Keithley 2450 Reference Manual page 129

Section 2: General operation Model 2450 Interactive SourceMeter® Instrument Reference Manual
Considerations for the 10 nA and 100 nA ranges
If you are operating the Model 2450 using the 10 nA and 100 nA ranges, you need to take extra
precautions to get accurate readings.
Be aware that these ranges may be selected if you have autoranging selected.
If you are using the 10 nA or 100 nA range, use the following precautions.
Never leave the FORCE HI terminal open to the environment. Current can couple into FORCE HI and
cause measurable current readings from unlikely sources. If you are operating from the rear
terminals, the triaxial connections will place guard around the FORCE HI and SENSE HI terminals so
that outside current is unlikely to affect the measurement.
Place the device under test inside a shielded enclosure (LO) so that the guards surrounding FORCE
HI and SENSE HI are inside the enclosure as close to the device under test as possible before
dropped.
If you are using the front panel connections, shield the FORCE HI terminal with LO. While this will not
prevent leakage, it will prevent noise and external signals from coupling into the FORCE HI terminal.
If you are using multiple instruments in an application, if possible, connect the LO terminals of each
instrument together. This prevents common-mode current from flowing through the measurement
circuits of the Model 2450. While the Model 2450 isolates measurement circuitry from earth ground, it
is still possible to have hundreds of nanoamps of earth-referenced (common mode) current at the LO
terminal. If the Model 2450 HI terminal is connected to the LO terminal of another instrument, the
common-mode current will flow into the Model 2450 measurement circuits, possibly driving the Model
2450 into source limit.
Another method for preventing common-mode current from affecting measurements is to provide an
earth reference for the measurement. The earth connection will allow the common-mode current of
the offending instrument to flow directly to earth, not through the measurement circuits of the Model
2450.
Selecting a specific source range
If you select a specific source range, the range must be large enough to source the value. If not, an overrange
condition can occur.
If an overrange condition occurs, an event is displayed and the change to the setting is ignored.
The fixed current source ranges are 10 nA, 100 nA, 1 µA, 10 µA, 100 µA, 1 mA, 10 mA, 100 mA, and 1 A.
The fixed voltage source ranges are 20 mV, 200 mV, 2 V, 20 V, and 200 V.
Using the front panel:
1. Press the HOME key.
2. Under SOURCE, select the button next to Range. The Select Range dialog box is displayed.
3. Select the range. You are returned to the Home screen.
Using SCPI commands:
To set the current source range, send the following command, with <n> set to the source range:
SOURce:CURRent:RANGe <n>
To set the voltage source range, send the following command, with <n> set to the source range:
SOURce:VOLTage:RANGe <n>
2-102 2450-901-01 Rev. B/September 2013