Regarding making your own extreme high-value resistors - any object that
has insulators and leads but with nothing connected inside will have
some high R that can be perhaps be measured, but won't be stable against
environment effects on the outer surfaces. There's not much point to
I'm guessing the application relates back to your leaf electrometer
project discussed earlier - trying to assess how the bias charge on the
capacitor holds up from leakage and use of the instrument. If this is
the case, then it's for a one-time use for design of the item, so
shouldn't be too
Yes Hendrik, same principle as the butterfly disk style, but mine use
cylinders - the field exposure is radial instead of axial. Ed
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Here's a simplistic view that may be sufficient. Some energy (in the
form of charge redistribution, which includes current flow) has to come
from the capacitor, and some from the input signal, to do the work
needed to push the leaf against gravity. When the input signal is
removed, some of the
I looked at that link that Brooke put up about Bohnenberger's
Electroscope. I don't know what your specific arrangement needs to be,
but it appears you need a plus and a minus HV wrt ground in the most
general form. If so, then this would mean having to split the voltage of
a single cap, or
Another thing I noticed in these instruments - the highest R value used
is E12, even though decades higher would have been appropriate in
certain ranges. It shows that was about the practical limit for somewhat
decent precision and cost. Filling in the desired higher ranges had to
be done by
Oops - forgot to mention a detail about microwave oven caps. Sometimes
they have built-in bleeder resistors, which would of course spoil this
kind of application. Ed
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Yup - Keithley 640 - that must be the one. This stirred my memory
somewhat, and I just located info on the model 642 also, which was
apparently newer. The 642 went to (or back to) ultra-low bias MOSFETs,
while keeping sapphire insulation and a separate input head. The MOSFETs
need all kinds of
For static bias, look up "electret" for ideas on some other possible
options.
I would recommend against your option 2 capacitor - that's a dangerous
amount of energy to store in something that may be fooled around with
experimentally. Also, even though it's a lot of C, being electrolytic,
Oops - I think I didn't send this message properly yesterday - here goes
again. Ed
Yes, David, unless you go to very extreme measures, you won't see real R
values that have any practical meaning beyond E12 ohms or so. Most
practical insulation Rs may be around E12-E14 tops, unless you go to
Coincidental to all this micro-ohms and nanovolt talk, I've been doing
some severe large scale garage cleaning to thin stuff out. I found that
audio amplifier that I mentioned earlier, that is good for some LIA
reference driver applications. I also found my low-level measurement
notebooks,
Hmm. Alternating the direction of the current repeatedly and processing
the results - sure seems like that is fundamentally an AC measurement
too, despite using DC measurement equipment.
Ed
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I just noticed this discussion recently, so I'm late to the party, but
that never stops me from adding my one-cent's worth.
David, regardless of the aluminum and other material issues, I think
your initial idea of using a lock-in analyzer is definitely the way to
go. I'm very fond of LIAs,
I would recommend against trying to use cadmium - it's very toxic, which
is why Cd-based solders are rare nowadays. They are probably still made,
but for lab or industrial use with proper handling. If you try to alloy
it with Sn yourself without proper handling, you could get poisoned. You
Haha, so it is legit - just a poorly decribed knock-off of the H-P unit.
I had never heard of this unit, but it looks like good info to have, to
replicate some equivalent reference inductors. Thanks for finding this
document.
Ed
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Sorry about the previous double-posting - had email problems.
There is another option that occurred to me last night, to get an
equivalent inductor from an accurate reference capacitor by using an
active circuit gyrator. The problem of course is that the circuit will
add errors too,
Yes, the input DC bias current can be compensated for, as long as it
doesn't change too quickly, or isn't too big to make the input amplifier
circuits run out of dynamic range. In a digital meter environment, all
sorts of auto-zeroing and multi-sloping things and digital signal
processing can
With modern digital readout meters, this can be very misleading in terms
of actual useable capability. In this case, using the specified highest
sensitivity 100 pA range, and six digits of digital resolution, gives
E-10 A/E6 or E-16 A, which is 100 aA for the last digit. But, looking at
the
You should include the effects of input bias current - the maximum
should be specified, and likely in the pA range at room temperature.
Just put a very high resistance from input to common, and read the
voltage to calculate the current at zero input. Likewise, you can
connect the resistor to
Yes, you are being a little too OCD about this. Instrument washing
issues come up often, and there are plenty of opinions available -
here are some of mine:
In the 419s that I have, the battery leakage crud has not gone beyond
the circuit boards or maybe the edge connectors. Cleaning the
I agree these caps are the most failure-prone parts in the 3456A, and
also the 3455A. Sometimes they also cause damage to other parts like
the associated three-terminal regulators. Don't be surprised if
there's still more to fix after cap replacement - but usually easy to find.
I had one
I agree with John on this one - put in another meter socket
downstream if you really want to experiment, and be sure to have a
main disconnect besides the utility's meter.
If you are changing the service entrance, you will likely have issues
with the local permits for it, and with the utility
Only specialized meters can provide virtually infinite input R at
voltages above the 10 to 20 V or so native range of conventional
amplifiers, so you have to use some kind of attenuator to cover the
higher ranges anyway. 10 megs and 1 meg (and sometimes 11) are the
traditional values used,
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Sometimes the old stuff is still the best. At room temperature, a
mechanical chopper combined with a high-ratio step up transformer and
high impedance follower amplifier is unbeatable for low noise, low
impedance signal amplification. The solid state devices that replaced
mechanical choppers
Andreas,
I think your expectations are not realistic - even if you could make
such a reference, you could not transport its voltage to the ADC without
thermoelectric effects causing error that would swamp the performance.
To keep everything below the 1 ppm/deg C range you would have to put
Yes, very impressive - Solartron should hire you as a consultant to
design or improve their current products.
Ed
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I just junked out a very beat up old Fluke 803 differential
voltmeter, and found deep within, an old-school Cd/Hg standard cell.
It was well protected in an aluminum box, and wrapped in foam and
foil. It looks brand-new, and still measures around 1.018... V. I'd
like to keep this one as
If you do modify the circuit, you will probably want to decrease the
series R. Even though the external loads are likely to be in the
megohm range, the feedback load is significant, and likely the
biggest part, except for fault conditions. For lowest noise and
suppression of effects due to
The simplest way to drop the output impedance without adding much
circuitry is to just change the series R to 100 ohms or so - that
would still give pretty good isolation from capacitive loading.
If the R is dropped to zero, the DC performance will be best, but
you'll have to worry about the
I'm not sure how much elaboration is needed, but here's some:
If you take all of the feedback from the output terminal, that's
better for DC accuracy by eliminating the voltage drop of the series
resistor, while still providing some overload protection to the
opamp. But, it also decreases
Yes, the effect can be estimated quite easily. Also, keep in mind
that the 0.13 nV is the RMS noise, so the peak to peak excursions can
be around six times that, or almost 1 nV p-p. If the FS is 1 mA, then
it's about 1 ppm - one count on a six digit DVM, or ten times more
with each additional
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