I just did a quickie experiment to get higher resolution. I rigged up an
external meter movement about the same size as the internal one, hooked
to the monitor out, which is 3V FS. The external meter is set up for
about ten times the sensitivity, so 300 mV FS. I wanted to see how the
noise looks, with similar meter ballistics, and expand the zone around
zero to assess the bias current. The net result is the external meter
reads about 10 fA FS, and the noise and drift are still low enough to be
quite usable. The latest bias current estimate is about 50-100 aA,
slowly dithering around less than plus one percent of FS, with
occasional jumps to about 500 aA or less. I can't tell yet if these
jumps are part of the 1/f noise, or line noise and transients getting
through the power supplies.
Anyway, presuming I didn't make any mistakes in my measuring and
figuring, this is quite impressive. I hope I just didn't get lucky with
one particular part, but I think the others will be similar. I will
eventually be checking them all, and other part types, in test setups.
For reference, I confirmed the scaling. With the 417 set up for maximum
sensitivity, it's 100 fA FS. Putting the suppression supply at 100 mV
through the top 1E12 resistor, it reads near full scale. Setting at 10
mV, the internal meter reads ten percent of FS, and the external meter
reads near its full scale. BTW I can set the suppression directly with
mV resolution. Long ago, I changed the last digit pot to a ten-turn
precision type with a kilodial indicator - it's nice.
This is now over a hundred times better than the original spec for the
417's grid current. When it's all said and done, I'll likely add a 10X
meter switch arrangement in the 417, to get a 10 fA FS range capability
too. It may be good even at much higher sensitivity, but 10X is pretty
good, and I likely won't need any more, practically speaking. Besides,
putting a DVM (and some filtering) on the output can expand it even
more, if ever necessary.
One problem I did find, is there's some line ripple appearing in the
output signal, which I need to investigate. It's pretty small - about
one percent FS - and the meters don't notice it, but it still bugs me,
since it's way bigger than the apparent random noise.
Ed
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