AppendixB DigitizerBasics
NI5911 User Manual B-4 ni.com
FigureB-4. TransferFunction of a 3-Bit ADC
ADC Resolution
ADC resolution limitst he accuracyof a m easurement. The higher the
resolution (numberof bits), the m ore accuratethe measurement. An 8-bit
ADC dividesthe vertical range of the input ampl ifierinto 256 discrete
levels.With a vertical range of 10 V, the 8-bit ADC cannot resolve voltage
differencessmaller than 39 mV. In comparison, a 12-bit ADC with 4,096
discrete levelscan resolve voltage differences as small as 2.4 mV.
Record Length
Record length refers to the amount of memory dedicated to storing
digitizedsamples for postprocessing or disp lay.In a digitizer, record length
limits the maximum duration of a single-shot acquisition. For example,
witha 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 bufferand a sample
rate of 20 MHz, the duration of acquisition is 5 ms (100,000 ×50 ns).
TriggeringOptions
Oneo f the biggest challenges of making a measurement is to successfully
triggerthe signal acquisition at the point ofinterest.Since most high-speed
digitizersactually record the signal for a fraction of the totaltime,they can
easily miss a signal anomaly if the trigger point is set incorrectly.The
NI5 911 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