2-2 Measurement Concepts Model 6485 Picoammeter Instruction Manual
Measurement overview
The basic measurement capabilities of Model 6485 are summarized in Table 2-1. Accu-
racy for each measurement function and range is listed in specifications (Appendix A).
Performance considerations
Warm-up period
Model 6485 can be used within one minute after it is turned on. However, the instrument
should be turned on and allowed to warm up for at least one hour before use to achieve
rated accuracy. If the instrument has been exposed to extreme temperatures, allow extra
time for the internal temperature to stabilize.
Autozero
To help maintain stability and accuracy over time and changes in temperature, the Model
6485 periodically measures internal voltages corresponding to offsets (zero) and amplifier
gains. These measurements are used in the algorithm to calculate the reading of the input
signal. This process is known as autozeroing.
When autozero is disabled, the offset and gain measurements are not performed. This
increases measurement speed up to 3 times. However, the zero and gain reference points
can eventually drift resulting in inaccurate readings of the input signal. It is recommended
that autozero only be disabled for short periods of time.
To disable autozero from the front panel, press the AZERO button. This button toggles
autozero on and off. It can also be enabled by restoring factory or GPIB default conditions.
When autozero is enabled, a colon will be displayed after the reading.
For example:
Autozero disabled: 0.00258 nA
Autozero enabled: 0.00258 nA:
Table 2-1
Basic measurement capabilities
Function Reading Range Available Ranges
Amps ±20fA to ±21mA 2nA, 20nA, 200nA, 2uA, 20uA, 200uA, 2mA and 20mA
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