Appendix D Optimizing Throughout and Reading Rate 329
the resolution of the measurement is dependent upon the number of samples, this
mode of operation is the least accurate and the slowest of the ACV functions for
high resolution. Aliasing (discussed in detail in the Digitizing Product Note
3458A-2) is avoided by a random selection of sampling intervals from 20 to 40
µs in 10 ns increments.
Comparison of ACV
Modes
With all three ACV modes of operation, the user has the option of selecting
accuracy versus speed if the input frequency allows. Referring to Table 31, the
frequency dependency of the reading rate is most pronounced for analog ACV: 1
reading per second from 10 Hz to 1 kHz, 10 readings per second from 1 kHz to
10 kHz, and 50 readings per second from 10 kHz to 2 MHz. These reading rates
pertain to the specified accuracies for analog ACV. The reading rates of all three
modes of operation can be increased by selecting either less resolution or by
decreasing the delay time from the default times to a time interval of ten times the
reciprocal of the highest frequency component present on the input signal. Hence,
to capture a signal of 1kHz, a delay time of at least 10 ms is needed for a
representative measurement of the wave form.
AC Current AC current measurements are made strictly through the analog ACV section with
the voltage input being supplied by the DCI shunts. While there is no real decision
to make regarding the mode of ACI measurement, you can decide to accept less
accuracy and speed up the reading rate by decreasing the integration and settling
time. As a rule of thumb, the AC to DC converter needs at least 10 cycles of the
input wave form to give representative rms measurements. Hence, the frequency
of the input has a direct impact on the reading rate. Characterization of the 3458A
may be necessary to fine tune the measurement throughput for either ACV or ACI
to fit your application.
Figure 47. Signal path
block diagram offers
three techniques for
ACV measurement.
Table 31: Compares the ACV modes.
Analog Synchronous Random
Bandwidth 10 Hz to 2
MHz
1 Hz to10
MHz
20 Hz to 10
MHz
Best
Accuracy
300 ppm 100 ppm 1000 ppm
Reading Rate 50 rdgs/s 10 rdgs/s 40 rgds/s
Crest Factor 5:1 5:1 5:1
Waveforms All Repetitive All
To Next Page
Loading...
Need help?
General
