I am having a little trouble calculating the upper cut off frequency of this Butterworth Filter in the Sallen and key configuration.

Below you can see the component values in the pic:

http://twitpic.com/152zmr

The upper cut off frequency here (according to my supervisors) is 28kHZ,

however using the equation I found:

fc=1/2pi(root(R1R2C1C2))

,it doesn't equal 28kHz, I am getting 44.209kHz?!

Am I using the wrong equation?

Any help would be much appreciated,

Regards,

Adam