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Appendix B Digitizer Basics
NI 5911 User Manual B-4 ni.com
Figure B-4. Transfer Function of a 3-Bit ADC
ADC Resolution
ADC resolution limits the accuracy of a measurement. The higher the
resolution (number of bits), the more accurate the measurement. An 8-bit
ADC divides the vertical range of the input amplifier into 256 discrete
levels. With a vertical range of 10 V, the 8-bit ADC cannot resolve voltage
differences smaller than 39 mV. In comparison, a 12-bit ADC with 4,096
discrete levels can resolve voltage differences as small as 2.4 mV.
Record Length
Record length refers to the amount of memory dedicated to storing
digitized samples for postprocessing or display. In adigitizer, record length
limits the maximum duration of a single-shot acquisition. For example,
with a 1,000-sample buffer and a sample rate of 20 MHz, the duration of
acquisition is 50 µs (the number of points multiplied by the acquisition
time/point or 1,000 × 50 ns). With a 100,000-sample buffer and a sample
rate of 20 MHz, the duration of acquisition is 5 ms (100,000 × 50 ns).
Triggering Options
One of the biggest challenges of making a measurement is to successfully
trigger the signal acquisition at the point of interest. Since most high-speed
digitizers actually record the signal for a fraction of the total time, they can
easily miss a signal anomaly if the trigger point is set incorrectly. The
NI 5911 is equipped with sophisticated triggering options, such as trigger
Range 0-5 V
Voltage Fluctuations
in This Region Will
Be Unnoticed
000
001
010
011
100
101
110
05V
111