Why does the wave signal from AD9850 / AD9851 DDS get cut off

ddssignalsine

I have the AD9850 and AD9851 version of this board.

I have written the necessary code to interface these with my microcontroller.

However, when I sample the sine output signal from the DDS it gets cut off like this:

Sampled sine wave signal from DDS

I have connected the wires like this:

GND <-> uC GND

D7 <-> uC GPIO

WCLK <-> uC GPIO

FQUP <-> uC GPIO

REST <-> uC GPIO

Vcc <-> 3.3V (Tried 5V as well)

V-ADJ <-> uC GND, Not Connected and different values between 0V and 3.3V has been tried.

I have been trying with and without jumper J3 and J2. Trying to change the potmeter etc. But there is no change except for when I use a DAC and send a few hundred millivolts signal in on V-ADJ, then the signal gets attenuated and eventually zero. I have not been able to increase the signal amplitude or change its offset so the signal don't get chopped.

The sine signal is directly connected to the microcontroller ADC pin.

I am running out of ideas and are therefore asking for help/tips/experiences.

I should have had an oscilloscope for debugging this signal, but none available at the moment.

The schematic is: enter image description here

Best Answer

Several things:

  • Make sure that your ADC minus(-) signal is GND. If it is by default something else you may see clipped signals like the one you are observing.
  • your ADC might not be able rail to rail input. It means that value lower than the minumum input voltage of the ADC would get clipped
  • There is a software bug somewhere.

Could you test with a DIY oscilloscope? (such as the one that uses your audio card of the PC. (if the frequency range falls in the audio range)

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