Model 6517B Electrometer Reference Manual
Section 4: Basic measurements
6517B-901-01 Rev. C / August 2015
4-15
Note that as R
S
decreases in value, the output noise increases. For example, when R
F
= R
S
, the input
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
F
value also depends on the measurement range.
Range
Minimum recommended
source resistance
pA
nA
µA
mA
1 G
to 100 G
1 M
to 100 M
1 k
to 100 k
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.
Figure 38: Source resistance and capacitance
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, our previous
noise gain formula must be modified as follows:
Here, Z
F
represents the feedback impedance made up of C
F
and R
F
, while Z
S
is the source
impedance formed by R
S
and C
S
. Furthermore,
and
