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Rset amplitude modulation in AD9951


after reading AN-423 I thought it could be a good idea to try the analog amplitude modulation with a similar structure as the one from figure 2. With a 3.3V digital signal, I generate at the gate of the transistor the same signal as it can be seen in figure 4, down at the right. Thus, I feed the transistor gate with a square signal between ~1.65 and ~3.3V. The problem is that the output of the DDS does not look like the figure 3 up at the right but it takes ~2.5 ms to get to its maximum amplitude. My measures at the output of the Rset pin conclude that the MOSFET is switching the proper way and that the problem is not a lack of current supply of my digital signal (which I have also measured and looks alright).

I expected to be able to modulate at >10 KHz, even maybe at 100 KHz renouncing to such a clean response which can be seen in that figure.

Can anybody tell me why is the DAC taking so long to recover? Can it be that I am forgetting anything?

  • AN-423 is associate with the AD9850 and AD9851 parts. I do not have direct knowledge of it working exactly the same for the AD9951. However, I would think the concept would work. Here's one thing to note. The AD9951 and AD9850/51 both have a pin called DAC bypass (pin23) . If it's populated on the AD9951 with a capacitor to ground trying removing and see if that helps. I know if the DAC circuit  is put to sleep having the DAC bypass capicator to ground it prolongs the wake up time.

  • Exactly, that dropped the wake-up time until around 4 us

    I am a bit afraid of the consequences of leaving DACBP floating due to amplitude noise... is there any information about the specific role of that pin?