Keithley 6517B Reference Manual page 83

Model 6517B Electrometer Reference Manual
Note that as R
noise is multiplied by a factor of two. Since decreasing the source resistance can have a detrimental
effect on noise performance, there are usually minimum recommended source resistance values
based on measurement range. The following table summarizes minimum recommended source
resistance values for various measurement ranges. Note that the recommended source resistance
varies by measurement range because the R
Minimum recommended
Range source resistance
pA
1 Gto 100 G
nA
1 Mto 100 M
µA
1 kto 100 k
mA
1 to 100 
Source capacitance
DUT source capacitance also affects the noise performance of the Model 6517B ammeter. In general,
as source capacitance increases, the noise also increases. To see how changes in source
capacitance can affect noise gain, let us again refer to the simplified ammeter model in the figure
below.
The elements of interest for this discussion are the source capacitance, C
capacitance C
noise gain formula must be modified as follows:
Here, Z
F
impedance formed by R
and
6517B-901-01 Rev. C / August 2015
decreases in value, the output noise increases. For example, when R
S
Figure 38: Source resistance and capacitance
. Taking into account the capacitive reactance of these two elements, our previous
F
represents the feedback impedance made up of C
and C . Furthermore,
S S
value also depends on the measurement range.
F
and R , while Z
F F
Section 4: Basic measurements
= R , the input
F S
and the feedback
S
is the source
S
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