Hello there,
I have a few questions regarding the parameters given in the datasheet.
1) Table 6.5: Standard deviation is given for Mode 2 only. What is it for Mode 1?
I understand that in Mode 1 the measurement is done by the ring oscillator only, so the measurement does not include the externally clocked coarse counter, which both removes an error source and on the other hand makes the whole measurement and not only fractional part dependent on the ring oscillator. The latter one I assume is less stable than the external clock. Does that mean standard deviation is larger in Mode 1?
2) Table 6.5 Accuracy is stated to be 28 ps which is only half of the LSB resolution. How can the device have a accuracy better than its resolution? If the accuracy is just a measure for the systematic error as defined in the footnote then 28 ps are added or substractet from the time interval to be measured and should thus lead to +/- 1 LSB accuracy??
3) The lower end of the measurement range in combined mode is stated as 250 ps in the Features section of the datasheet. Where does this lower limit come from? In principle I don't see why one shouldn't be able to measure down to 0 ps. Of course the shorter the measurement interval the more dominating are the error sources and it can be discussed whether a measurement of 0 ps +/- (2*LSB + 2*std dev) makes any sense, but I don't see where the lower limit of 250 ps comes from?
Thanks in advance for a clarification
Klaus
