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DAC8734: DAC8734 noise is too good

Justin Scott
Intellectual 545 points
Part Number: DAC8734

I have an DAC8734 set for a gain of 2 with output +/-15V. I have it set to 0V out (midscale). The DAC output is connected to a low-noise preamplifier that has a 100KHz low pass filter. When I measure the output of this preamp, I get 12 uVrms.

Is this expected? The datasheet spec is 40 uVrms in this band with a gain of 4, +/-10V out.

  • 0
    TI__Guru 52663 points

    Hi Justin,

    The 40µVrms is likely somewhat conservative, as well as a typical value.  If you were in a gain of 4, you would probably see ~24µVrms.  It is possible that our spec value included some noise from the reference we used to test the system. If you integrate the noise density (45nV/rtHz) over 100kHz bandwidth, you get about 14µVrms, which is about what you are seeing.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Intellectual 545 points

    It's true that if you integrate the noise density over 100KHz you get 14 uVrms, but that ignores 1/f noise. So I would expect that with a gain of 2, the noise would still be much higher than 12 uVrms for a 0.1 to 100 KHz span. But if you think the spec in the datasheet includes some noise from the reference, as well as some extra to account for typical variation, then I understand.

  • 0 in reply to Justin Scott
    TI__Guru 52663 points

    It likely does.  We do show 0.1 to 10Hz noise on the typical characteristics curve, which is maybe 4uVpp.

    We specify the 1/f curve as about 500Hz.  So it is at least bandwidth limited to that point (before broadband noise dominates).