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Appendix B Digitizer Basics
NI 5911 User Manual B-4
©
National Instruments Corporation
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 a digitizer, 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 x 50 ns). With a 100,000-sample buffer and a sample
rate of 20 MHz, the duration of acquisition is 5 ms (100,000 x 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
thresholds, programmable hysteresis values, and trigger hold-off. The
NI 5911 also has two digital triggers that give you more flexibility in
triggering by allowing you to connect a TTL/CMOS digital signal to trigger
the acquisition.
Making Accurate Measurements
For accurate measurements, you should use the right settings when
acquiring data with your NI 5911. Knowing the characteristics of the
signal in consideration helps you to choose the correct settings. Such
characteristics include:
• Peak-to-peak value—This parameter, in units of volts, reflects the
maximum change in signal voltage. If V is the signal voltage at any
given time, then V
pk-to-pk
= V
max
-V
min
. The peak-to-peak value
affects the vertical sensitivity or gain of the input amplifier. If you do
not know the peak-to-peak value, start with the smallest gain
(maximum input range) and increase it until the waveform is digitized
using the maximum dynamic range without clipping the signal. Refer
CBIHWum.book Page 4 Thursday, October 29, 1998 1:56 PM