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Low noise opamps without input diodes?

Discussion in 'Electronic Design' started by Joerg, Mar 20, 2007.

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  1. Joerg

    Joerg Guest

    The OPA series seems hard to buy these days and I've heard rumors of
    some difficulties. So, I am looking for non-OPA varieties:

    <5nV/rtHz would be nice. +/-15V supplies. No back-to-back diodes on the
    inputs like the AD8674 has, need to use some of them as comparators and
    trackers. Can't easily buffer and they must track some high-Z nodes. I
    don't need the full VEE to VDD swing but at least 5V differential or so.
     
  2. Ban

    Ban Guest

    Yes, there are some...
    shit, I remember you were boycotting National and Phillips because of their
    fancy websites, they are cheaper, but what to do! Maxim and LT also -cazzo,
    there they go
    then maybe AD? ADA4004, OP467
    Otherwise just thow together some discretes, because those mentioned are
    above 80c.
     
  3. Joerg

    Joerg Guest


    I had checked National (via Digikey because this time they really goofed
    their web site...), Philips and LT. Didn't find much there. Maxim is the
    only one I didn't check because clients had too many "can't deliver for
    first production run" events.


    Thanks! I had looked at the ADA4004 before but it was out of stock. Now
    Digikey has 20 in SO, not enough but AD says they'll have more this
    month. The OP467 is 6nV/rtHz but that might be ok here, have to run the
    numbers one more time. Interestingly, AD had not suggested either as an
    alternative. Eight bucks street price, ouch. Oh well, in this case it
    might work.


    Believe it or not but this time it's all about performance. Well,
    almost. But you are right, these kinds of chips are usually outside my
    league from a cost point of view.
     
  4. JFET amplifiers may be your best choice. LTC's LT1792 has only
    4.2nV/rt-Hz typical and allows a wide differential swing, AFAICT.
    You didn't mention bandwidth requirements. BTW, you can make a
    composite two-opamp opamp to get the bandwidth you might need,
    and the input performance you need. OTOH, if you want superior
    performance and rock-bottom prices, remember, somebody has to
    pay the bills for the development of today's high-performance ICs.
     
  5. Phil Allison

    Phil Allison Guest

    "Joerg"


    ** The Natsemi LM833 is dual, low noise ( 4.5 nV), no diodes.

    Bout 40 cents each.





    ........ Phil
     
  6. colin

    colin Guest

    That leaves out a lot of choices, next best thing to OPA from TI at least
    seems to be TLC070 etc
    I was going to suggest TLV274 wich I just used but looked up vn is 39nv
    the other chip I use is opa2354,
    I too needed to use them as comparators so no input diodes.

    Colin =^.^=
     
  7. Joerg

    Joerg Guest

    Thanks, Win. It's a great amp. Unfortunately it seems to come only in
    singles and then the roughly $2.50 a pop begin to hurt. Ban suggested
    the OP467 which is a bit higher with its 6nv/rtHz but still not bad at all.

    Bandwidth is low, a few kHz and mostly used as integrators, limiters etc.

    This one doesn't have to be rock-bottom in cost, just reasonable. Of
    course I was quietly hoping there would be a supercharged TL084
    somewhere for just a few pennies more :)
     
  8. ["Followup-To:" header set to sci.electronics.design.]
    Whoa! Now that's a great amp. Hadn't heard of it before. What makes me
    cringe though is that +/-15V supplies are now marketed as some kind of
    specialty "high voltage" feature. It used to be standard for all analog
    stuff (and as far as I'm concerned it still is).

    robert
     
  9. Joerg,
    together with the noise you have to specify the
    bandwidth. Most very low noise opamps have
    extensive bandwidths, actually all opamps I
    remember with GBWs from a few 100MHz are
    specified as very low noise with figures in the
    range of 3nV/rtHz and below

    Rene
     
  10. Joerg

    Joerg Guest

    That makes me cringe as well. For lots of dynamic range it's hard to
    live with 5V or so. What good does a really low-noise performance do
    when the top of the dynamic range is chopped off?
     
  11. Joerg

    Joerg Guest

    Neat! Thanks. Comes only in duals but heck, can't beat that price. And
    it is 2nd sourced, found it also at ST and ON-Semi. This one ought to be
    really popular with high end audio guys yet for some reason I have never
    heard them talk about it.
     
  12. Joerg

    Joerg Guest

    It's below audio in this case. High GBW would not be a problem but the
    few that are in the +/-15V range such as some THS devices cannot
    tolerate much differential input.
     
  13. Joerg

    Joerg Guest

    I am going to check out Phil's suggestion, the LM833. Best of all, that
    one is multi-sourced.
     
  14. ["Followup-To:" header set to sci.electronics.design.]
    It's just too cheap.

    robert
     
  15. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :
    Plus those high GBW opamps generally are low noise at high frequency and
    have a pretty high 1/F corner frequency. Even bipolar opamps.
     
  16. Eeyore

    Eeyore Guest

    They don't make designers any more like they used to in the old days !

    Graham
     
  17. Eeyore

    Eeyore Guest

    I thought its input current might be too high. Maybe not ?

    Good point. I came across it quite independently by chance when clicking on the
    Audio Op-amp link at national.com. I didn't know it was that decent. I'm curious
    that National themselves haven't promoted it more before. Of course it's been
    overtaken by more exotic stuff since. The NJM4580 for example is also
    inexpensive.

    Graham
     
  18. Joerg

    Joerg Guest

    Yeah, 1uA is a bit highish and so is the offset. Then again, at that
    price I am not complaining. We'll have four in a row and the first two
    can be calibrated out.
    Digikey calls one of the NJM4580 versions "noiseless". Now that's a bit
    brazen but the main problem is the stock situation. Zilch.

    It's also quite high in input current, at 0.5uA. Says differential
    +/-30V and then (note). But I couldn't see any note other than the usual
    disclaimer.

    BTW, why did they call themselves "New" Japan Radio Corporation? I
    already liked the old one, got some of their gear here in the lab.
     
  19. Werty

    Werty Guest

    2 kinds of Op Amps ,

    Mosfet and

    Bipolar .

    Bipolar has almost no change in input current.
    Thus the voltage drop across the input R's\
    is no problem . Accurate .

    MOSFETs have a extreme TEMPCO , thus
    their input Z is meaningless . It may as well
    be 50K ohm , to swamp out the extreme
    input current changes .
    They are 4 times higher B.W. .

    Noise is very low in Bipolar OpAmps .
    But low noise is also very low B.W.

    Have you noticed the old Bipolar Op Amps
    are still very useful ? MOSFETs did not obsolete
    Bipolars ..
     
  20. Joerg

    Joerg Guest

    Tell me about it. I am still in disbelief about the percentage of fresh
    grads who can't even understand the function of a circuit from a schematic.
     
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