Optimizing Through The Dcv Path - Agilent Technologies 3458A User Manual

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
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
Through the DCV
Johnson noise associated with real resistive components in the input path of the
Path
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
l/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 l/2 digits at 500 nanoseconds aperture to 8 1/2 digits at 1
seconds aperture and anywhere in between in 100 nanosecond steps. Figure 44
shows the aperture versus measurement speed, noise, resolution and accuracy.
From the graph in Figure 44, one can see the influence of the actual aperture or
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 convenience. The
HPML commands that most affect the throughput speed from a measurement
viewpoint are:
FUNC<DCV, DCI, OHM, FOHM>,<range>,<resolution in %>
NPLC #
APER<integration period s>
RES <resolution in %>
AZERO,<on or off>
In Table 30 you'll see that NPLC and APER commands are somewhat
interchangeable. The significant difference between these two commands is that
NPLC actually uses the power line frequency to establish the integration period
for the chosen multiple or submultiple of the line frequency. The APER command
sets the integration period in fundamental units of seconds from 500 ns to 1s in
100 ns steps. Operating at 60 Hz line frequency, for example, the choice of NPLC
1 is equal to APER 0.016666.
324 Appendix D Optimizing Throughout and Reading Rate