Basic Sampling - Keithley 194 Instruction Manual

GETTING STARTED
Measurement-a series of samples stored in internal
memory. You can control the number of samples and how
fast they are taken with the SAMPLES and RATE keys.
Reading-a measurement that is mathematically processed
in some way and then displayed on the front panel
OI
transmitted over the IEEE-488 bus. Typical processes in-
clude waveform (display a single sample), average (average
the samples in the measurement and display the result),
and standard deviation (take the standard deviation of the
measurement and display it).
2.6.2 Sampling Discussion
Using the procedure discussed in paragraph 2.5, the Model
194 appears to operate much like an ordinary DMM, in
that a reading immediately appears on the display. Actual-
ly, the instrument is taking a number of samples, digitiz-
ing the analog value, and storing the result in its internal
memory. The resulting sequence of samples is called a
measurement. While in the continuous trigger mode (as
in this example), the display is continuously updated with
the sample stored in the memory location at the trigger
point (assuming the waveform mode is in effect). Samples
stored in the remaining locations can be accessed by us-
ing the RECALL button.
Figure 2-9 demonstrates the basic idea behind signal
sampling. Here, a time-varying signal with the amplitude
shown is being sampled at specific intervals. As each sam-
ple is taken, it is digitized into an S-bit or 16.bit binary value
(depending on the sampling rate and number of samples
in the measurement) and stored in memory. The complete
sampling sequence is a measurement, as defined above.
Once the measurement sequence is performed, a reading
can be generated by processing the block of samples in
s"me way. For example, to obtain the true RMS value of
the measurement (remember a measurement is a series
of samples), you would use the TRMS function. By using
the instrument in a single trigger mode, you could apply
a variety of different mathematical functions to a single
measurement. Conversely, a single mathematical process
could be applied to a variety of different signals by using
the instrument in the continuous trigger mode. A single
sample can be displayed by using the waveform mode (in
which case the sample at the trigger point is displayed),
or by using the RECALL button to display individual
SCl"lpleS.
v
Figure 2-9. Basic Sampling
2.7 TYPICAL OPERATING MODES
The following information will help you t" select the
various operating modes for commonly encountered
voltages and waveforms. Keep in mind that these are in-
tended only as a starting point. Some experimentation may
be required to determine the optimum instrument con-
figuration for a particular measuring situation based on
your particular waveform analysis requirements.
Table 2-5 summarizes recommended mode selection for
four c"mm"n voltages or waveforms: a DC voltage of 3Ov;
a 60Hz, 2.82V RMS sine wave; a ZV, lkHz symmetrical rec-
tangular waveform; and a 50V peak, 15.734kHz sawtooth
waveform.
When selecting operating modes, the following points
should be kept in mind:
1. The selected range should be high enough to handle
the peak value of the waveform you are measuring. For
example, the peak value of a 30V RMS sine wave is
42.4V. Thus, you would have to place the instrument
on the ZOOV range to properly measure this waveform.
Keep in mind that the instrument will normally display
the OFLO (overrange) err"r message even if only one
sample is overrange.
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