DAC39J84: Sync Toggling

Hi Anirudh,

Not sure why SYNC toggles. We have not seen it do that before.

Is it toggling at the sysref frequency by chance?

Do you have some bit unmasked and each sysref pulse is asserting CGS restart?

Regards,

Rob

Hi Rob,

The sync toggling issue has been resolved when I changed the register 0x49 to 0x0000 which is the JESD link configuration setting. 

Now to check the DAC output, I am configuring the NCO to generate the 280 MHz signal which is given as one of the inputs to the mixer and giving a constant signal like 7F7F7F7F as another input to the mixer. The corresponding registers to enable the mixer, NCO and Frequency control word are written. The sampling rate, clocks and other parameters are same as mentioned above i.e. they are:

     sampling rate = 500 MHz,

      linerate = 5 GSPS,

      DACCLK = 500 MHz,

     JESD Ref clk = 125 MHz,

     SYSREF = 1.953125 MHz,

     LMFS = 8411,

     K = 32

I can observe the output as 220 MHz due to aliasing. I can also observe the image frequencies.

When I try to enable the 2x interpolation, I can see the output as 140 MHz. I am enabling the 2x interpolation by writing the 0x00 register with data 0x011C. Generally when 2x interpolation is enabled, the sampling rate will be doubled and I need to get the output as 280 MHz. But this is not the case with my output.

Am I missing any register or will there be any change in the DAC input rate and DACCLK when interpolation is enabled such that I can get the desired output.

Regards 

P. Anirudh Reddy

Hey Anirudh, 

Are you also doubling the clock rate of the DAC when you change the interpolation to 2x? Interpolation is what determines the rate between the input data and the output data. So simply increasing the interpolation from 1x to 2x will just cause the input rate to be halved. Typically changing the interpolation also requires adjusting the SERDES settings as the input rate changes with interpolation. 

Regards, 

Matt 

Hi Matthew,

After doubling my DACCLK (input data rate) to 1000 MHz, I am able to get the correct output of 280 MHz. 

Now I am trying to feed the DAC with IQ samples. The following parameters are set to implement this configuration :

               LMFS         =    8212

Sampling Frequency =  500 MHz

                DACCLK   =    1000 MHz

               SYSREF    =     1.953125 MHZ

        JESD Ref_Clk  =  125 MHz

                linerate     =    5 GSPS

           Interpolation  =    2x

I am generating the IQ samples for input frequency 250 MHz using Matlab by setting the sampling frequency to 1000 MHz (as interpolation is enabled).
I am writing the following set of registers to configure the DAC:

     Addr     Data
	 0x00    0x011C
	 0x01    0x0800
	 0x02    0x2082
	 0x03    0xF380
	 0x04    0x00FF
	 0x05    0xFF0D
	 0x06    0xFFFF
	 0x07    0x0000
	 0x08    0x0000
	 0x09    0x0000
	 0x0A    0x0000
	 0x0B    0x0000
	 0x0C    0x0400
	 0x0D    0x0400
	 0x0E    0x0400
	 0x0F    0x0400
	 0x10    0x0000
	 0x11    0x0000
	 0x12    0x0000
	 0x13    0x0000
	 0x14    0x0000
	 0x15    0x0000
	 0x16    0x0000
	 0x17    0x0000
	 0x18    0x0000
	 0x19    0x0000
	 0x1A    0x0020
	 0x1B    0x0000
	 0x1E    0x1111
	 0x1F    0x99C0
	 0x20    0x0800
	 0x22    0x1B1B
	 0x23    0x01FF
	 0x24    0x0020
	 0x25    0x4000
	 0x26    0x0000
	 0x2D    0x0001
	 0x2E    0xFFFF
	 0x2F    0x0004
	 0x30    0x0000
	 0x31    0x7030
	 0x32    0x0700
	 0x33    0x7FC8
	 0x34    0x0000
	 0x3B    0x1800
	 0x3C    0x9028
	 0x3D    0x0088
	 0x3E    0x0108
	 0x3F    0x0000
	 0x46    0x0044
	 0x47    0x190A
	 0x48    0x31C3
	 0x49    0x0000
	 0x4A    0xFF1E
	 0x4B    0x1F00
	 0x4C    0x1F07
	 0x4D    0x0300
	 0x4E    0x0F6F
	 0x4F    0x1CC1
	 0x50    0x0000
	 0x51    0x00DF
	 0x52    0x00DF
	 0x53    0x0000
	 0x54    0x00FF
	 0x55    0x00FF
	 0x56    0x0000
	 0x57    0x00FF
	 0x58    0x00FF
	 0x59    0x0000
	 0x5A    0x00FF
	 0x5B    0x00FF
	 0x5C    0x1155
	 0x5E    0x0000
	 0x5F    0x0123
	 0x60    0x4567
	 0x61    0x0003
	 0x64    0x0000
	 0x65    0x0000
	 0x66    0x0000
	 0x67    0x0000
	 0x68    0x0000
	 0x69    0x0000
	 0x6A    0x0000
	 0x6B    0x0000
	 0x6C    0x0000
	 0x6D    0x0000
	 0x6E    0x6A70
	 0x6F    0x0325
	 0x70    0x076F
	 0x71    0x01D6
	 0x72    0x016E
	 0x73    0x5974
	 0x74    0xD6D8
	 0x75    0x7000
	 0x76    0x6A70
	 0x77    0x0325
	 0x78    0x076F
	 0x79    0x01D6
	 0x7A    0x016E
	 0x7B    0x5974
	 0x7C    0xD6D8
     0x7D    0x7000

I am feeding only IQ data to the DAC and the mixer and NCO are disabled. When I observed the output in the spectrum, I cannot see any peak.

Later I have enabled the mixer and written the NCO frequency word to generate 0.1 MHz frequency. So one of the inputs to the mixer will be the IQ data
and the other input will be the 0.1 MHz. Now I can observe the peak at 250 MHz in the spectrum.

Can't I get the single tone output without enabling the NCO by giving only the IQ data or do I need to configure any other register for that ?