Automatic Reference Measurements - Keithley 2450 Reference Manual

Section 2: General operation
Using SCPI:
To query the upper limit for voltage measurements, send the command:
:SENSe:VOLTage:RANGe:AUTO:ULIMit?
To set the upper limit for resistance measurements, send the command:
:SENSe:RESistance:RANGe:AUTO:ULIMit <n>
Where <n> is the highest resistance measurement range that can be used.
Using TSP commands:
To view the upper limit for voltage measurements, send the commands:
smu.measure.func = smu.FUNC_DC_VOLTAGE
smu.measure.autorange = smu.ON
print(smu.measure.highrange)
Where highRange is the highest voltage measurement range that can be used.
To set the upper limit for resistance measurements, send the commands:
smu.measure.func = smu.FUNC_RESISTANCE
smu.measure.autorange = smu.ON
smu.measure.highrange = highRange
Where highRange is the highest resistance measurement range that can be used.

Automatic reference measurements

To ensure the accuracy of readings, the instrument must periodically get new measurements of its
internal ground and voltage reference. The time interval between updates to these reference
measurements is determined by the integration aperture that is being used for measurements. The
Model 2450 uses separate reference and zero measurements for each aperture.
By default, the instrument automatically checks the reference measurements whenever a signal
measurement is made. If the reference measurements have expired when a signal measurement is
made, the instrument automatically takes two more readings, one for the internal ground and one for
the voltage reference, before returning the result. This can cause some measurements to take longer
than normal.
This additional time can cause problems in sweeps and other test sequences in which measurement
timing is critical. To avoid the time that is needed for the reference measurements, you can disable
the automatic reference measurements.
When automatic reference measurements are turned off, the instrument may gradually drift out of
specification. To prevent inaccurate readings, you can send a command that gets autozero
information on a one-time basis.
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Model 2450 Interactive SourceMeter® Instrument Reference Manual
2450-901-01 Rev. B/September 2013