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DAC7811: Output noise spectrum

Katsuhiko Kaizawa
Intellectual 2045 points
Part Number: DAC7811


Hello support team,

Would it be possible to share below measurment data and condition for DAC7811?

 1) Output noise spectrum. I would like to know the noise spectral density at 1 kHz.

 2) What was Op Amp device used for the output noise evaluation?

I would appreciate your support.

Best regards,

Kaizawa

  • 0
    TI__Genius 14220 points
    Hi Kaizawa,

    Good to hear from you again. I am checking the details with the design team. Will get back as I get a response.

    Regards,
    Uttam Sahu
    Applications Engineer, Precision DACs
  • 0 in reply to Uttam Sahu
    TI__Genius 14220 points
    Hi Kaizawa,

    Please find below the response from the design team. I am putting here as is:

    For this question it is good to understand that the noise spec on our pds is just from the resistor network of the device. Essentially the network from a noise standpoint looks like 2 independent 10K resistors. Thus the noise is the rms sum of the two 10K resistor noises. One 10K will have a flat noise spectrum of 12.6nV/rt Hz. The rms sum will be sqrt(2) * 12.6nV/rt Hz = 17.9 nV/rt Hz, same as on our pds.

    Now considering the output op amp… For whatever op amp the customer wishes to use, find the 1KHz noise value for it. In the MDAC configuration for the op amp, the maximum noise gain for the op amp will be about 2. So take the op amp noise spec and multiply it by 2. Then from that value rms it with the 18nV/rt Hz noise of the DAC7811 resistor network. For example, lets say the op amp chosen has a noise of 5nV/rt Hz @ 1KHz. The gained up noise would be 10nV/rt Hz. The rms noise with the DAC7811 resistors would then be calculated as sqrt (18^2 + 10^2) = 20.6nV/rt Hz. Of course if the op amp input noise is higher it will dominate more.

    Regards,
    Uttam
  • 0 in reply to Uttam Sahu
    TI__Intellectual 2045 points
    Hi Uttam,

    Thanks for your answer. Let me see if I understand it correctly.
    The output noise voltage of DAC7811 is just determined by the internal 10Kohm resistors. Is the spec value of 18nV/rt Hz theoritical value or mesurement result?

    Best regards,
    Kaizawa
  • 0 in reply to Katsuhiko Kaizawa
    TI__Intellectual 2045 points
    Hi Uttam,

    I have one more question.
    Generally, I think noise gain of transimpedance amp is 1 v/v. But in case of the MDAC, it is configured as the noise gain controlled transimpedence amp with Rfb and R-2R resistor network. Consequently, the noise gain is 2 v/v. Is my understanding correct?

    Best regards,
    Kaizawa
  • 0 in reply to Katsuhiko Kaizawa
    TI__Genius 14220 points
    Hi Kaizawa,

    I will get back to you on the first question after checking with the design team.

    Regarding the second question, you are right. As the output opamp is configured as a transimpedence amplifier with the R-2R and RFB, the noise gain becomes 2.

    Regards,
    Uttam
  • 0 in reply to Uttam Sahu
    TI__Genius 14220 points
    Hi Kaizawa,

    The noise value on the datasheet is theoretical calculation since the MDAC network is essentially just 2 equivalent resistors. However, the team had confirmed that the noise follows the behavior by measurements.

    Regards,
    Uttam
  • 0 in reply to Uttam Sahu
    TI__Intellectual 2045 points
    Hi Uttam,

    Thank you. I understand.

    Best regards,
    Kaizawa