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OPA211 Noise

David Dorfan
Prodigy 30 points

Other Parts Discussed in Thread: OPA211

I have been trying an OP211 as an input buffer and find it to be far noisier than advertised.  I have tried 4 or 5 and I get a voltage noise of about 2.3nv/roothz instead of the 1.1nv  claimed.  I do not think it is my setup as using other low noise op amps gives their advertised values.   I am running broadband so the1/f noise is irrelevant.  Has anyone else experienced similar problems with the 211?  It is not an oscillation as it goes down as the inverse of the square root of the number of traces I average, just the way noise should.

  • 0
    Guru 21975 points

    David,

    Can you please provide a schematic of your measurement circuit and method?

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 30 points

    Hi Bruce,

    We used the setup below.  The 50 Ohm in the feedback was just there for another reason but it could not have caused all the noise we saw and the A/D amps were all fine.

    dave.

  • 0 in reply to David Dorfan
    Guru 21975 points

    Dave,

    I'm not familiar with the Anadyne amplifier you are using. I'm curious how you are "subtracting" the noise of this amplifier as it is a significant contributor in the measurement. What is the logarithmic function of this amplifier?

    The phase compensation in the OPA211 causes a rise in the noise density in the 1MHz to 100MHz range. This will be a contributor in your 20MHz measurement. It peaks to around 3.8nV/rt-Hz at 20MHz. This may be what you are seeing. The folks who characterized the device and wrote the data sheet had a noise measurement system that went to 100kHz and did not see this effect. Furthermore, most applications for the device operate in substantial gain, limiting the banswidth so this effect is not seen.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 30 points

    Bruce,

    Thanks a lot, that explains it.  It never occurred to me to actually look at the range measured in the spec graphs. Our signal source actually includes a larger term than the 211  so we can still probably use it.  I just was not comfortable using it when the noise was so far off spec.  The Anadyne log amp is an L-17D, which is DC coupled and has a dynamic range of about 92dbv with a rise time around 6 ns.  When the inputs are small the amplifier is essentially linear with a gain that can be adjusted, but is usually around 4000, for the smallest inputs that are logged.  3nv/rthz is huge compared to 1.1.  With so much gain it is not difficult for a scope RMS to read accurately. Even by eye though it was patently obvious that we were not adding two noise voltages around 1nv in quadrature which is why we made more careful measurements.  Anyhow, thanks a lot, things make perfect sense.

    Cheers,

    Dave