Joy:
how do I know what frequency input is what output voltage..The datasheet is not clear to me...
Check out the equation given:
Vout = fin * Vcc * R1 * C1
So just for the sake of argument say you wanted the full-scale reading (Vout = Vcc) to be at 2000 Hz (2000 rpm in your case as I understand it):
R1 * C1 = 1/2000 = .0005
On p. 7 of the data sheet there are some guidelines for choosing R1 and C1, although as you say it's not exactly clear--maybe just experiment.
Joe