The droop rate of the sample and hold determines how long the sampled voltage will be held without much drop in the voltage. Hold times of 100 us with < 100 uV drop in the sampled voltage is possible with the S&H component.
For how long does the sample and hold is holding the sampled voltage for SAR ADC to convert ?
Can you send the screen shot of the S&H output ? What is the BW of the input signal that is fed to S&H ?