DAC38J84: issue with self test

lakshmi sumala grandhe

Part Number: DAC38J84

Hi,

I tried setting DAC38J84 in self test mode and generating required NCO output.

DAC CLK=518.4MHz

reg_map(0) <= x"00_0000";
reg_map(1) <= x"2f_0001";
reg_map(2) <= x"01_0003";
reg_map(3) <= x"02_2050";
reg_map(4) <= x"12_0000";
reg_map(5) <= x"13_0000";
reg_map(6) <= x"14_8E39"; --NCO phase ab lower --72M
reg_map(7) <= x"15_38E3"; --NCO phase ab middle
reg_map(8) <= x"16_238E";
reg_map(9) <= x"17_0000";
reg_map(10) <= x"18_0000";
reg_map(11) <= x"19_0000";
reg_map(12) <= x"1A_0020";
reg_map(13) <= x"1B_0000";
reg_map(14) <= x"1E_9999";
reg_map(15) <= x"1F_9980";
reg_map(16) <= x"1F_9982";
reg_map(17) <= x"1F_9980";

Tried generating 68MHz,70MHz, 72MHz. In all three cases apart from the desired signal some dominant signal at some other frequency is observed as shown in the images below.

Can someone please tell me the reason for such output.

Thanks in advance.

Regards,

suma

over 5 years ago

0 Ebenezer Dwobeng over 5 years ago

TI__Genius 9695 points

Hi Suma,

I am looking into this.

Thanks,
Eben.

0 Ebenezer Dwobeng over 5 years ago in reply to Ebenezer Dwobeng

TI__Genius 9695 points

Hi Suma,

I checked the exact configuration you have and I am able to generate a tone as expected. I load the configuration after resetting the DAC so all other registers are at their default values.

Can you do a readback after every SPI write to confirm you are writing the correct values to the DAC?

Thanks,
Eben.

0 lakshmi sumala grandhe over 5 years ago in reply to Ebenezer Dwobeng

Prodigy 130 points

Hi Eben,

I have done readback of registers, its correct.

Thanks,

suma

0 Ebenezer Dwobeng over 5 years ago in reply to lakshmi sumala grandhe

TI__Genius 9695 points

Hi Suma,

Can you also confirm that all other register values are their default by doing a hardware reset before you configure the DAC? Are there other spurious signals if you increase the span on the spectrum analyzer?

Thanks,
Eben.

0 lakshmi sumala grandhe over 5 years ago in reply to Ebenezer Dwobeng

Prodigy 130 points

Hi Eben,

I am giving DAC reset (resetb pin of DAC) before programming it. I will check and let you know about the spurious signals on increasing the span of the spectrum analyzer.

Thanks and Regards,

suma

0 lakshmi sumala grandhe over 5 years ago in reply to Ebenezer Dwobeng

Prodigy 130 points

Hi Eben,

I have increased the span of spectrum analyzer and captured. please check the below images.

Thanks and Regards,

suma

0 Ebenezer Dwobeng over 5 years ago in reply to lakshmi sumala grandhe

TI__Genius 9695 points

Hi Suma,

Can you check your clock source, it may require some filtering to remove spurious tones. I suspect you may have high level spurs or sub-harmonics that are creating these extra spurs within the 1st Nyqusit zone. Ideally, there should not be any other tone in the spectrum of the clock source from DC to twice the clock frequency.

Thanks,
Eben.

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DAC38J84: issue with self test