118 Chapter 4 Making Measurements
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.
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.
10 OPTION 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
80 OUTPUT 722;"TRIG SYN" !SYN TRIGGER EVENT
90 ENTER 722;Rdgs(*) !SYN EVENT, ENTER NULL CORRECTED READINGS
100 PRINT Rdgs(*) !PRINT NULL CORRECTED READINGS
110 END
The following program performs the post-process NULL operation on 20
readings. After executing the MMATH NULL command, 21 readings are
taken and stored in reading memory in FIFO mode. Line 80 recalls the first
reading taken which is stored in the OFFSET register. The value in the
OFFSET register is then changed to 3.05. The remaining 20 readings in
memory are recalled and the NULL operation is performed on each.
10 OPTION 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;"MEM FIFO" !ENABLE READING MEMORY, FIFO MODE
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