Hmmm, on second thought it does work.
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I don't see how, since in the first place you'll have to measure
frequency, which can't be done instantaneously.
But, even given that, as a freebie, two measurements need to be made
to determine the slope of the sinusoid at the "point" of interest
and, therefore, to determine its amplitude.
Consider, for example: A single measurement is made and it happens
to coincide with the zero-crossing of the sine wave. is there any
information there which can be used to determine the amplitude of
the signal?
No.
Now let's say another single, separate measurement is made which
yields a reading of 1 volt. is there any information there which
will allow us to determine the amplitude of the signal?
No.
Going back to the first example, If frequency is known and a
measurement is made which yields zero volts there will still be
nothing known about the amplitude of the signal. However, if
another measurement is made, after the first, in the smallest
increment of time possible, and the time between the two
measurements is known, the slope of the sine wave can be
determined and its amplitude calculated.
The same is true anywhere up and down the sine wave, but the fact
that _two_ measurements, displaced in time, need to be made to
determine the slope means that the amplitude can't be determined
from a single instantaneous measurement.