The LMP2012 is a ch ================== "The 6001 is just guaranteed to be less than 20fA due to a special test. They are no longer hand-tested (that was the old metal can version), but they are still screened with the long-term test. That is why they are so expensive - long test time and "value added".
"N" suffixes are the old DIP's - which we do not make anymore, except for specific parts where there is still huge DIP volume. "M" suffixes are the SOIC package. The "A" and "B" suffixes are the offset grade (A=better). The "I" means industrial temp range.
Otherwise, the LMC660/662 is still the king of the cheap, low Ibias CMOS op amps. Use the second channel of the 662, mid VCM, and you will be in the low fA's. The older devices with the 500V HBM ESD rating have the lowest leakage.
The LMP2011 is a CMOS device, but the "chopper" CMOS input switches cause charge transient spikes on the inputs, which will disturb a very high-impedance circuit. This can happen with any chopper and is not specific to the 2011. ======================
There you have it from the source, and Paul noted there is link in the upper right of National's page to go back to HTML tables.
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 ***: ========================== > but I want to respond about the op-amp thread as well. Is it correct to say: >
**** 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. > Maybe the LMC6484 but the real deal is that the LMC6001 is just a hand-selected LMC660.
*** 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. > "Any LMC part would be OK as well as JFET parts like the LF411. Input bias doubles with every 10 deg C in temp so keep it cool."
*** 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 =========================== I hope Bob's comments above are helpful.
Thanks very mich to Paul Rako for his two helpful postings about LMC6001 and other amplifiers. I'll be better able to look at the National Semiconductor website when I get to a library computer. The actual signals will be of the order of nanoamps but in some of the experiments it will be desirable to measure them very accurately.
Basically, I'm reading the descriptions of how to do the Franck-Hertz experiment in Melissinos' book and trying to figure out how to do it without "simply" purchasing the whole Franck-Hertz kit from Leybold or its representative for more money than I can possibly afford. I want to purchase a gas filled tetrode from a contemporary tube vendor and use it as described in the experiment. Since the current through the tube will be in picoamps or tens of picoamps, that's what has to be measured. For measuring multiple excitations, it doesn't have to be extremely accurate, but for measuring single excitations it is desirable to have very accurate measurements to be able to detect excitations to other low lying excited states. The various op amps mentioned are probably good enough, but I'll have to study them carefully. The more thought I invest in it, the less money I'm likely to waste on the wrong parts and I don't have much to waste.
--
Ignorantly,
Allan Adler
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston.
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