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Paul Rako
Allan said:Perhaps. Being a mathematician, I gravitate to the ordering LCM
(least common multiple). The national semiconductor site hangs
my browser. I tried it in lynx and got some information. They are
talking in terms of selling them in lots of 1000, with a $9.46
rail charge. Jameco doesn't turn up the part. National Semiconductor
describes the part as obsolete. What is used now?
I would probably only need to measure nanoamps, since Melissinos' book
says that the Franck-Hertz tube would produce nanoamps under the
conditions of the experiment. On the other hand, one wants a good
picture of the dips, so I feel better about picoamps.
When I asked Paul Grohe about this in my previous pose I also
copied Bob Pease. He responded with some important clarifications
to my statements. My comments are preceded by a >, Bob uses ***:
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measuring picoamperes.but I want to respond about the op-amp thread as well. Is it correct to say:
"The LMP2012 is a chopper amp and would be unsuitable for
**** Actually, the LMP2011/ LMP2012 are reasonably good
for measuring moderate numbers or DOZENS of picoamps,
but like most Chopper-Stabilized amplifiers, it's a little
noisy, especially for frequencies above 10 kHz. It has
superior VOLTAGE offset errors, but so-so current offsets
and noises. At a 1 nA full-scale ( i.e. with Rf = 100
or 1000 megohms) it's a little noisy. The CMOS dc
(non-chopper) amplifiers are MUCH quieter at that range,
and much less Ib and Ios.
is just a hand-selected LMC660.Maybe the LMC6484 but the real deal is that the LMC6001
*** Not exactly. The LMC6001 is a SINGLE Amplifier
so it is similar to 1/4 LMC6484 or 1/2 LMC6482 (dual).
But it's a different mask-set, for sure. Just similar.
Input bias doubles with every 10 deg C in temp so keep it cool.""Any LMC part would be OK as well as JFET parts like the LF411.
*** LF411's can be typically as good as 50 pA, but even the
LF411A grade is only guaranteed better than 200 pA. Our
CMOS -input amplifiers are normally 1000 to 10,000 x less
input current than that. The reason is, their input protect
diodes are TINY junctions; the BiFETs have the reverse
leakage of the whole TUB, which is physically a much larger
diode. More than 100x bigger.
*** As with many other signals, the best accuracy depends
on the actual signal range of V and I, and the signal's
impedance, and the feedback resistor's value.
*** If you are trying to measure accurately a dc signal of
100 or 1000 pA, some of the amplifiers listed above might
be OK. But if you are trying to measure 1 pA, you want
one of the better CMOS amplifiers. The LMC6001 is guaranteed
less than 0.02 pA at room temp; the 1/2 x LMC662 is normally
4 x better, BECAUSE it has smaller input protect diodes.
(Which is why its ESD rating is only 800 volts, compared
to the more robust ratings of the LMC6001) . So the choice
of the "best" amplifier DEPENDS on how big your signal is,
in V, I and Z - as is true for most every amplifier application.
There is no one amplifier that is "best" for EVERYTHING.
After all, we are talking about a LARGE number of orders of
magnitude of I and V... On the other hand, if one of the
amplifiers is "good enough", and is cheap, save your money
and buy the cheap one.
*** Paul Grohe's comments on this are quite right, as he
and I did a LOT of measurements on the old CMOS amplifiers.
*** I hope this clarifies the question of "good, better,
best". / Best regards. / rap / Robert A. Pease / Staff Sientist, NSC
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I hope Bob's comments above are helpful.