When using ADCs for direct sampling, we actually need to frequency plan them as well. Because, as the question “Are mixers and ADCs related? As mentioned, you can also think of an ADC as a mixer-like thing.
This is because ADCs are also analogue devices and also produce non-linear products, such as harmonics. Although it may not be as severe as the higher order spurious of a mixer, it still requires a bit of planning.
Interference is divided into in-band and out-of-band interference. Frequency planning, on the other hand, is for in-band interference.
As with the IF selection principle, when the useful signal and the in-band interference signal enter the ADC together, the interference signal passes through the ADC and the resulting higher harmonics are folded to within DC~fs/2.
Just as in the selection of the receiver IF, due to the non-ideal nature of the mixer, the in-band interfering signal passing through the mixer may generate MXN higher order spurious with the local oscillation and fall into the useful signal band. And when the interfering signal is large enough, it may affect the sensitivity of the receiver.
A similar principle applies to the selection of the IF frequency for the ADC. So it becomes necessary to plan the sampling frequency rationally to get the optimum performance.
The frequency planning software is available on the TI website by sending a message “Frequency Planning” in the background.
