Keithley 6521, Instruction Manual, Page: 35 / 65

Operation
3-7
3.5.3 Card configuration
Settling time
Use the STIMe command to program the channel settling
time, which can be programmed in the range of 0sec to
999.999sec. For example, to program a 50msec settling time,
send the following command:
:ROUTe:SCAN:STIMe 50E-3
Scan method
The scanner card can be configured for either voltage or cur-
rent operation. When in the voltage mode, the card is config
ured for break-before-make operation. When in the current
mode, the HI and LO terminals of each input channel remain
shorted together when the channels are open.
Scan method is programmed with the SMEThod command.
For example, to configure the card for current operation,
send the following command:
:ROUT:SCAN:SMET CURR
Similarly, send the following command to configure the card
for voltage operation:
:ROUT:SCAN:SMET VOLT
Setting the voltage source limit
The maximum recommended voltage for the Model 6522
card is 200V, while the Model 6521 is limited to 30V for
safety reasons. Use the VSLimit command to program the
voltage limit to 200V. For example, the following command
sets the voltage limit to 200V:
:ROUTe:SCAN:VSLimit 1
3.5.4 Bus scanning
Follow the steps below to perform scanning over the bus.
Step 1: Program internal scan list
Use the :ROUT:SCAN:INT command to program a scan list
(channels you wish to scan). For example, assume that you
wish to scan channels 1, 3, and 5. This scan list would be pro-
grammed by sending the following command:
:ROUT:SCAN:INT (@ 1,3,5)
Step 2: Configure scanner card
Configure the scanner card as outlined previously. For exam-
ple, the following commands will program the card for a set-
tling time of 100msec, current operation, and a voltage
source limit of 200V:
:ROUT:SCAN:STIM 0.1
:ROUT:SCAN:SMET CURR
:ROUT:SCAN:VSL 1
Step 3: Enable scan
After programming the scan list and functions, use the
:ROUT:SCAN:LSET INT command to enable scanning as
follows:
:ROUT:SCAN:LSET INT
The scanning sequence will then begin using the scan list and
other scan parameters previously programmed.
To disable a scan sequence in progress, send the following
command:
:ROUT:SCAN:LSET NONE
3.6 Measurement considerations
Many measurements made with the aid of the Model 6521/
22 Scanner Cards are subject to various effects that can seri-
ously affect low-level measurement accuracy. The following
paragraphs discuss some of these effects and ways to mini-
mize them.
For further information on making accurate low-level mea-
surements, refer to the
Low Level Measurements
handbook
available from Keithley Instruments. Contact the Keithley
Instruments Applications Department for more information.
3.6.1 Generated currents
Any extraneous generated currents in a test system will add
to the desired measured current, degrading sensitive current
measurements. The following paragraphs discuss the more
commonly encountered generated currents and methods to
minimize their effects when using the Model 6521/22 Scan-
ner Card.
Offset currents
Offset currents can be generated from a variety of internal or
external sources. Internal offsets can be generated within the
meter or scanner card itself, while external offsets can be
present in connecting cables, connectors, or test fixtures