Model 6485 Picoammeter Instruction Manual
Applications Guide
I-5
Figure I-2
Voltage burden considerations
Noise and source impedance
Noise can seriously affect sensitive current measurements. The following paragraphs dis-
cuss how source resistance and input capacitance affect noise performance.
Source resistance
The source resistance of the DUT will affect the noise performance of current measure-
ments. As the source resistance is reduced, the noise gain of the picoammeter will
increase, as we will now discuss.
shows a simplified model of the feedback picoammeter. R
S
and C
S
represent the
source resistance and source capacitance, V
S
is the source voltage, and V
NOISE
is the noise
voltage. Finally, R
F
and C
F
are the feedback resistance and capacitance respectively.
The source noise gain of the circuit can be given by the following equation:
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.
recommended source resistance values for various measurement ranges. Note that the rec-
ommended source resistance varies by measurement range because the R
F
value also
depends on the measurement range.
Rs
Vs
I
M
=
R
S
V
S
- V
B
I
V
B
(Voltage
Burden)
+
-
+
-
Programmable
V-Source
6485
Picoammeter
Output V
NOISE
Input V
NOISE
1
R
F
/R
S
+
(
)
=
Содержание 6485
Страница 1: ...Model 6485Picoammeter Instruction Manual A G R E A T E R M E A S U R E O F C O N F I D E N C E ...
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Страница 44: ...1 20 Getting Started Model 6485 Picoammeter Instruction Manual ...
Страница 66: ...3 6 Measurements Model 6485 Picoammeter Instruction Manual ...
Страница 138: ...9 20 Remote Operation Model 6485 Picoammeter Instruction Manual ...
Страница 159: ...11 CommonCommands ...
Страница 164: ...11 6 Common Commands Model 6485 Picoammeter Instruction Manual ...
Страница 165: ...12 SCPISignalOriented MeasurementCommands ...
Страница 190: ...14 12 SCPI Reference Tables Model 6485 Picoammeter Instruction Manual ...
Страница 214: ...16 14 Calibration Model 6485 Instruction Manual ...
Страница 219: ...A Specifications ...
Страница 221: ...B StatusandErrorMessages ...
Страница 227: ...C GeneralMeasurement Considerations ...
Страница 233: ...D DDCEmulationCommands ...
Страница 247: ...E ExamplePrograms ...
Страница 250: ...E 4 Example Programs Model 6485 Picoammeter Instruction Manual ...
Страница 251: ...F IEEE 488BusOverview ...
Страница 266: ...F 16 IEEE 488 Bus Overview Model 6485 Picoammeter Instruction Manual ...
Страница 267: ...G IEEE 488andSCPI ConformanceInformation ...
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