Iphone accelerometer sampling frequency

Some applications are available for measuring frequencies of alternating currents up to 200 Hz using magnetometers and designed for Iphones.

I collected data from Iphone 5s magnetometers and the minimum average value of the acquisition period I recorded was about 0,02 sec, in other words the average sampling frequency was 50 Hz. In my opinion and in this case, it's possible to measure only upto 25 Hz. How is possible to measure alternating current up to 200 Hz ? Did I write something wrong? Is there a trick to decrease the acqusition period? Thank's a lot for you support. Best regards.

Jeff 974