Current Measurements; Current Ranges; Autorange; Zero Check And Correct - Keithley 486 Instruction Manual

SECTION3
Front Panel Operation
3.5 CURRENT MEASUREMENTS
3.5.1 Current Ranges
VRANGEA
El El
AUTO
RANGE
Range selection provides control over the sensitivity of
the measurement. The available current ranges of the
Model 4&j/487 are listed in Table 3-5. Also included in
the table for each range are maximum reading and maxi-
mum input voltage overload.
Table 3-5. Current Ranges
2nA 219999nA
35OVdc
z&IA 21.9999nA
35OVdc
2ochA 219.999nA 35OVdc
lipA
2.19999pA 3SOVdc
21.9999pA 5OVdc
2wA
219.999&4 5OVdc
2.19999mA 5OVdc
wgiler voltages require current to be limited to
3mA.
CALITION
Do not exceed the maximum input voltage as
specified in Table 3-5. Otherwise, damage to
the Model 486/487 may result
NOTE
After an overload occurs, it may take several
minutes for the instrument to recover and dis-
play a settled reading. In general, the more se-
vere the overload, the longer it take to re-
COVeI-.
Manual range selection is accomplished with the two
ranging keys. Each press and release of the RANGE A
key upranges the instrument to the next higher current
range. Conversely, RANGE v downranges the instru-
ment to the next lower current range. Range selection can
also be controlled automatically and is discussed in the
next paragraph.
3.5.2 Autorange
SHIFT
El
AUTO
RANGE
When autorange is enabled (AUTO RANGE indicator
on), the instrument will automaticaliy go to the lowest
possible (most sensitive) current range to make the meas-
urement. Autorange is enabled by pressing SHIFT and
then AUTO RANGE in that order. To disable autorange,
again press SHIFT and then AUTO RANGE. Autorange
will also disable by manually selecting a different range
using the 'I and A range keys.
CAUTION
To avoid possible damage to the instnunent,
do not apply more than 5OVdc to the input
when autorange is enabled (unless an exter-
nal series resistor to limit current to 3mA is
used).
3.5.3 Zero Check and Correct
--..-
SHIFT
CHECK
q El
CORRECT
When zero check is enabled, the input amplifier is inter-
nally discomected from the input connector of the in-
strument. A 1COkQ resistor shunts the input connector,
and the instrument is configured to measure and display
the offset of the selected range.
When zero correction is performed, the offset is meas-
ured and algebraicaLly subtracted from every subsequent
reading Cmhding zero checked readings). This correc-
tion value is "remembered" and used until a new correc-
tion value is established by again performing zero correc-
tion. Also, an established correction value can be retained
as a default condition on power up (see paragraph 3.8.6).
Zero check is enabled by pressing ZERO CHECK (ZERO
CHECK indicator turns on). Pressing ZERO CHECK a
second time disables the feature.
3-5