NetDAQ Users Manual

B-1
Appendix B
Noise, Shielding, and Crosstalk
Considerations
Introduction B-1.
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 B-2.
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 2640A/2645A 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 B-3.
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