Chapter3 DigitizerBasics
©NationalInstruments Corporation 3-7 NI5102 User Manual
buffering.See the Understanding the Probe and Its Effect s
on Your Waveform sectionlater in this chapterfor more information.
In addition to thei nputresistance, all digitizers, DSOs, and probes
presentsome input capacitance in parallel with the resistance. This
capacitancecan interfere with your measurement in much the same
wayas the resistance does. You can reduce this capacitance by using
an attenuating probe (X10, X100, or X1000) or an active probe. See
Appendix A, Specifications, or your probe specificationsfor accurate
input capacitance numbers.
•Input frequency—If your sample ratei sless than t wice the highest
frequencycomponen t at the input, the frequency components above
half your sample rate will alias in the passband at lower frequencies,
indistinguishable from other frequencies in the passband. If the
signal’shighest frequency is unknown, you should start with the
digitizer’smaximum sample rate to prevent aliasing and reduce the
digitizer’ssample rate until the display shows either enough cycles of
the waveformor the information you need.
•Generals ignal shape—Some signals, such as sinusoidal, triangular,
square,and saw tooth waves are easy to capture with ordinary
triggering methods.
Some of the more elusivewaveforms, such as irregular pulse trains,
runt pulses, and transients, may be more difficultto capture. You can
solvethis problem without using complicated signal processing
techniquesby using trigger hold-off, which lets you specify a time
fromthe end of the last acquisition during which additional triggers are
ignored.
•Input coupling—Youcan configure the input channels on your
NI 5102 to be DC coupled or ACcoupled. DC couplin g allowsthe
DC and low-frequencycomponents of a signal to pass through without
attenuation. In contrast, ACcoupling removes DC offsets and
attenuates the low-frequencycomponents of a signal. This feature
can be exploitedto z oom in on ACsignals with large DC offsets,
such asswi tching noise on a 12 V powersupply. Refer to Appendix A,
Specifications, for the input limits that must be observed regardless
ofco upling.