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
(
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
11M
Ω
200nA or 2
µ
A
3.5M
Ω
20
µ
A or 200
µ
A
50k
Ω
2mA or 20mA
500
Ω
The series limiting resistor should have a minimum power rating of:
Example: If measuring 100G
Ω
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 M
Ω
= 14M
Ω
minimum power rating = (500V)
2
/14M
Ω
=18mW
NOTE
The 14M
Ω
in series will increase the measured resistance to 100.014G
Ω
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
.
MinR
series
SourceVoltage
220V
–
220V
-----------------------------------------------------------
R
in
=
MinPowerRating
SourceVoltage
2
R
series
⁄
=
Содержание 6485
Страница 1: ...Model 6485Picoammeter Instruction Manual A G R E A T E R M E A S U R E O F C O N F I D E N C E ...
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Страница 44: ...1 20 Getting Started Model 6485 Picoammeter Instruction Manual ...
Страница 66: ...3 6 Measurements Model 6485 Picoammeter Instruction Manual ...
Страница 138: ...9 20 Remote Operation Model 6485 Picoammeter Instruction Manual ...
Страница 159: ...11 CommonCommands ...
Страница 164: ...11 6 Common Commands Model 6485 Picoammeter Instruction Manual ...
Страница 165: ...12 SCPISignalOriented MeasurementCommands ...
Страница 190: ...14 12 SCPI Reference Tables Model 6485 Picoammeter Instruction Manual ...
Страница 214: ...16 14 Calibration Model 6485 Instruction Manual ...
Страница 219: ...A Specifications ...
Страница 221: ...B StatusandErrorMessages ...
Страница 227: ...C GeneralMeasurement Considerations ...
Страница 233: ...D DDCEmulationCommands ...
Страница 247: ...E ExamplePrograms ...
Страница 250: ...E 4 Example Programs Model 6485 Picoammeter Instruction Manual ...
Страница 251: ...F IEEE 488BusOverview ...
Страница 266: ...F 16 IEEE 488 Bus Overview Model 6485 Picoammeter Instruction Manual ...
Страница 267: ...G IEEE 488andSCPI ConformanceInformation ...
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