Keithley 6487 - Page 312

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