Hello Satoshi-san,
Sorry I did not realize the link I had sent you was for an internal post. Anyway the question in the post is very similar to yours - how slow can SCLK be on the ADS795x? The reply states that SCLK frequency can be as low as 200KHz without causing performance degradation. For frequencies much lower, the longer conversion time causes the ADC's linearity to degrade due to non-linear voltage droop at the ADC input caused by leakage current through the input sample-and-hold capacitor.
Best Regards,
Harsha