battery voltage goes down, but you won't see it if your meter resolution is at the mV level).
Due to the very low bias current of the LM662, you should hopefully find that you reach very close to the same result (zero setting), its offset voltage should dominate the adjustment
The difference at the output should be in the low uV range.
I think to date that's why I haven't noticed anything other than 0 volts output after completing the zeroing: I wasn't looking for anything in the low uV range. What would that low microvoltage represent though? Does each microvolt represent 1 pa of bias current, which, theoretically, I would need to add to the final picoamp measurement (assuming single digit pa measurements are possible), measured in the millivolt range?
No, you probably won't notice a difference - the external zeroing link just makes it much easier to get the offset voltage spot-on zero, without having to worry about noisy readings or the shielding.
Microvolt outputs represent FemtoAmps (pA/1000)
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The
typical spec for the LM662 bias current is 3fA. With the way I restricted the adjustment range of the offset pot, it's possible to zero the opamp to 0uV (assuming that you have a meter with that resolution). The reading is always 1pA/mV.
Luckily, with the input shorted to the -ve output, you don't need to worry about screening - you are effectively creating a low impedance voltage follower which follows the offset adjustment pot wiper, and then zeroing the offset voltage relative to ground (the +ve output).
By "you don't need to worry about screening," do you mean that shielding during the zeroing isn't important after all? See post directly above.
As above, if you have the external link in place, you're bypassing the 1G resistor with a short, so it doesn't care about shielding any more - it's not seeing fA or pA in that state.
Ok, I modified the picoammeter to use BNC connectors:
I blocked off the unused hole with a piece of single sided copper PCB, held in place with scotch tape. Meh, not ideal, but hopefully it will do for the short-term.
Looking good.
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