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RE: measuring volts


Ludwik and others:

I think you're seeing two different behaviors caused by the same basic
design parameters, once in the ULI and once in the CBL.

I have more details at hand on the CBL, so I'll discuss that device. You
said

What is this 2.00 V? Can somebody confirm these observations? By the way
the electrolytic capacitor (whose leakage resistance is certainly much
smaller than for C=2 nF) behaves "normally"; I can see how it discharges
(mostly through its own R) after being disconnected from the battery.

If you look at the CBL system guidebook (a task all in itself, I'll grant)
you find that the input impedance of the high volt range (+/- 10 V) is about
780K when the input is above 2V. That is, the input is not very high
impedance. But things get worse below 2V due to a floating reference
voltage. An open-circuited CBL will read 2V with nothing attached. I've
found in practice that if you want to get a really good RC decay curve with
a CBL, you need to have no more than about 50Kohms across the input. 10K is
better. Otherwise, the rather low effective input impedance of the CBL will
swamp the R of your RC circuit when the potential is below 2V.

All this comes from the fact that the input of the CBL (and the ULI) was
really optimized for use with probes. If you use a direct connection to an
experiment, you must be careful.

The ULI has a similar sort of variable effective input resistance, and an
open input (no connection) will be reported as a noisy value between 1 and 2
V typically. Never trust the input of any A/D when the input device has an
extremely high impedance. Many apparently quiet converters actually have
small resistors across the input to pull the input to ground when the input
would otherwise float.

The discussion of graphing calculators has been interesting. I've not chimed
in as yet because I've been travelling without access to my email, and just
returned to find the thread. I disagree with many of the things stated, and
will post a proper response in a couple of days.

JEG