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Digital Signal Processing
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Digital Signal Processing |
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Time Skewing
Data acquired from various channels are often functionally related. In stress analysis work, for example, three separate channels might provide strain data from the three grids of a strain gage rosette, which are used together to calculate principal stresses and strains. In this case, when the measurements were made is important if the three signals vary with time. When such signals are sampled sequentially, the resulting data are all taken at different times, and are said to be skewed. The errors produced by this skewing depends upon the nature of the signal, the scanning interval (inverse of the scanning rate) and the number of intervals between the sampling of any two data points. For sinusoidal signals with sequential sampling, the worst-case errors will occur as the signal is crossing through the inflection points. At these points, the maximum frequency that can be sampled without any detectable skewing (signal change of 1 LSB, or less, of the full scale signal over a single scanning interval) is a function of the sampling rate and the number of "bits" into which the ADC digitizes the full-scale signal. These frequencies are a few hertz at best, even for relatively high sampling rates. And, of course, the situation worsens as the number of intervals between data points increases. (continued...) 
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