Keithley SourceMeter 2600 Series Reference Manual page 143

Series 2600 System SourceMeters 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
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are out of date when a signal measurement is made, the instrument will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
Return to Section 4 topics
Basic Operation
Table 4-4. By
2600S-901-01 Rev. A / May 2006
4-7