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Old 31st December 2010, 07:09 PM   #1
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Default Lowest voltage noise dual JFET op-amp

Hi,

The lowest noise dual JFETop-amp that I can find is the OPA1642 with 5.1 nV/root-Hz of voltage noise. I need the low current noise too (it's 0.6 fA/root-Hz for the aforementioned op amp).

Does anyone know of a lower voltage noise dual JFET op amp? I know there are lower noise dual op amps, but all that I've seen have much higher current noise than my application can tolerate.

Oh, I'd like one that runs with +/- 15 volt supplies too.

Other spec I need is at least 10 MHz GBP.


Regards,

Ken
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Old 31st December 2010, 09:12 PM   #2
jcx is online now jcx  United States
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where does the noise need to be low - A weighting?, what source/feedback Z?

The lowest noise jfet input is the single AD743/5

if lowest possible 1/f corner frequency isn't important then some CMOS parts may be useful

plot from thread new interesting CMOS op-amps
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Old 31st December 2010, 10:25 PM   #3
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Thanks,

The AD743 has too high input C for my application, otherwise the 2.9 nV/root-Hz is attractive. The high C increases the noise gain of the first stage.

My application isn't audio, it's a transimpedance amplifier that's used to amplify a photo diode. I need ultra-low noise for this application. I attached a schematic to this reply. It has a unique topology in that I use two capacitors to multiply the photodiode AC current by a factor of 30, noise-free. I get a total effective transimpedance of 41 MOhm, but only get noise that's equivalent to a 1.34 MOhm resistor.

Gain peaking occurs around 1 kHz and the Vn starts to become a problem because the noise gain of the op-amps starts to get large. Current noise needs to be low too, so a JFET (or CMOS) op-amp is the only choice.

Ken
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Old 31st December 2010, 10:37 PM   #4
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Hi,

I looked at a few CMOS op-amps from the thread you sent me, however they operate from low supply voltages. I need a dual that operates from +/-15V.

Also, my circuit's bandwidth is 100 Hz to 10 KHz.

Regards,

Ken
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Old 31st December 2010, 11:24 PM   #5
jcx is online now jcx  United States
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you might seach photodiode transimpedance bootstrapping

I remember seeing descrete jfet Q in front of op amp photodiode transimpedance amps

I need to spend some time to see if this paper's bjt cascode/bootstrap is a good idea http://www.electrooptical.net/www/fr.../frontends.pdf

also you shouldn't let Vsupply prevent you from using the best op amps for the applicaiton - higher speed processes are lower V, you can make a compostie op amp circuit with a 2nd hgier V output chip if you need the Vout

Last edited by jcx; 31st December 2010 at 11:32 PM.
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Old 31st December 2010, 11:30 PM   #6
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JFET bootstrap is a good idea, I thought of that too, may have to go that way if I can't find a dual with Vn < 3 nV/root-Hz. Real estate is kind of a problem since the PCB is has to be about the size of a two pieces of Dentyne gum.
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Old 1st January 2011, 06:35 AM   #7
Previously: Kuei Yang Wang
 
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Hi,

Quote:
Originally Posted by kaquino215 View Post
Hi,

The lowest noise dual JFETop-amp that I can find is the OPA1642 with 5.1 nV/root-Hz of voltage noise. I need the low current noise too (it's 0.6 fA/root-Hz for the aforementioned op amp).

Does anyone know of a lower voltage noise dual JFET op amp?
Sounds like you need to use a NE5532 with a pair of discrete dual J-Fet's type 2SK389 (Toshiba) or LSK389 (Linear Systems) and make a hybrid.

This should manage < 1nV |/Hz J-fet Dual. Only needs the dual J-Fet and 3 SMD resistors added per op-amp, survivable even in a confined space, SMD versions of Op-Amp and J-Fets are available.

Ciao T

Last edited by ThorstenL; 1st January 2011 at 07:01 AM.
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Old 1st January 2011, 11:53 AM   #8
AndrewT is offline AndrewT  Scotland
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could one assemble a pair of lsk389 and a pair of 5534, all smd size, onto a converter board that fits an 8 pin dil - dual opamp socket?

The 389 would feed into pins 5&8 of the 5534, thus removing the BJT input pair. Check which pins go to the BJT collectors !
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regards Andrew T.
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Old 2nd January 2011, 12:54 PM   #9
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Thanks Andrew,
Collectors of the NE5534 come out to pins 1 and 8.

Thanks everyone for the suggestions.

Ken
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