Source Capacitance - Keithley 2500 User Manual

Models 2500 and 2502 User's Manual
Note that as R
R = R
F
resistance can have a detrimental effect on noise performance, there are usually minimum
recommended source resistance values based on measurement range.
rizes minimum recommended source resistance values for various measurement ranges for
the Model 2500 ammeters. Note that the recommended source resistance varies by mea-
surement range because the R
Table F-1
Minimum recommended source resistance values
I-measure range
1nA – 200nA
2µA – 200µA
2mA and 20mA

Source capacitance

DUT source capacitance will also affect the noise performance of the Model 2500 amme-
ters. In general, as source capacitance increases, the noise gain also increases.
The elements of interest for this discussion are the capacitance (C
internal feedback capacitance (C
reactance of these two elements, our previous noise gain formula must be modified as
follows:
Output V
where;
• Output V
• Input V
• Z
F
• Z
DUT
Furthermore,
Z =
F
and
Z
DUT
Note that as C
gain. Again, at the point where Z
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decreases in value, the output noise increases. For example, when
DUT
, the input noise is multiplied by a factor of two. Since decreasing the source
DUT
F
Minimum recommended
source resistance
1MΩ to 100MΩ
1kΩ to 100kΩ
1Ω to 100Ω
= Input V
NOISE NOISE
is the noise seen at the output of the ammeter.
NOISE
is the noise seen at the input of the ammeter.
NOISE
is the internal feedback impedance for the ammeter that is formed by C
is the internal impedance of the DUT that is formed by C
R
F
------------------------------------------- -
( )
2
2πf R C + 1
F F
R
S
= ------------------------------------------- -
( )
2
2πf R C + 1
S S
increases in value, Z
S
Measurement Considerations
value also depends on the measurement range.
) for the ammeter. Taking into account the capacitive
F
(1 + Z /Z )
F DUT
decreases in value, thereby increasing the noise
DUT
= Z , the input noise is amplified by a factor of two.
DUT F
F-3
Table F-1 summa-
) of the DUT and the
DUT
and R .
F F
and R .
DUT DUT