Model 6487 Reference Manual Measurement Concepts and Connections 2-7
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 2μA3.5MΩ
20μA or 200μA50kΩ
2mA or 20mA 510Ω
The series limiting resistor should have a minimum power rating of:
Example: If measuring 100GΩ resistances using an external voltage source of 750V, 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
= (750V - 505V)/505V×11 MΩ = 12.25MΩ
minimum power rating = (750V)
2
/14MΩ =41mW
klqb The 12.25MΩ in series will increase the measured resistance to 100.012GΩ
The Model 6487 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 505V–
505V
-----------------------------------------------------------
⎝⎠
⎛⎞
R
in
=
MinPowerRating SourceVoltage
2
R
series
⁄=
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