Hello Martin
If you set the ADC in continuous conversion mode with highest accuracy, slowest ADC clock, maximum HW averaging and longest time added it will fire at about 650Hz.
Therefore, the method to use to get accurate 50Hz triggering is to use hardware trigger mode based on a PIT timer.
Regards
Mark
Kinetis: µTasker Kinetis support
KL25: µTasker Kinetis FRDM-KL25Z support / µTasker Kinetis TWR-KL25Z48M support
ADC/DAC: http://www.utasker.com/docs/uTasker/uTaskerADC.pdf
For the complete "out-of-the-box" Kinetis experience and faster time to market