Keithley 6485, User Manual

Page: 72 / 115

4-2 Range, Units, Digits, Rate, and Filters Model 6485 and 6487 User’s Manual
Range, units, and digits
Range
The ranges for current measurements are listed in Table 4-1 .
Table 4-1
Measurement ranges
nA
μ
A mA
2nA 2
μ
A 2mA
20nA 20
μ
A 20mA
200nA 200
μ
A
The full scale readings for every measurement range are 5% over range. For example, on the
20
μ
A range, the maximum input current is ± 21
μ
A. Input values that exceed the maximum
readings cause the overflow message (OVRFLOW) to be displayed.
Manual ranging
To select a range, press the desired manual RANGE key. The instrument changes one
range per key-press. If the instrument displays the OVRFLOW message on a particular
range, select a higher range until an on-range reading is displayed. Use the lowest range
possible without causing an overflow to ensure best accuracy and resolution.
Autoranging
When using autorange, the instrument automatically goes to the most sensitive available
range to measure the applied signal. Up-ranging occurs at 105% of range, while
down-ranging occurs at the range value. The AUTO key toggles the instrument between
manual ranging and autoranging. The AUTO annunciator turns on when autoranging is
selected. To disable autoranging, press AUTO or a manual RANGE key. Pressing AUTO
to disable autoranging leaves the instrument on the present range.
Every time an autorange occurs, a search for every available range of the selected function
is performed. The time it takes to perform the search could slow down range change speed
significantly. Setting upper and/or lower autorange limits can reduce search time.
Autorange limits
Search time for finding the correct amps range can be reduced by setting upper and/or
lower autorange limits. To set upper and/or lower autorange limits, press CONFIG then
either manual RANGE key (for UPPER or LOWER), then scroll through the available
range limits using the RANGE keys. Press ENTER when the desired range is flashing.