I have an ADC12D1800RF card that I would like to use for sampling a ~1GHz wide IF band centered at fc=3.0GHz (ie. 2.5 to 3.5 GHz).
I am assuming I should use Fs=3.6GHz and 2nd Nyquist band.... or should I possibly consider using Fs=2.4GHz and 3rd Nyquist band?
Which mode would be the best for this configuration (non-DES, DESI, DESIQ or DESCLKIQ)?
Hello Skip,
Running the ADC at a lower clock rate can help improve dynamic performance as well as reduce power consumption (~500mW). There are some plots in the ADC12D1800RF versus clock frequency. Additionally a sample rate of 2.4Gsps (clock frequency of 1.2GHz) would fold your signal nicely to 100MHz to 1.1GHz, leaving only 100MHz on the upper and lower end. Sampling with 3.6Gsps (clock frequency of 1.8GHz) would be wasting 700MHz on the upper end. In either case, because the input signal is such a high input frequency we would recommend either DESIQ or DESCLKIQ modes which have a larger input bandwidth, but there will be some attenuation though at the upper frequency end. Please take a look at figure 5-26 of the ADC12D1800RF datasheet below.
There are also performance tradeoffs between the different modes based on external factors. For more information on driving the ADC in the different DES modes, please review the following TI Design:
Driving GSPS ADCs in Single-Channel or Dual-Channel Mode for High Bandwidth Applications: 
Regards,
Luke LaPointe
High Speed Data Converters
