Too man variations to chose among: EMI Absorbers: Electronic Solutions : 3M United States There are more exotic available and they are heavier than they look.
There are some other options including a roll material that may be much easier to use.
Paralleling opamps-
One of these http://www.ti.com.cn/cn/lit/ds/symlink/lme49724.pdf gives you 4 2.1 nV/rtHz opamps to parallel but that only gets you close to .7 nV? (too much for my head at this hour), but since they are all on the same substrate they may not need any balancing stuff. They are cheap as well ($3.30 USD at Digikey) That and correlation of several would get pretty deep into the noise for measurement.
Scott:
What do you mean by transformer feedback to get low noise? It sounds intriguing.
if he needs less than 7 ohms of source resistance, that gives you the upper bound of the gate resistor values you can use...
Whether for measurements or audio applications, in real world, there are additional headaches than 1/f , flicker, thermal ( R ) noise.
http://www.ti.com/lit/an/snoa497b/snoa497b.pdf
http://www.ti.com/lit/an/sboa128/sboa128.pdf
TI has acknowledged the issue and is releasing RF hardened OPAs. Has anyone worked with such ICs?
George
PS: In urban environments, RF pollution starts very low (hundreds of kHz).
http://www.ti.com/lit/an/snoa497b/snoa497b.pdf
http://www.ti.com/lit/an/sboa128/sboa128.pdf
TI has acknowledged the issue and is releasing RF hardened OPAs. Has anyone worked with such ICs?
George
PS: In urban environments, RF pollution starts very low (hundreds of kHz).
PMA answered this for you already Patrick. Noise from a 1k resistor resistor is very significant in the context of a low noise active device.
The SMD ferrites are great because you have very low DC resistance at LF (so up to a few hundred kHz) but above that Z rises quickly - there's plenty of options that will give you hundreds of Ohms in MHz region (they usually quote Z at 100 MHz IIRC).
If you are using LM4562/49710 opamps and similar on input stages, a ferrite followed by a suitable cap to ground gives a very effective RFI filter. A quick test is to place you mobile phone on top of your pre or power amp and if you hear the characteristic zzz . . zzz . . .zzz you have a problem. BTW, not many devices pass this test - its brutal.
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I understand the benefits of the ferrite, but aren't you worried about linearity or saturation ?
I know that the gate has no current, and it is very small signal, and probably noise is much more than lineairty in this case.
But still, I have a prejudice against ferrite I find difficult to overcome.
Maybe I should just try it one day to convince myself, especially SMD .....
Patrick
I know that the gate has no current, and it is very small signal, and probably noise is much more than lineairty in this case.
But still, I have a prejudice against ferrite I find difficult to overcome.
Maybe I should just try it one day to convince myself, especially SMD .....
Patrick
Courtesy of Dr. Hill of Horowitz and Hill, every detail of the design is revealed but I could never find the pot cores they use except in large quantities.
http://www.picovolt.com/win/elec/articles/Lepaisant_preamp-xfmrs_RSI.pdf
I discussed it with Ovidos when he was doing his preamp. One SMD inductor (.06 Ohms DC) for each 4 - BF862's and there were no problems. No sure of the exact ones he used. It's not just RFI, massively parallel devices can be underdamped and make (usually) something like an RF Colpits oscillator with the interconnects acting as L's. The lossy L on the gate kills the forward gain at MHZ. If you obsess 10 turns of #32 wire on a 1K might work too.
A customer had a 1024 channel EGC (JFET op-amps) board spontaneously oscillate which was immediately fixed by gate dampers.
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Thanks - Ovidiu's approach is the one I'm looking at - with a bit of Denis Colin and DS as well although I wont use an opamp to remove the offset between the front end ant the opamp stage - I'll use a good quality film cap for that.
Charlie Hansen of Ayre Audio recommends against ferrites use anywhere in a signal path
Use your own judgement, weight the scientific principles using your own intelligence. Don't let the gurus slip one by you.
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