352 Appendix E High Resolution Digitizing With the 3458A
Using the
Track-and-Hold
Path for Direct or
Sequential
Sampling
The track-and-hold path is the solution to capturing the amplitude of narrow
pulses. This path has a bandwidth of 12 MHz and a fixed aperture of 2 ns. With
trigger jitter of 2 ns, you can, with a little searching, capture the peak amplitude
of a pulse as narrow as 40 ns without measurement degradation, as indicated in
Figure 53. Rise times of less than 10 ns will cause overshoot in a digitized
measurement; hence, if it is likely that signals with these frequency components
will be applied to the input of the 3458A, then bandlimit the signal by filtering.
Direct digitizing with the track-and-hold path allows the capture of signals with
frequency components up to 12 MHz. The same path is used to subsample
repetitive signals up to 12 MHz.
Programming the 3458A for direct or subsampled (sequential) digitizing using
the track-and-hold path is simple. Only one command is required. For example,
DSAC provides direct sampling, AC coupled, or SSAC provides sequential
sampling, AC coupled. These commands automatically use default parameters
that can be changed.
Capturing the Data
The 3458A can be triggered to commence the measurement cycle by the level and
slope of the input signal, by a zero voltage level crossing of the power line, by the
GET (group execute trigger) command on the GPIB, by an external TTL signal,
by an internally generated trigger signal (for burst measurements, this can be
paced), and by the computer asking for a reading.
The 3458A provides all the tools you need to catch the signal of interest by offering
three levels of triggering and up to eight conditions to satisfy including the wave
form's level and slope. The hierarchy of trigger levels is trigger arming (TARM),
trigger (TRIG), and number of readings per trigger (NRDGS). Focused at
Figure 52. The 3458A
Multimeter provides
two different digitizing
paths, the standard
DCV path and a
track-and-hold path.
Figure 53. Capturing
the pulse amplitude of
narrow pulses requires
the use of the 12 MHz
track-and-hold path.
Note, the minimum time
between sample
acquisition and trigger
event is 175
nanoseconds.
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