Models 2500 and 2502 User’s Manual Measurement Considerations F-3
Note that as R
DUT
decreases in value, the output noise increases. For example, when
R
F
= R
DUT
, 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. Table F-1 summa-
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
F
value also depends on the measurement range.
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
DUT
) of the DUT and the
internal feedback capacitance (C
F
) for the ammeter. Taking into account the capacitive
reactance of these two elements, our previous noise gain formula must be modified as
follows:
Output V
NOISE
= Input V
NOISE
(1 + Z
F
/Z
DUT
)
where;
• Output V
NOISE
is the noise seen at the output of the ammeter.
• Input V
NOISE
is the noise seen at the input of the ammeter.
•Z
F
is the internal feedback impedance for the ammeter that is formed by C
F
and R
F
.
•Z
DUT
is the internal impedance of the DUT that is formed by C
DUT
and R
DUT
.
Furthermore,
and
Note that as C
S
increases in value, Z
DUT
decreases in value, thereby increasing the noise
gain. Again, at the point where Z
DUT
= Z
F
, the input noise is amplified by a factor of two.
Table F-1
Minimum recommended source resistance values
I-measure range
Minimum recommended
source resistance
1nA – 200nA
2µA – 200µA
2mA and 20mA
1MΩ to 100MΩ
1kΩ to 100kΩ
1Ω to 100Ω
Z
F
R
F
2πfR
F
C
F
()
2
1+
--------------------------------------------=
Z
DUT
R
S
2πfR
S
C
S
()
2
1+
--------------------------------------------=
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