AppendixB DigitizerBasics
©NationalInstruments Corporation B-9 NI5911 User Manual
•Source impedance—Most digitizers and digital storage oscilloscopes
(DSOs)havea1MΩ inputresistance in th e passband. If the source
impedanceis large, the signal will be atte nuated at the amplifierinpu t
and themeasurement willbe inaccurate. If the source impedance is
unknownbut suspected to be high, change the attenuation ratio on
your probe and acquire data. In addition to the input resistance, all
digitizers,DSOs, and probes present some input capacitance in parallel
with the resistance.This capacitance can interfere with your
measurementin m uch thesame wayas the resistance does.
•Input frequency—If your sample ratei sless than t wice the highest
frequencycomponen t at the input, the frequency components above
half your sample rate will alias in the passband at lower frequencies,
indistinguishable from other frequencies in the passband. If the
signal’shighest frequency is unknown, you should start with the
digitizer’smaximum sample rate to prevent aliasing and reduce the
digitizer’ssample rate until the display shows either enough cycles
oft hewaveform or the information you need.
•Generals ignal shape—Some signals are easy to capture byordinary
triggeringmeth ods.A fewiterations on the trigger level finally render
asteady display. This method works for sinusoidal, triangular, square,
and sawtooth waves. Some of the more elusive waveforms, such as
irregularpulse trains, runt pulses, and transients, may be m ore difficult
tocapture. Figure B-8 shows an example of a difficult pulse-train
trigger.