Hello,
I've been experimenting with amplitude modulation of AD9834, but I have rather failed but don't know why.
Based on this application note for a similar DDS:
https://www.analog.com/media/en/technical-documentation/application-notes/AN-423.pdfand based on the datasheet of AD9834 directly, I have concluded the FSADJ pin leads to some kind of a current mirror, which sets the DAC FS current, but 18 times smaller. (the DAC full scale current is 18*1.15V/Rset)
So what I've done was enhanced the circuit from that AN-423 with some precision. However the result was quite not expected. The DDS can not be fully modulated and it does weird sh!t.
Below is the schematic of my experiment, below that a DSO capture of the voltage when the required modulation depth is small ) - the voltage on FSADJ does not change much (which is expected), the source voltage copies the input one well.
When I try to apply more modulation, the FSADJ pin starts to go to GND, then to normal, then to AVDD (+5V) - what the hell is going on there? In such case, the output is not much modulated and modulation envelope is fairly distorted. I can get a maximum of about 20% modulation depth.
I even should not be exceeding the maximum FSADJ current. Max DAC current is 4mA, which is 4/18 = 220uA FSADJ current. Scope shows about 400mV peak at R7, equaling 180uA peak current (or 3.24mA respectively).
(FSADJ pin yellow, source of the 2N7002 in blue).
Tnx for any hits with this.