Do you need to detect the amplitude of each cycle of a given 2kHz input or is one 2kHz source going to have fixed amplitude which may be different from another 2kHz source?
If it's the latter you might consider a peak detector circuit; allow it to sample 'N' peaks (characterize what number 'N' is to get a representative peak output corresponding to the amplitude of the input) and then take an ADC reading of the PD output. Use a digital output to trigger the MOSFET to reset the peak value.
