Update.
The 417 has been running out in the garage for some time, and I've been
occasionally monitoring operation. It's uncontrolled temperature and
humidity, so I get to see a decent range of conditions. In the mornings,
after cooler ambient, the bias current appears about 1 fA or less, then
as things warm up, it gets down to the 0-+/-200 aA range at some ideal
condition, then increase to about 1 fA max in the other direction later
in the day.
On top of this are occasional jumps up to an extra fA or two in either
direction. Also some 120 Hz line ripple that was about 1% FS p-p, which
I have reduced greatly with some temporary changes in the wiring. Again,
the drift and jumps may be from many factors that I can't pinpoint yet.
Besides temperature, there is the random noise and 1/f noise at the
front, and line regulation and transient susceptibility to consider, and
possibly noise at the back end gain ranging switch contact noise, and
connection pin noise. The 417 is a plug-in type deal, and the business
end connects to the main box through a DB-15 connector, and has
provision to run on an extension cable too. Everything adds to the
situation, and the signals are at very low levels, so difficult to
assess cause and effect.
One thing that I think I've determined, is that some of the jumps may be
due to occasional ionizing radiation hits. Last night it was running
around 600 aA, and I placed a bag of thorium mantles on top of the
plug-in's back end, where all the action is. It promptly ran up to about
3 fA and stayed, then went back to the original range when it was removed.
Also, days ago, it seemed to get into a high leakage mode sometimes
especially after being cool over night. The bias would stick around 2-3
fA then much later in the day finally settle down. I believe I have
fixed this problem by changing the first stage compensation cap. It's
hard to say for sure the effect of changes on things like this, because
sometimes the act of simply taking something apart and putting it back
together (necessary each time plug-in is worked on) is actually what
makes the difference.
The opamp front end is temporarily way over-damped with a small cap
(actually two stages of C) from the output to inverting input, so I
could get stable DC operation. It's mechanically not possible to work on
the plug-in in place, so each group of changes I want to try means
removing it and opening the case, doing the stuff, then putting it all
back. Ideally, this could be done while on a proper extension cable,
which I need to build. I tried a standard DB-15 straight-through cable,
but it all oscillated horribly. The cable needs to be built the right
way to eliminate crosstalk Until then, I do only a little at a time,
with no live testing available.
The compensation cap is a 1 pF glass type (because I don't have any
polystyrene ones that low). With cleaning, but no silicone treatment,
its body resistance is probably around E11-E13 ohms tops, comparable to
the highest feedback R E12 ohms. To isolate the opamp's DC output from
the glass resistance, I used an extra stage 1000 pF, then a 100 meg R to
common/guard, then the 1 pF cap. This way, the glass sees nearly zero DC
at the output end, minimizing leakage current, while the capacitive
current goes right through. I suspect that the mica cap I used initially
may have been way too leaky (virtually all my parts of all types are old
used pulls, so may be of questionable condition). Replacing it with a
well washed polystyrene one seems to have fixed it, and I haven't seen
the behavior since - although it may be just coincidence, as outlined above.
I'll have more to say later. I pulled out some of my other electrometers
too, and have some interesting observations.
Ed
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