AppendixB DigitizerBasics
©NationalInstruments Corporation B-5 NI5911 User Manual
thresholds, programmable hysteresis values,and trig ger hold-off.The
NI 5911 also has two digital triggers that giveyou more flexibility in
triggeringby allowing you to connect a TTL/CMOS digital signal to trigger
the acquisition.
Random Interleaved Sampling
Random InterleavedSampling (RIS) is a form of Equivalent Time
Sampling(ETS) that allows acquisition of pretriggered data. ETS refers to
anymethod used to sample signals in such a way that the apparent sampling
rateis higher than the real sampling rate. ETS is accomplished by sampling
differentpoints along the waveform for each occurrence of the trigger, and
thenreconstructing the waveform from the data acquired over many cycles.
InRIS, the arrival of the waveform trigger point occurs at some time
randomly distributed between two sampling instants. The time from the
triggerto the next sampling instant is measured, and this measurement
allowsthe waveform to be reconstructed. Figure B-5 shows three
occurrencesof a waveform. In Frame 1, the dotted points are sampled, and
the triggeroccurs time t1before the nextsample. In Frame 2, the square
pointsare sampled, and the trigger occurs time t2before the next sample. In
Frame3, the triangular points are sampled, and the trigger occurs time t3
beforethe next sample. With knowledge of the three times, t1,t
2,andt
3,you
canreco nstruct the waveformas if it had been sampled at a high er rate, as
shownat the bottom of the figure.