Keithley 6485 - Source Capacitance

I-6 Applications Guide Model 6485 Picoammeter Instruction Manual
Figure I-3
Simplified model of a feedback picoammeter
Source capacitance
DUT source capacitance will also affect the noise performance of the Model 6485 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 I-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 reac-
tance of these two elements, the previous noise gain formula must be modified as follows:
Table I-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
6485
Picoammeter
Current
Source
Output V
NOISE
Input V
NOISE
1Z
F
/Z
S
+()=