I agree with the mail of Donatas.

You must consider the Nyquist (or Shannon) frequency. You need to
sample at twice the higher usefull frequency (cut band) of your sensor.
This frequency is given by data-sheets of your sensor.

When you know your sample frequency, it is simple to program one
timer of your controller wich deliver an interrupt for sampling. So you
work with a sampling frequency deduced of system clock.

You have a choice of MCS51 controllers with integrated ADC :

10 bits : Philips 80C552, Dallas 87C550 (max sampling @ 10/20 KHz)
12 bits : Cygnal 8051C00x, Analog Devices ADuC812 (smpl @ 100 KHz)
24 bits : Analog Devices ADuC824 (max sampling < 1 KHz)

Don't forget to put a low pass ANALOG filter (and not a CC filter wich
generates sampling noise) before input of your ADC, with a cutting
frequency equal to Nyquist frequency.