Hi Fernando,
First of all an advice: try to write in english, in this way you will have a lot more persons that could help you.
Now related to your question, can you give more details of the application? if you are looking for which NI Hardware is for you the best choice, you should tell us the characteristics of the signals to be acquired (exampe: 3 voltage signals, range +-5V).
Regarding the analysis of the signals, once you do the acquisition (the A/D conversion) you can build a simple post-processing VI to calculate the frequency spectrum and see the harmonics. You can also implement this processing in the same acquisition VI in order to see the harmonics during the acquisition procedure.
Best regards,
CJMV