3-2 Measurements Model 6485 Picoammeter Instruction Manual
Measurement overview
Measurements — Model 6485 can make amps measurements from 20fA to 21mA using
8 measurement ranges; 2nA, 20nA, 200nA, 2µA, 20µA, 200µA, 2mA, and 20mA.
NOTE Accuracy specifications are provided in Appendix A.
Procedure
WARNING The maximum safe voltage between picoammeter LO and chassis
ground (common mode voltage) is 42V. The Model 6485 does not inter-
nally limit the LO to chassis voltage. Exceeding 42V can create a shock
hazard.
If it is possible for the DUT or external supply to present more than
42V to the input HI, it is imperative that the connection between input
LO and the external voltage source be sufficiently low impedance and
capable of carrying the short-circuit current of the source, in order that
the LO not exceed 42V.
CAUTION The LO to chassis breakdown voltage is 500V. Exceeding this voltage
may cause damage to the instrument.
The maximum input voltage and current to Model 6485 is 220V peak
and 21mA. Exceeding either of these values may cause damage to the
instrument that is not covered by the warranty.
To achieve optimum precision for low-level current measurements, input bias current and
voltage burden can be minimized by performing the offset correction procedure. Informa-
tion about these offsets are provided in “Measurement considerations,”on page I-2.
NOTE After overloading with high voltage, it may take several minutes for the input
current to drop to within specified limits. Input current can be verified by plac-
ing the protection cap on the input BNC connector and then use the ground link
to connect COMMON and CHASSIS ground. With the instrument on the 2nA
range and zero check disabled, allow the reading to settle until the input bias
current is within specifications. The specifications for input bias current are
included in the offset portion of the accuracy specification listed in Appendix A.
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