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Noisest Opamp?


Now that I've got your attention...no, no, this is real.

My company wants to sell into China without having to wait months for an export license, we can accomplish this by making the resolution of our systems worse, especially in the bandwidth below 100 Hz.  I know that FET amps tend to have the most noise down in that area, but what is the noisest one?  We have amps in the SOIC-8 package (duals).  I'm also going to sprinkle some carbon comp resistors in there to boost the noise.

I know that I can do a redesign to add a noise generator, but I'm trying to get around this without a redesign and re-layout.

Thanks...Steve

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  • Hi Steve,

    Ok looked through some old databooks, pretty much everything (including older FET designs) is designed to LM741 noise figures. Your source resistance tends to favour FETs anyway. Better off looking at amps designed for lower Rs.

    Two other choices

    (a) use a video amp , these are designed for Zin=100ohm , thay also have 1/f knees in 1kHz to 10kHz range , they can have really low voltage noise above 10kHz, BUT current noise is much higher.

    So take for example the OPA2674 (this only gets to +/-12V but there are more out there) it has an impressive voltage noise of 2nV/rtHz, but current noise of 100nA/rtHz at 100Hz ,

    so at 100R source resistance, at 100Hz its 12.8nv/rtHz

             ~3K source resistance will yield 300nV/rtHz at 100Hz

             ~30k source resistance yields 3000nV/rtHz

    So you can tailor the noise by changing the input/feedback resistors (and the caps).

    Also a possibility the video amp may oscillate (might fix this by lowering C26, C27 and swap R27 R28 for ferrite beads (Zo~ 2k)

    (b) Use a push-pull comparator (or differential line receivers) (with same pinout) these aren't intended as opamps, so would expect higher noise, you will need to test them first as they may oscillate, and they will have some crossover distortion, and possibly slower slewrate.

Reply
  • Hi Steve,

    Ok looked through some old databooks, pretty much everything (including older FET designs) is designed to LM741 noise figures. Your source resistance tends to favour FETs anyway. Better off looking at amps designed for lower Rs.

    Two other choices

    (a) use a video amp , these are designed for Zin=100ohm , thay also have 1/f knees in 1kHz to 10kHz range , they can have really low voltage noise above 10kHz, BUT current noise is much higher.

    So take for example the OPA2674 (this only gets to +/-12V but there are more out there) it has an impressive voltage noise of 2nV/rtHz, but current noise of 100nA/rtHz at 100Hz ,

    so at 100R source resistance, at 100Hz its 12.8nv/rtHz

             ~3K source resistance will yield 300nV/rtHz at 100Hz

             ~30k source resistance yields 3000nV/rtHz

    So you can tailor the noise by changing the input/feedback resistors (and the caps).

    Also a possibility the video amp may oscillate (might fix this by lowering C26, C27 and swap R27 R28 for ferrite beads (Zo~ 2k)

    (b) Use a push-pull comparator (or differential line receivers) (with same pinout) these aren't intended as opamps, so would expect higher noise, you will need to test them first as they may oscillate, and they will have some crossover distortion, and possibly slower slewrate.

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