On 4/14/22 3:00 PM, ed breya wrote:
It depends on context and your definition of low noise. I was picturing ones commensurate with the 1/f discussion, trying to get down toward DC. Many are spec'd for so many nV or uV p-p over 0.1 to 10 Hz. The OP-07 is a good example of this class.A lot of opamps are spec'd ok for their intended use, but you have to look fully at the charts and numbers for your actual use. For instance, the ADA4817 looks great as a HF/RF amp, with 4 nV/rt-Hz, but that's at 100 kHz. Similar for the OPA656. At 10 Hz, it looks like it's well over 100 nV/rt-Hz.The JFET types are fine for high-Z amplifiers and such, but can't reach the near-DC region in the few nV/rt-Hz (at 10 Hz, say).The CFAs may be OK too - they're built for low-Z, so may tend to be operating the + inputs at fairly high bias currents, like the low noise opamps.The good part overall, is that to get near the nV/rt-Hz range, the source resistance has to be very low anyway, so at least the bias currents are manageable, although zero would be nicer.
of course, if your low noise frequencies of interest start at 100kHz, then whether the 1/f crossover is at 100Hz or 10kHz is less important :)
My recreational complaining gripe is that there's a lot of interesting low noise parts intended for the microwave market, but they don't even bother giving data below 50 MHz. So it becomes a matter of tribal lore - and you have the risk of "we got a really good batch" or "we don't make those anymore".
Vishay Siliconix *used* to make JFETs (e.g. JANTXV2N4858) with good noise properties (3 nV/sqrt(Hz) at 1 kHz) - but they're not in the JFET business any more. They even put the noise on the datasheet, although with a "not guaranteed JEDEC spec", footnote. And they still sell them, maybe (It says "contact factory for JFET products" - factory stock, I suppose). Or you can scrounge around and find 4 or 8 from some dealer. And you can buy plenty of 2n4858s from other people, but *they* don't say anything about the noise. And how do you find parts like this? Word of mouth from someone who spent a bunch of time looking at data sheets, bought a bunch of different kinds, and tested them.
I suppose that's why this is a recreational complaint - I (or anyone) who needed low noise FETs basically has to suck it up and go through datasheets, but parts, and test them. There's no "easy fix"
JANTXV2N4858 Vishay 70244.pdf
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