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Section 4: Basic measurements
Model 6517B Electrometer Reference Manual
AUTO-DISCHARGE
The AUTO-DISCHARGE option is used to enable or disable auto discharge. When enabled, auto
discharge resets the charge reading to zero at the specified level. After the integrator resets, the
charge measurement process simply restarts at zero.
The AUTO-DISCHARGE selections are as follows:
OFF: Use this selection to disable auto discharge. With auto discharge disabled, you can use zero
check to reset the integrator.
ON: Use this selection to enable auto discharge. After selecting ON, you are prompted to enter the
discharge level. The charge reading resets every time the specified charge level is reached. Note that
if you specify a level that exceeds the measurement range, the display overflows before the integrator
resets.
AUTORANGE
The AUTORANGE option is used to configure autorange for the coulombs function. This option
allows you to speed up the autoranging search process by eliminating the low (2 nC and 20 nC) or
high (200 nC and 2 µC) measurement ranges. For example, if you know that the readings do not
exceed 10 nC, you can select LO autorange limits. When the instrument autoranges (assuming
AUTO range is enabled), it does not search into the high ranges.
LO (2 nC-20 nC): Use this option to limit the autorange search to the low measurement ranges.
HIGH (200 nC-2 µC): Use this option to limit the autorange search to the high measurement ranges.
Charge measurement considerations
Some considerations for making accurate charge measurements are summarized in the following
paragraphs (see
Other measurement considerations
(on page 4-35) for additional measurement
considerations). For comprehensive information on precision measurements, refer to the Low Level
Measurements Handbook, which is available from Keithley Instruments.
Input bias current
A primary consideration when making charge measurements is the input bias (offset) current of the
integrating amplifier. Any such current is integrated along with the input signal and reflected in the
final reading. The Model 6517B has a maximum input bias of 4fA (4 10
-15
A) for change at 23°C.
This input offset translates into a charge of 4fC per second at a temperature of 23°C. This value must
be subtracted from the final reading to obtain the correct value.
Input bias current may be reduced by performing the offset adjustment procedure (OFFSET-ADJ).
External voltage source
When using an external voltage source, the input current should be limited to less than 1 mA by
placing a resistor in series with the high input lead. The value of this resistor should be at least:
R = 1000 V (ohms)
Where V is the voltage across the resistor, or the compliance of the current being integrated.
4-34
6517B-901-01 Rev. C / August 2015
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