AD9694 vs AD9695 DDC/NCOs

When using the DDC/NCO, can the AD9694 ADC operate in the 2nd Nyquist zone in the same way for the AD9695?

I have tested the AD9595 eval PWB; Clock Input is 400M, and decimation rate =8, digital output is 50M bandwidth, centered around the NCO freq selected. The NCO may be set below 200M (ADC clk/2)-(1st Nyq zone) or above 200M (2nd Nyq zone). The measured results from eval PWB match the performance in the Virtual Eval app.

However, when the AD9694 is selected in the Virtual Eval app, it does not permit the NCO to be set above ADC clk/2. Is this an indication that DDC/NCO in the AD9694 is different from the AD9595, so digital mixing/filtering only in 1st Nyq zone is allowed, Yes, the AD9695 has more decimation rates, and a higher sampling clock, but description of the DDC/NCO looks the same, so why the apparent restriction in Virtual Eval app?

  • 0
    •  Analog Employees 
    on Nov 5, 2019 9:43 PM

    Analog input frequency higher than Nyquist frequency would fold back to first Nyquist. For example, Ain frequency = fs - x would fold back to frequency x. To move frequency fs - x to DC, where fs - x is in second Nyquist, you can set the NCO to x.