Section 7: Command reference
Model 2651A High Power System SourceMeter® Instrument Reference Manual
7-192
2651A-901-01 Rev. A / March 2011
smuX.measure.autozero
This attribute sets the behavior of the source-measure unit's (SMU's) A/D internal reference measurements
(autozero).
Type
TSP-Link accessible
Affected by
Where saved
Default value
Attribute (RW)
Yes
Instrument reset
SMU reset
Recall
Saved setups
smuX.AUTOZERO_AUTO
Usage
azMode
= smu
X
.measure.autozero
smu
X
.measure.autozero =
azMode
azMode
Indicates status of autozero; set to one of the following values:
0
or
smu
X
.AUTOZERO_OFF
: Autozero disabled
1
or
smu
X
.AUTOZERO_ONCE
: Performs autozero once, then disables autozero
2
or
smu
X
.AUTOZERO_AUTO
: Automatic checking of reference and zero
measurements; an autozero is performed when needed
X
SMU channel (for example,
smua.measure.autozero
applies to SMU channel A)
Details
The integrating ADC uses a ratio metric A/D conversion technique. To ensure the accuracy of readings, the
instrument must periodically obtain fresh measurements of its internal ground and voltage reference. The time
interval between updates to these reference measurements is determined by the integration aperture being used
for measurements. Separate reference and zero measurements are used for each aperture.
By default, the instrument automatically checks these reference measurements whenever a signal measurement
is made. If the reference measurements have expired 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 additional 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
smu
X
.measure.autozero
attribute can be used to disable the automatic reference measurements. Keep in
mind that disabling automatic reference measurements may allow the instrument to 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
smu
X
.AUTOZERO_ONCE
setting can be used to force a refresh of the reference and zero measurements
used for the current aperture setting.
Autozero reference measurements for the last 10 used NPLC settings are stored in a reference cache. If an
NPLC setting is selected and an entry for it is not in the cache, the oldest (least recently used) entry will be
discarded to make room for the new entry.
The fast ADC (analog to digital converter) does not need or use the reference measurements associated with
this attribute (only the integrating ADC). When this attribute is set to
smuX.AUTOZERO_AUTO
, the fast ADC will
not require these reference measurements. However, if the fast ADC is selected, setting this attribute to
smu
X
.AUTOZERO_ONCE
will cause the integrating ADC to acquire a new set of reference measurements.
Example
smua.measure.autozero = 1
Performs autozero once for SMU channel A. Alternatively,
the value 1 may be replaced with
smua.AUTOZERO_ONCE
.
