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2600S-901-01 Rev. C / January 2008
Section 4: Basic Operation
Series 2600 System SourceMeter® Instruments Reference Manual
Operation considerations
The following paragraphs discuss the warm-up period and auto zero.
Warm-up
The SourceMeter must be turned on and allowed to warm up for at least two hours to achieve
rated accuracies. See Appendix A for specifications.
Auto zero
The Series 2600 SourceMeters use a ratiometric A/D conversion technique. To ensure accuracy of
readings, the instrument must periodically obtain fresh measurements of its internal ground and
voltage reference. The time interval between needing to update these reference measurements is
determined by the integration aperture being used for measurements. Separate reference and
zero measurements are used for each aperture.
There are three different settings for auto zero as summarized in
. By default, the
instrument automatically checks these reference measurements whenever a signal measurement
is made (AUTO). If the reference measurements are out of date when a signal measurement is
made, the instrument will automatically take two more A/D conversions, one for the reference and
one for the zero, before returning the result. Thus, occasionally, a measurement takes longer than
normal.
This extra time can cause problems in sweeps and other test sequences in which measurement
timing is critical. To avoid the extra time for the reference measurements in these situations, the
OFF selection can be used to disable the automatic reference measurements. Keep in mind that
with automatic reference measurements disabled, the instrument may gradually drift out of
specification.
To minimize the drift, a reference and zero measurement should be made just prior to the critical
test sequence. The ONCE setting can be used to force a refresh of the reference and zero
measurements used for the current aperture setting.
NPLC caching
NPLC caching speeds up operation by caching A/D reference and zero values for up to the five
most recent measurement function settings. Whenever the integration rate is changed via the
SPEED key, or a user setup is recalled, NPLC caching will occur. If the integration rate is already
stored in the cache, the stored reference and zero values are recalled and used. Otherwise, a
reference and zero value are acquired and stored in the cache. If there are already five NPLC
values stored, the oldest one will be overwritten by the newest one. When auto zero is off, NPLC
values stored in the cache will be used regardless of how old they are. If there are no entries in the
cache for the aperture being used, the unit will acquire them when the first measurement is made.
Table 4-4
Auto zero settings
Auto zero
setting
Description
OFF
Turns automatic reference measurements off.
ONCE
Turns automatic reference measurements on, forcing one reference
and one zero measurement.
AUTO
Automatically takes new acquisitions when processor determines refer-
ence and zero values are out-of-date.
