Chapter 3 Digitizer Basics
© National Instruments Corporation 3-7 NI 5102 User Manual
buffering. See the Understanding the Probe and Its Effects
on Your Waveform section later in this chapter for more information.
In addition to the input resistance, all digitizers, DSOs, and probes
present some input capacitance in parallel with the resistance. This
capacitance can interfere with your measurement in much the same
way as 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 specifications for accurate
input capacitance numbers.
• Input frequency—If your sample rate is less than twice the highest
frequency component 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’s highest frequency is unknown, you should start with the
digitizer’s maximum sample rate to prevent aliasing and reduce the
digitizer’s sample rate until the display shows either enough cycles of
the waveform or the information you need.
• General signal shape—Some signals, such as sinusoidal, triangular,
square, and saw tooth waves are easy to capture with ordinary
triggering methods.
Some of the more elusive waveforms, such as irregular pulse trains,
runt pulses, and transients, may be more difficult to capture. You can
solve this problem without using complicated signal processing
techniques by using trigger hold-off, which lets you specify a time
from the end ofthe last acquisition during which additional triggers are
ignored.
• Input coupling—You can configure the input channels on your
NI 5102 to be DC coupled or AC coupled. DC coupling allows the
DC and low-frequency components of a signal to pass through without
attenuation. In contrast, AC coupling removes DC offsets and
attenuates the low-frequency components of a signal. This feature
can be exploited to zoom in on AC signals with large DC offsets,
such as switching noise on a 12 V power supply. Refer to Appendix A,
Specifications, for the input limits that must be observed regardless
of coupling.
