B Noise, Shielding, And Crosstalk Considerations; Noise, Shielding, And Crosstalk Considerations - Fluke 2680A User Manual

Noise, Shielding, and Crosstalk
Introduction
This appendix supplies information on how you can setup your instruments to
minimize measurement errors. The topics include static versus dynamic
measurement, noise and shielding, and how to minimize crosstalk effects
Static Versus Dynamic Measurements
Some signals change value slowly. Other change rapidly. At some point, things
change quickly enough that we call the signals dynamic rather than static.
Frequently, one wishes to analyze dynamic signals in the frequency domain.
However, if you wish to do that, you must use an anti-aliasing filter (a low pass
filter with a sharp cutoff above the highest frequency of interest) ahead of every
input that is to be digitized and you must sample at a frequency at least two times
that of the transition band of the filter.
The 2680 Series instruments do not have an anti-aliasing filter built-in. They are
not suitable for digitizing dynamic signals unless you put a suitable anti-aliasing
filter in series with each input. Note that a suitable anti-aliasing filter is a low-pass
filter with at least 3 poles and a fairly sharp cutoff characteristic. Good anti-
aliasing filters have as many as 8 poles.
Noise and Shielding
An additional manifestation of aliasing can appear even in static readings. If there
is noise coupled into the signal wiring from power mains wiring, it usually appears
as noise in the readings. However, it is possible to accidentally combine a fast
Appendix B
Considerations
B-1