©NationalInstruments Corporation B-1 NI5911 User Manual
BDigitizer Basics
This appendix explainsbasic information you need to understand about
making measurements with digitizers, including important terminology.
Understanding Digitizers
Tounderstand how digitizers work, you should be familiar with the Nyquist
theoremand how it affects analog bandwidth and sample rate. You should
also understandterms including vertical sensitivity, analog-to-digital
converter(ADC) resolution, record length, and triggering option s.
Nyquist Theorem
TheNyqui st theorem states that a signal must be sampled at least twice as
fastas the bandwidth of the signal to accurately reconstruct the waveform;
otherwise, the high-frequency content will alias at a frequency inside the
spectrumof interest(passband). An aliasis afalse lower frequency
componentthat appears in sampled data acquired at too low a sampling
rate.Figure B-1shows a 5 MHz sine wave digitized by a 6 MS/s ADC. The
dottedline indicates the aliased signal recorded by the ADC at that sample
rate.
FigureB-1. Sine Wave Demonstrating the Nyquist Frequency
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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