Front Panel Operation
2-45
2.8.3 Charge measurement considerations
Some considerations for making accurate charge measure-
ments are summarized in the following paragraphs. Addi-
tional measurement considerations are summarized in
paragraph 2.21. For comprehensive information on precision
measurements, refer to the Low Level Measurements hand-
book, which is available from Keithley.
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 6517A 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 explained in paragraph 2.19.3 (OFF-
SET-ADJ).
Table 2-13
CONFIGURE COULOMBS menu structure
Menu item Description
SPEED
NORMAL
FAST
MEDIUM
HIACCURACY
SET-SPEED-EXACTLY
SET-BY-RSLN
Measurement speed (integration time) menu:
Select 1 PLC (power line cycle, 16.67msec for 60Hz, 20msec for 50Hz and 400Hz).
Select 0.01 PLC.
Select 0.1 PLC.
Select 10 PLC.
Set integration time in PLC (0.01-10).
Default to setting appropriate for resolution.
FILTER
AVERAGING
TYPE
NONE
AVERAGING
ADVANCED
AVERAGING-MODE
MEDIAN
DISABLE
ENABLE
Filter menu:
Configure digital averaging filter:
Select type of average filter:
No average filtering performed.
Program a simple average filter (1-100 rdgs).
Program a simple average filter (1-100 rdgs) with noise tolerance window (0-100% of
range).
Select moving average or repeating average mode.
Configure median filter:
Disable median filter.
Enable median filter and specify rank (1-5).
RESOLUTION
AUTO
3.5d, 4.5d, 5.5d, 6.5d
Display resolution menu:
Default to resolution appropriate for integration time.
Select a specific resolution.
AUTO-DISCHARGE
AUTORANGE
Enable (specify level) or disable auto discharge.
Select autorange limits (high or low).
EXTERNAL VOLTAGE SOURCE
When using an external voltage source, the input current
should be limited to less than 1mA by placing a resistor in se-
ries 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.
MEASUREMENT TIMES
Long measurement times may degrade charge measurement
accuracy. See the Model 6517A coulombs specifications in
Appendix A.
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