s/h ckt design

how to select input signal frequency of sample and hold circuit.
my op amp specifications:gain=22500
u.g.b.=6.5MHz
slew rate=10v/us

Whell there is much subject matter on this but without getting into Nyquist (Old Bell Labs dude), Wiki Nyquist Sampling Theory, the sampling rate should be twice the sampling frequency of the highest frequency you want to represent.

So in you case your unity gain frequency is 6.5MHz, then you should sample no less than 13 MS/s (mega samples per second). Now just because your unity gain is 6.5MHz, does't limit you from sampling faster, it just means that the frequencies above 6.5MHz are attenuated by your network.

So 13MS/s is the bare minimum.

here'a an article about sample & hold circuit you might want to consider and some circuit diagram using the 741 and Ca3140