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Amplitude modulation of AD9834 DDS - failed, but why?
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RES:
Yansi:
Update: I have tried modulating using the 6k8 resistor fed from the bottom.  Well, guess what! It didn't work either!   :box:

After a bit of thinking and poking around with a probe, I have noticed the COMP pin voltage is slighthly following the modulation current from FSADJ.

I have tried deliberately increasing the COMP pin decoupling capacitor and guess what - it has stopped working correctly even at 100Hz, where it has worked before.

I may be onto something here!

Sad thing is the documentation of what the COMP pin is used for, is only vague at best. But it definitely seems to set a corner frequency for the rate of change of the bias current.

Strange is, many designs seem to omit this capacitor entirely - but why? There isn't any info available on the purpose of the cap, other than "DAC Bias Pin. This pin is used for decoupling the DAC bias voltage.".  Can I omit it then to have the modulation possible? Or can't it be omitted? This is starting to seem more like a question for Analog Devices.

Including your design, RES! Why don't you have a cap at the COMP pin?

//EDIT: I have written question to the ADI Support. Let's see where that'll bring us...
OwO:

--- Quote from: Yansi on April 08, 2019, 02:39:26 pm ---There isn't any info available on the purpose of the cap, other than "DAC Bias Pin. This pin is used for decoupling the DAC bias voltage.".  Can I omit it then to have the modulation possible? Or can't it be omitted? This is starting to seem more like a question for Analog Devices.

--- End quote ---

That is a pretty big hint. In a DDS any noise on the DAC reference/bias source results in AM noise at the output, so this is usually brought out to a pin where it can be bypassed with a big cap. In your case you want AM so you can omit the cap.
Yansi:
I guess so, but I'd rather have that confirmed. Also I'd be interested how to estimate value of the cap to set a specific corner frequency. (you do not want to let it modulate all crap from above, if you just need a couple kHz BW).
OwO:
Well, it's not a simple RC filter, since you already saw that increasing frequency results in nonlinear behavior. I would guess it's more like a (possibly asymmetric) current limited source driving the cap, which results in slew rate limits. I would either omit the cap or see about driving the bias pin directly.
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