©NationalInstruments Corporation 3-1 NI5102 User Manual
3
Digitizer Basics
This chapter explains basic information about using digitizers, including
important terminology and use of the probe.

Understanding Digitizers

To understand how digitizers work, you should be familiar with the
Nyquisttheorem and how it affectsanalog bandwidth and sample rate.
You should also understand vertical sensitivity, analog-to-digital converter
(ADC) resolution, record length, and triggering options.

Nyquist Theorem

The Nyquist theoremstates that a signal must be sampled at least twice
asfast as its bandwidth in order to accurately reconstruct the waveform;
otherwise, the high-frequency content will alias at a frequency inside
the spectrum of interest (passband).An alias is a false lower frequency
componentthat appears in sampled data acquiredat too low a sampling
rate. Figure 3-1 shows a 5 MHz sine wave digitized by a 6 MS/s ADC.
The dotted lineindicates the aliased signal recorded by the ADC at that
sample rate.
Figure3-1. Aliased Sine Wave When Waveform is Under Sampled
The5 MHz frequency aliases back in the passband, falsely appearing as if
itwere a 1 MHz sine wave. To prevent aliasing in the passband, a lowpass
filterlimits the frequency content of the input signal above the Nyquist rate.
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