I have been considering couple of ways of realizing ASK with AD9834, and I am wondering which one is the best choice.
In circuit note CN-0156, a DAC(SPI interface) is utilized to control AD9834's output amplitude. However, what I want to achieve is to use a GPIO pin of a microcontroller to realize ASK. So here are some methods I have been considering:
1. Use 2 DACs, and a digitally controlled SPDT switch to control the current out of the FS ADJUST pin of AD9834
2. Use 2 resistors of different value, and a SPDT to interface with the FS ADJUST pin of AD9834. It's a less expensive implementation, but less flexible solution compared to 1.
3. The third method is use two synchronized AD9834 chips and program them for different phase. Then feed the two outputs to the same resistor, essentially summing the output. By toggling one AD9834's phase select pin, I will be able to obtain two different amplitudes( Trig Identity)
For solution 1 and 2, what is the requirement for the switching time? During the transitioning period of the switch, the FS ADJUST pin will see an open, thus the output amplitude might drop to zero briefly.
Any comment or suggestions would be greatly appreciated !