Hi, right now I am trying to do amplitude modulation on the AD9838 using a similar method as shown in this circuit note:
http://www.analog.com/static/imported-files/circuit_notes/CN0156.pdf
I've built up the circuit and it is working for the most part, but I am finding out the modulation bandwidth is severely limited.
I believe I've tracked it down to the FSADJUST pin, and for some reason it isn't able to supply current out fast enough. See the scope trace below attached for what I am talking about.
In the picture, the YELLOW trace is the voltage at the FSADJUST pin (before the 6.8k resistor), and the TEAL trace is the voltage on the pin coming out of the DAC (in my case I am actually using a pwm that is filtered, hence some noise on the yellow and teal traces). The purple trace is just simply TEAL-YELLOW, to give the current going through the 6.8k resistor.
Finally, the pink trace is the output of the 9838. You can see that it SEVERELY lags the FSADJUST pin. From my understanding, the pink trace should be ramping up exactly at the teal trace drops down, instead it is taking 3-4ms to react. Any idea why?
I've tried the same setup using a simple push button switch to gnd and ungnd the FSADUST pin, and I get the exact same behavior. Is this just a limitation of these chips?
Craig
