Model 6485 Picoammeter Instruction Manual Measurement Concepts 2-7
To prevent accidental damage, a series resistor should be added to the test setup. The min-
imum value of this series resistor depends on the lowest current range to be used in the
measurement. If it will not be necessary to use the lower measurement ranges, a smaller
series resistor can be used, reducing the effect it will have on measurement accuracy. The
lowest necessary measurement range can be determined from the measurement range
accuracy specs, the applied voltage and largest resistance desired to measure. If using auto
range, program the Model 6485 to not use its lowest ranges when autoranging.
To set the auto range lower limit from the front panel:
1. Press the CONFIG key.
2. Press the down range key ().
3. Use the and range keys to scroll through the available lower limit settings.
4. Press ENTER to save the displayed value as the lower limit. Press EXIT to return
to the previous setting.
To set the auto range lower limit over the bus, use [CURRent]:RANGe:AUTO:LLIMit
(Section 4).
Use the following formula to determine the minimum resistance for proper current limit-
ing resistors:
Lowest range to be used R
in
2nA or 20nA 11MΩ
200nA or 2µA 3.5MΩ
20µA or 200µA 50kΩ
2mA or 20mA 500Ω
The series limiting resistor should have a minimum power rating of:
Example: If measuring 100GΩ resistances using an external voltage source of 500V, and
thus, a lowest necessary current range of 20nA, the minimum series resistance that will
prevent damage in the case of a shorted resistor would be:
minimum R
series
= (500V - 220V)/220V×11 MΩ = 14MΩ
minimum power rating = (500V)
2
/14MΩ =18mW
NOTE The 14MΩ in series will increase the measured resistance to 100.014GΩ
The 6485 can be programmed to calculate the resistance and subtract the series resistance.
Using the M/X+B function, in the example above, one would set M to 500, B to -14e6, and
the units character to “omega”. For more details on the M/X+B function, see Section 5.
MinR
series
SourceVoltage 220V–
220V
-----------------------------------------------------------
R
in
=
MinPowerRating SourceVoltage
2
R
series
⁄=
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