Then use a schmitt trigger.
All the datasheets I've seen quote rise an fall times of 10 to 15 ns for the range of frequencies you want. What rise/fall time is specified as required for your ADS 805 ADC?
If your oscilloscope doesn't have a bandwidth many times (say 10 times) the frequency of the square wave you're trying to look at, then it will appear to have far slower rise and fall times.
the crystal osc that was mentioned(4 pin metal package) gave a sinusoidal types wave.
it doesnot give a exact square wave. I need it to operate ADS 805 ADC