User's Manual

Appendix B Digitizer Basics
© National Instruments Corporation B-5 NI 5911 User Manual
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.
Random Interleaved Sampling
Random Interleaved Sampling (RIS) is a form of Equivalent Time
Sampling (ETS) that allows acquisition of pretriggered data. ETS refers to
any method used to sample signals in such a way that the apparent sampling
rate is higher than the real sampling rate. ETS is accomplished by sampling
different points along the waveform for each occurrence of the trigger, and
then reconstructing the waveform from the data acquired over many cycles.
In RIS, the arrival of the waveform trigger point occurs at some time
randomly distributed between two sampling instants. The time from the
trigger to the next sampling instant is measured, and this measurement
allows the waveform to be reconstructed. Figure B-5 shows three
occurrences of a waveform. In Frame 1, the dotted points are sampled, and
the trigger occurs time t
1
before the next sample. In Frame 2, the square
points are sampled, and the trigger occurs time t
2
before the next sample. In
Frame 3, the triangular points are sampled, and the trigger occurs time t
3
before the next sample. With knowledge of the three times, t
1
,t
2
,andt
3
,you
can reconstruct the waveform as if it had been sampled at a higher rate, as
shown at the bottom of the figure.