I am using the AD9958 to generate two signal at the same frequency. Channel 0 is modulated in PSK mode with a low frequency square wave applied on pin 42 (P2 used as CH0 two-level modulation input, Phase "180°" is written in its CW1 register). Channel 1 is not modulated, but I tune its phase manually with its phase offset word CPOW0. In the selected modulation mode, its modulation input (P3 pin 43) is grounded because unused. Everything works fine.
Now, I want to adjust the output amplitude independently on both channels.
I make some coarse adjustment of these amplitude with variable "DAC full-scale current control" data written in CFR's. Both channels works independently: after addressing correct "Channel enable" bits in CSR, I got variation on 1-2-4-8 scales.
I try to make fine adjustments with variable "Amplitude scale factor" data written in ACR's. I got variable amplitude only on the modulated Channel (Ch0). I modified my software to write exactly the same things in both Channel registers (same configuration, same data using always both "Channel enable" bits in CSR). I got amplitude variation only on the modulated channel. Because it works on this channel, the registers contents are correct.
So, I guess that amplitude is updated only on modulation signal input transitions. Am I true ?
If so, I will have to cut the grouding trace on P3 pin, to tie it with the P2 pin with modulates CH0. With zero phase modulation written in channel 1 CW1 register, it will give really no modulation on this channel despite of the configuration and modulation signal applied ... But cutting this trace will be so delicate ...
Thanks for help