AD9779A close in noise

Hi, All!

I have measured the output spectrum of the AD9779A running at 155.52MHz (the internal PLL is DISABLED). The DAC is clocked from the clean source (ABLNO-V-155.52 crystal oscillator). I see a very strange noise shape close in to the carrier (see at +-200kHz region). Usually such noise shape is expected when PLL is used or power supply is noisy. But there is no PLL and the CVDD18 and AVDD33 power domains are supplied from the clean LT3042 regulators.

So, the question is it normal for this DAC? What is (or can be) the nature of the noise close in to the carrier?

See the FFT with the calculated noise values for different offsets (NOTE: the ADC used to capture AD9779A output has sufficient SNR to measure noise lower then -150dBc/Hz at 20kHz offset, so the DAC noise prevail close in to the carrier, the spectrum shown combined PM/AM noise)

Thanks!
Oleg

Top Replies

    •  Analog Employees 
    Jun 21, 2021 in reply to Alexey_V +1 suggested

    Hello   , Hello ,

     

    My sincere apologies for the delayed response.

    Unfortunately, the close in noise for this Legacy DAC was not characterized.

    May I know what are your target application for…

Parents
  • Hi, All!

    I have a custom board with an AD9779A. The DAC is clocked from a VCXO 80MHz AT-cut level 3-rd overtone with PLL frequency multiplication inside the DAC and a final frequency of 320MHz. For analog power, I used an LDO LT1963, which has noise below 10nV/sqrt (Hz) from 50kHz and above. But the problem with analog noise in the 10-100 kHz range also occurs.

    Best regards,

    Alexey.

Reply
  • Hi, All!

    I have a custom board with an AD9779A. The DAC is clocked from a VCXO 80MHz AT-cut level 3-rd overtone with PLL frequency multiplication inside the DAC and a final frequency of 320MHz. For analog power, I used an LDO LT1963, which has noise below 10nV/sqrt (Hz) from 50kHz and above. But the problem with analog noise in the 10-100 kHz range also occurs.

    Best regards,

    Alexey.

Children