Measurement Considerations
F-7
open. This current is known as the input offset current, and it is caused by bias currents of
active devices as well as by leakage currents through insulators within the instrument.
The internal input offset current adds to the source current so that the meter measures the
sum of the two currents:
I
M
= I
S
+ I
I0
where; I
M
is the measured current.
I
S
is the source current.
I
I0
is the internal input offset current.
Internal current noise
—
Peak-to-peak noise over any one minute interval will be within
400aA during 90% of the interval for following conditions:
•
The triax input connectors of the Remote PreAmp capped, and on the 1pA range for at
least three minutes.
•
Auto
fi
lter on.
•
Auto zero off.
•
Source delay set to 0 sec.
External offset current
—
Offset currents can also be generated from external effects, such
as electrochemical effect (discussed below). The external offset current also adds to the source
current, and the ammeter again measures the sum of the currents:
I
M
= I
S
+ I
I0
+ I
EO
where; I
EO
is the external offset current.
As long as the internal and external offsets remain stable for a reasonable period of time, the
Relative feature of the SourceMeter can be used to zero out offset current. With the source cur-
rent (I
S
) set to zero, the ammeter will only measure and display the offset current reading.
When REL is enabled, the display will zero. What happens is that the offset current reading is
acquired as the rel value and is subtracted from present and future readings. When the source
current is applied, the displayed reading will not include the offset:
I
M
= I
S
+ I
I0
+ I
EO
- rel
I
M
= I
S
where; rel is the rel value (I
I0
+ I
EO
).
As long as the rel value equals the sum of the offsets, only the source current will be dis-
played. See Section 7 for details on Relative.
Offset current drift is a function of time and/or temperature. Therefore, when offset current
drifts signi
fi
cantly, you will have to re-zero the ammeter.
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: ......