First I would like to thank people for the helpful pushes in what I hope is the right direction.
The frequency varies very little from 5.5Khz maybe 200 Hz either way. The amplitude peaks just hit 500mv pp and drops to zero over a few cycles. There is no constant repetition of the input. There may be less than a second to a minute or more between bursts.
I assume that a sample & hold will get me the input voltage at the peak. Which leaves me with the time. How do I integrate the input to get a value for this ?
Is it a matter of summing the samples from the output of the ADC ?