Thank you KW, you helped me from I don't know anything about ADC to I know something about ADC. I think my problem is the ADS 6145 at 125MHz clock rate couldn't well captured 70 MHz signal, because ADS 6145 only captures around 3 samples every 2 period cycles. Some spikes appeared in lower frequency signal. They are bit errors, I consider it's acceptable and I can use matlab to apply a lowpass filter to moveout error bits.
I really appreciated your help and patient. ^_^
Zaidi Zhu