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The extended ohms feature of the 3468A is available only via the F7 remote pro-
gramming
command.
With extended
ohms, the 3468A
can be used to measure
resistances
in excess
of 30
Mohms.
When
in the extended
ohms
mode,
the
3468A
goes to the 30 Mohm
range, 2-wire mode. An internal resistance of ap-
proximately
10 Mohms
is placed in parallel (internally) with the input terminals. If
this resistance is measured first and then your unknown resistor connected to the
input terminals; the parallel combination can be measured. A calculation can then
be performed
to to determine the approximate value of the unknown
resistance.
The formula for the calculation is:
Ri * Rt
Rx = i Rt
Rx is the unknown
resistance, Ri is the measured value of the internal 10 Mohm
resistor and Rt is the measured
value of the parallel combination.
The test leads
used should be very short, preferably a shielded twisted pair, to minimize noise
pick up.
A program
to make
the necessary
measurements,
perform the calculations,
and
display the value of the unknown resistor is given below. Follow the instructions
displayed on the -hp- Model 41C/41CV. First it will indicate: OPEN TERMS. This
means
to remove
the test leads from the 3468A.
When
the message
- APPLY
RESIST.
- appears, connect the unknown
resistance to the 3468A.
The value of
that resistance will be displayed on the calculator. Notice the use of MXOHM
sub-
program in lines O5 and
11.
Label
-
Display
Extended
49
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