Keysight Technologies 3458A Guide D'utilisation page 438

D Optimizing Throughout and Reading Rate
DC Volts, DC Current and Resistance
The 3458A offers two separate measurement paths: the standard DCV path direct
to the Analog to Digital Converter and a path to the track-and hold circuit
(track-and-hold path). The DCV path is limited to 150 kHz bandwidth, the
track-and-hold path can accept signals up to 12 MHz. The track-and-hold path is
limited to 16 bits of resolution unless repeated measurements are made. The DCV
path can present up to 8 1/2 digits (27 bits) resolution.
Optimizing through the DCV path
The classic trade-offs one can make with the 3458A are measurement speed
versus measurement resolution. Because of early design decisions to reduce the
intrinsic Johnson noise associated with real resistive components in the input path
of the 3458A, the resolution of the integrated measurement is 3 times better than
with dmms of previous generations. For example, with the 3457A one may make a
6 1/2 digit (3,000,000 count) measurement with one power line cycle of
integration (PLC) or 17 ms; with the equivalent integration period, the 3458A may
make a 7 1/2 digit measurement (12,000,000 counts). Similarly, extreme care is
taken to insure the linearity is excellent, a factor of 10 times better than the
3457A. The result is faster, more accurate measurements than ever before. It also
means that one can take advantage of the increased accuracy and resolution and
make measurements at 1 PLC with the 3458A that previously would have taken 10
PLC.
For the measurement that requires only high speed, or a trade-off of resolution
and accuracy without line noise as an issue, the 3458A provides a range of
alternatives from 4 1/2 digits at 500 nanoseconds aperture to 8 1/2 digits at 1
seconds aperture and anywhere in between in 100 nanosecond steps.
shows the aperture versus measurement speed, noise, resolution and accuracy.
From the graph in
integration period on reading rate; hidden is the influence of the HPML
commands on throughput and some of the basic operating methods of the 3458A.
HPML is an application-oriented command set. The basic philosophy behind this
command set is that you don't need know what the 3458A is doing to make the
measurement but need only to understand the measurement you want to make.
To optimize throughput for any complex application, however, requires more
understanding of the operation of the 3458A than simply to make a measurement.
Many of the trade-offs you will make involve trading speed for accuracy and
438
Figure D-1, one can see the influence of the actual aperture or
Figure D-1
Keysight 3458A User's Guide