Thanks guys,
Pommie ive actually tried with the comparator and its lead me here, the hysteresis on the comparator is not suitable for this application because while each transition of the comparator output increments the register, the comparator was experiencing multiple transitions for each input above the specified voltage, so the register increment would be way in excess of what was expected. the result was better with hysteresis than without but still not sufficient or even a consistent value that could be adjusted through software.
The transient analysis i with the comparator values calculated using the formulas in the document that u uploaded provided showed me that theoretically the hysteresis should have worked but, the practical outcome was not the same.
Thanks dougie, you're right, i was of the opinion that acquisition time was for time between samples. things are a bit clearer now