Document created by analog-archivist on Feb 23, 2016
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Q

Can I use the AD96685 to convert a 100MHz analogue input to a 100MHz TTL signal
accurately?

A

Can I use the AD96685 to convert a 100MHz analogue input to a 100MHz TTL signal
accurately?
A 100MHz signal has a period of 10ns. The AD96685 has a propagation delay of
about 2.5ns, which means that the output will start to change about 2.5ns after
you apply an input voltage which exceeds the threshold. Now if it were a
constant propagation delay of 2.5ns you would get an accurate representation of
the input frequency at the output but it would be delayed in time (or shifted
in phase) by 2.5ns (90 degrees). The key parameter when trying to accurately
reproduce frequency using comparators is the propagation delay dispersion which
for the AD96685 is 50ps. This means that the propagation delay for a given
input signal might be 2.55ns or it could be 2.45ns depending on how much the
input is overdriven. For a input waveform period of 10ns, a propagation delay
dispersion of 50ps corresponds to an error in the output period of +-50ps (or
1.8 degrees). The instantaneous output frequency could be anywhere between
99.5MHz and 100.5MHz. It's important to note I use the word "instantaneous"
output frequency. If you average the period of the output waveform over a large
number of periods, you will get a more accurate representation of the input
frequency since propagation delay dispersion is statistic in nature and while
the propagation delay can very by as much as 50ps, it will have an average
value around the mean. Note also that the propagation delay is related to the
amount of overdrive you apply to the input (i.e. the amount by which the input
signal exceeds the threshold voltage)  and you will get better figures for the
propagation delay dispersion if the input is of constant amplitude. If you can
handle this kind of error then the AD96685 will do the job for you.