Measurement Considerations
F-5
Noise and source impedance
Noise can seriously affect sensitive current measurements. This section discusses how DUT
(device under test) resistance and capacitance affect noise performance.
DUT resistance
The resistance of the DUT will affect the noise performance of the ammeter. As the DUT
resistance is reduced, the noise gain of the ammeter will increase. Noise gain can be given by
the following equation:
Output V
NOISE
= Input V
NOISE
(1 + R
F
/R
DUT
)
where;
•
Output V
NOISE
is noise seen at the output of the ammeter.
•
Input V
NOISE
is the noise seen at the input of the ammeter.
•
R
F
is the internal feedback resistance for the ammeter.
•
R
DUT
is the resistance of the DUT.
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 resis-
tance can have a detrimental effect on noise performance, there are usually minimum recom-
mended source resistance values based on measurement range. Table F-1 summarizes
minimum recommended source resistance values for various measurement ranges for the
ammeter. Note that the recommended source resistance varies by measurement range because
the R
F
value also depends on the measurement range.
Table F-1
Minimum recommended source resistance values
I-measure range
Minimum recommended
source resistance
1pA
–
100pA
1nA
–
100nA
1µA
–
100µA
1mA
–
100mA
1G
Ω
to 100G
Ω
1M
Ω
to 100M
Ω
1k
Ω
to 100k
Ω
1
Ω
to 100
Ω
Summary of Contents for 6430
Page 26: ......
Page 32: ......
Page 78: ...2 14 Connections ...
Page 98: ...3 20 Basic Source Measure Operation ...
Page 138: ...5 30 Source Measure Concepts ...
Page 156: ...6 18 Range Digits Speed and Filters ...
Page 168: ...7 12 Relative and Math ...
Page 176: ...8 8 Data Store ...
Page 202: ...9 26 Sweep Operation ...
Page 248: ...11 22 Limit Testing ...
Page 310: ...16 6 SCPI Signal Oriented Measurement Commands ...
Page 418: ...17 108 SCPI Command Reference ...
Page 450: ...18 32 Performance Verification ...
Page 477: ...A Specifications ...
Page 489: ...B StatusandErrorMessages ...
Page 498: ...B 10 Status and Error Messages ...
Page 499: ...C DataFlow ...
Page 503: ...D IEEE 488BusOverview ...
Page 518: ...D 16 IEEE 488 Bus Overview ...
Page 519: ...E IEEE 488andSCPI ConformanceInformation ...
Page 523: ...F MeasurementConsiderations ...
Page 539: ...G GPIB488 1Protocol ...
Page 557: ......