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4 Measurement Tutorials
Digitizing measurements
The digitize mode is available on both DAQ970A/DAQ973A with the DMM Digitize data acquisition mode. Digitizing
is the process of converting a continuous analog signal, such as a sine wave, into a series of discrete samples
(readings). The figure below shows the result of digitizing a sine wave. This chapter discusses the various ways to
digitize signals. The importance of the sampling rate, and how to use level triggering.
Sampling rate
The Nyquist or Sampling Theorem states: If a continuous, bandwidth-limited signal contains no frequency
components higher than F, then the original signal can be recovered without distortion (aliasing) if it is sampled at a
rate that is greater than 2F samples per second.
In practice, the multimeter's sampling rate must be greater than twice the highest frequency component of the
signal being measured. From the front panel you can select a sample rate in samples per second, or select a sample
interval (time between samples) by toggling the Sample softkey.
The figure below shows a sine wave sampled at a rate slightly less than 2F. As shown by the dashed line, the result is
an alias frequency which is much different than the frequency of the signal being measured. Some digitizers have a
built-in anti-aliasing low-pass filter with a sharp cutoff at a frequency equal to l/2 the digitizer's sampling rate. This
limits the bandwidth of the input signal so that aliasing cannot occur. Since the multimeter has a variable sample
rate for DCV digitizing, and to preserve the upper bandwidth for high-frequency measurements, no anti-aliasing
filter is provided in the multimeter. If you are concerned about aliasing, you should add an external anti-aliasing
filter.
Keysight DAQ970A/DAQ973A User's Guide
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