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G-6
Applications Guide
Table G-1
Minimum recommended source resistance values
2nA, 20nA
200nA, 2µA
20µA, 200µA
2mA, 20mA
Source capacitance
DUT source capacitance will also affect the noise performance of the Model 6487 picoam-
meter. In general, as source capacitance increases, the noise also increases. To see how
changes in source capacitance can affect noise gain, again refer to the simplified picoam-
meter model in
capacitance (C
reactance of these two elements, the previous noise gain formula must be modified as
follows:
Output V
Range
Minimum recommended source resistance
1 MΩ to 1 GΩ
10 kΩ to 10 MΩ
100Ω to 100 kΩ
10Ω to 1kΩ
Figure G-3
Simplified model of a feedback picoammeter
C
S
Z
R
S
S
V
S
Current
Source
Figure
G-3. The elements of interest for this discussion are the source
) and the feedback capacitance (C
S
=
Input V
NOISE
NOISE
C
F
R
F
-
+
V
noise
6487
Picoammeter
). Taking into account the capacitive
F
(
)
+
1
Z
/Z
F
S
Model 6487 Reference Manual
Z
F
V
O
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