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DAC39J84: qmc_gain changes not effective in DAC39J84

Part Number: DAC39J84

I have enabled qmc by settting qmc_corrab_en and qmc_corrcd_en.  I see a slight bump in output amplitude when I do this.  However changes to the qmc_gain registers (offsets 0xC-0xF) don't seem to have any effect on output amplitude, including setting all four qmc_gain registers to 0.

Are there any known issues with regard to qmc on this device?

  • Bill

    With the enables set (address 0x00 data 0x3018) for both, I can easily adjust the gain of the output using bits 10:0 of registers 0x0C, 0x0D, 0x0E and 0x0F.

    Set these to 0x00 to verify the output signal goes away, then set them to 0x400 for a gain of 1 then 0x7FF for max gain.

    There are no other register settings required.



  • Yeah that procedure works for me too on our 38J84 evaluation board.

    The 39J84 board in question is a COTS FMC board and FPGA carrier, and I'm using the board vendor's register setup/modification utility. In general register setups seem to work OK with that utility and everything else I've tried works, so I would be surprised if there is some bug that only affects those registers.  They read back as written.

    Just for the record, the output goes up 0.5 dB when I turn on qmc_corrab_en.  I haven't checked C/D channels, but A and B behave the same.

    A weird bug in the vendor's SPI/utility? Faulty DAC? Something else?

  • Hi Bill,

    One thing you can try is setting the QMC phase register after setting the QMC gain register. The DAC may expect a value for it because enabling QMC correction is for both the gain correction and phase correction. If you do not need any phase correction you can write 0x0 to the QMC phase correction registers. 



  • The register was already set to 0, and resetting to 0 makes no difference.

  • Actually the more I experiment I do see a requirement to re-write the phase register after adjusting the gain registers. The gain control takes effect once the phase register is written.  I'm not sure why my first attempt didn't show that.

    Try to get the details added to the datasheet, please, but thanks!