4-2 Range, Units, Digits, Rate, and Filters Model 6485 Picoammeter Instruction Manual
Range, units, and digits
Range
The ranges for current measurements are listed in Table 4-1.
The full scale readings for every measurement range are 5% over range. For example, on
the 20µA range, the maximum input current is ± 21µA. Input values that exceed the maxi-
mum readings cause the overflow message OVRFLOW to be displayed.
Manual ranging
To select a range, press the RANGE or 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 with-
out 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. For example, if on the 20µA range, the instrument
will go up to the 200µA range when the input signal exceeds 21µA. While on the 200µA
range, the instrument will go down to the 20µA range when the input level goes below
20µA.
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 the RANGE or 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.
NOTE Range limits and groups are not in effect for manual ranging. Every range is
accessible with manual range selection.
Table 4-1
Measurement ranges
nA µA mA
2nA 2µA 2mA
20nA 20µA 20mA
200nA 200µA
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