Optimizing Through The Track-And-Hold Path; (Direct Sampling And Subsampling); Ac Volts And Ac Current; Analog Acv - Agilent Technologies 3458A User Manual

Optimizing
Through the
Track-and-Hold
Path (Direct
Sampling and
Subsampling)

AC Volts and AC Current

Analog ACV

Synchronous ACV

Random ACV

328 Appendix D Optimizing Throughout and Reading Rate
As stated earlier, the standard DCV path directs the signal to the A to D Converter.
This path exhibits 150 kHz bandwidth and selectable resolution from 4 1/2 to 8
1/2 digits. The track-and- hold path exhibits 12 MHz bandwidth and 4 1/2 digits
of resolution. This path uses a 16 bit track-and-hold circuit between the input and
the A to D to take a "snapshot" of the input. DCV may be measured up to a
maximum reading rate of 50,000 readings per second through this path. The
commands for this choice of path are:
DSAC (direct sample, AC coupled)
DSDC (direct sample, DC coupled)
SSAC (subsampled, AC coupled)
SSDC (subsampled, DC coupled)
The Product Note 3458A-2, High Resolution Digitizing with the 3458A System
Multimeter, covers the use of these commands in detail along with their associated
trigger commands and constraints. In general, the aspects of these commands that
most influence throughput are those associated with ACV, where the 3458A
handles the task of measuring the rms value of either repetitive wave forms with
the synchronous ACV or noise measurements with random ACV. A detailed look
at the techniques and the trade-offs of the three methods of rms ACV measurement
is in the next section.
The 3458A Multimeter has the unique capability of offering the user three
different ways of measuring equivalent DCV heating value of an input wave form
(true root-mean-square value) : analog ACV, synchronous ACV and random
ACV. The input signal follows the track-and-hold path (see Figure 47) where it
may be routed into the analog AC-to-DC converter or the track-and-hold circuit.
The analog ACV offers broadband 10Hz to 2 MHz rms capability utilizing a
monolithic AC to DC converter. Its accuracy, while good, is not as good as the
synchronous ACV's; its bandwidth, while also good, is not as good as the random
or the synchronous ACV's. But, it does offer the ability to measure more
accurately, faster than either of the other methods over its measurement
bandwidth. And, it can measure either repetitive wave forms or noise signals.
Synchronous ACV offers 1 Hz to 10 MHz bandwidth with excellent 100 ppm best
accuracy, but the input wave form must be repetitive. The reading rate is
determined by the frequency of the input wave form and the desired accuracy and
resolution. The technique is straightforward: a frequency measurement is made
on the input wave form, the decision to sample the input sequentially, or in bursts
at 20 µs intervals, is made based on the value of the frequency, and the
measurements are processed statistically for the rms value. The number of samples
taken, which is a measure of the speed, is determined by the resolution selected
and also determines the accuracy of the measurement.
Random ACV offers the same upper measurement bandwidth that synchronous
ACV offers, but the wave form can be noise or any non-repetitive signal. Since