The sampling rate should be about 10 times the highest frequency component in the measured signal to avvoid aliasing.
If the only noise available in the signal is the 60Hz noise, then the sample rate should be at least 600 samples per second (at least 120 to meet Nyquist criterion).
<<" i.e why 2500000
is used as sampling rate ">>
Are you sampling the signal at 2 500 000 Hz ?, if so then this this the value to use. If you sample at 1000 Hz, then the value should be 1000. etc....
<<"If I reduce the CUt off
freq from 10hz to 5hz the amplitude of sine reduced considerably & increased
low cutoff freq from 10hz to 60hz the sine wave got distorted">>
The low cutoff Frequency should be way higher than the frequency of interest in the neasurement.
An IDEAL filter has a flat (constant) response from 0 to Fcutoff, and then goes to zero above Fcutoff. However, all filters are not ideal and their response droops dpending on the kinfd of filter and its parameters. so you should have the cutoff frequency an order highercthan the highest freqiuency of interest.
Also, you may want to increase the order of the filter (2 to 4). It can improve the response. If you are using LabVIEW 7.X, try using the filter Xpress vi's.