AD9833 - Not oscillating 300mv O/P

Hi,

I've pretty much copied the UG-272 schematic and made a board up, but when testing I just get a static 300mV output.

On power up there's 300mV, drops down to about 100mV, then down to 0V and back to 300mV.

I've followed the AN-1070 and followed their basic example to create a 400Hz sine wave. I'm using CCS compiler on a dspic33, below are some snip-its:

// SPI Config

#USE SPI (DI=PIN_F7, DO=PIN_F8, CLK=PIN_F6, ENABLE=PIN_B2, MSB_FIRST, STREAM=AD9833, ENABLE_ACTIVE=0, MODE=1, BAUD=400000, BITS=16) 

// setup sinewave

spi_xfer(AD9833, 0x2100, 16);
delay_ms(2000);
spi_xfer(AD9833, 0x50C7, 16);
delay_ms(200);
spi_xfer(AD9833, 0x4000, 16);
delay_ms(200);
spi_xfer(AD9833, 0xC000, 16);
delay_ms(200);
spi_xfer(AD9833, 0x2000, 16);

Here's my schematic, my oscillator is 16MHz so I'm not expecting to get 400Hz as per the app note. Oscillator running fine, 3V3 present and I've examined the SPI data which is correct, the only thing left to try is replace the IC, any ideas what's wrong anyone please?

  • Hi,

    Could you take the part from reset mode before program any register, please?

    0x2100

    0x2000

    Let me know if this fix the problem.

    Regards,

    Miguel

  • I found on the Eval board they had a 10nF cap on the output, I had 100nF, removed this and checked output, then I had a 600mV signal oscillating, but not sine wave, put the 10nF on and now 350mV flat again! Included the layout for ref too.

  • Hi Miguel,

    Thanks for your reply, I tried what you suggested and now it is oscillating!

    With the code that I had after taking part from reset mode, I now have 350mV level with 60mV P-P at 166KHz, quite far away from 400Hz, and any changes to the code don't change frequency or wave shape.

    I think I have since damaged the part anyway, so need to replace, i read on a forum that you should use a 220 ohm load resistor even though the datasheet says you don't, so I tried this, which just pulled the output voltage down and accidentally caught the resistor on the 3v3 pin, putting 3v3 into the output. Strangely enough it started oscillating a lovely sine wave about 3 or 5V P-P at around 300-400Hz, but I could not adjust the frequency or wave shape.

    Thanks

    James

  • Hi,

    I have since replaced device and is working correctly!

    Thanks

    James

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