Basic Source-Measure Operation 3-7
Temperature changes across components within the instrument can cause the reference and
zero values for the A/D converter to drift due to thermo-electric effects. Auto zero acts to negate
the effects of drift in order to maintain measurement accuracy over time. Without auto zero
enabled, measurements can drift and become erroneous.
Front panel auto zero
Set the auto zero from the front panel as follows:
1. Press the MENU key.
2. Select A/D CTRL from the main menu, then press ENTER.
3. Select AUTO ZERO, then press ENTER.
4. Select ENABLE or DISABLE as appropriate, then press ENTER.
5. Press EXIT as necessary to return to normal display.
Remote command auto zero
Use the :SYSTem:AZERo command to enable or disable auto zero via remote. For example,
send the following command to disable auto zero:
:SYST:AZER ON
Conversely, send this command to disable auto-zero:
:SYST:AZER OFF
NPLC caching
NPLC caching speeds up source memory sweeps by caching A/D reference and zero values.
When NPLC caching is enabled (using the NPLC-CACHE/ENABLE menu selection), the A/D
reference and zero values will be saved for up to the 10 most recent voltage, current, and resis-
tance measurement function settings. Whenever the integration rate is changed via the SPEED
key, a recalled user setup (using the SAVESETUP/RESTORE menu selection), or during a
source memory recall (either with the SOURCE-MEMORY/RESTORE menu or during a
source memory sweep), 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 10 NPLC values stored, the
oldest one will be overwritten by the newest one.
NOT
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Auto zero should be disabled for maximum source memory sweep speed; otherwise
the cache is of little use. With auto zero enabled, new A/D reference and zero values
are taken for every reading and saved into the cache, slowing down sweep operation.
However, with auto zero disabled, measurements may drift and become erroneous.
To minimize drift when using NPLC caching with auto zero disabled, periodically
select AUTO-ZERO/ONCE in the A/D-CTRL menu to force an immediate auto zero
update.
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