Keysight Technologies 3458A Guide D'utilisation page 178

4 Making Measurements
NULL
The NULL operation subtracts a value from each reading (following the first
reading). The equation is:
After you select the NULL operation, the first reading made (real-time) or the first
reading taken from memory (post-process) is stored in the OFFSET register. The
value of this reading is then subtracted from all subsequent readings. If you do not
want the first reading to be the null value, you can write another value to the
OFFSET register using the SMATH command. You must wait, however, until after
the first reading is made (real-time) or recalled (post-process) before changing
the value.
A typical application of the NULL operation is in making more accurate 2-wire
ohms measurements. To do this, select 2-wire ohms (OHM command) and short
the ends of the test leads together. Now enable the NULL operation. The first
reading taken (the lead resistance) is stored in the OFFSET register. Connect the
test leads to the unknown resistance to be measured. The multimeter then
subtracts the value in the OFFSET register from all subsequent readings until the
math NULL operation is disabled. This method is not as accurate as 4-wire ohms
because the resistance of the test leads connected together probably will not be
the same as when they are connected to the unknown resistance. Also, the
resistance of the test leads is checked only once for a series of measurements and
the test lead resistance may change.
The following program performs the real-time NULL math operation on 20
readings. After executing the NULL command, the first reading is triggered by line
50. The value in the OFFSET register is then changed to 3.05. The 20 readings are
triggered by line 90 and 3.05 is subtracted from each reading.
10OPTION BASE 1!COMPUTER ARRAY NUMBERING STARTS AT 1
20 DIM Rdgs(20)!DIMENSION ARRAY FOR 20 READINGS
30 OUTPUT 722;"PRESET NORM"!PRESET, NRDGS 1,AUTO, DCV 10
40 OUTPUT 722;"MATH NULL"!ENABLE REAL-TIME NULL MATH OPERATION
50 OUTPUT 722;"TRIG SGL"!TRIGGER 1 READING, STORED IN OFFSET
60 OUTPUT 722;"SMATH OFFSET,3.05"!WRITE 3.05 TO OFFSET REGISTER
70 OUTPUT 722;"NRDGS 20"!20 READINGS PER TRIGGER
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Result = Reading - OFFSET
Where:
OFFSET is the value stored in the OFFSET register (typically the first reading).
Reading is any reading following the first reading.
Keysight 3458A User's Guide