HP 3468A - Page 54

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
AL®LBL
“DXOHN*
Ohms
18
GTO
68
82 ¥EG
“INI6Q*
Execute
initialization
program
iQ 1k
@3
“OPEN
TERNS.“
ii
:
110
‘Moh
28
1
£6
84
PROMPT
ERO
ies
MOM
Fa,
ke
Calculate
value
of
unknown
@5
SEQ
“MXOHN“
BLOCKS
program
22¢4LBL
a2
resistor
Store
inverted
value
in Reg.
23 FIR
2
ee
24 RAB
SIST.)
Measure
parallel
resistor
25 CLA
combination
26
ARCL
¥
18+
BL
81
Se
MAOHM.
fom:
BLORKS
27
“F
MDHN"
>
Display
value
of
resistor
1 xE@
-wxoHH:
— } PYO8
28 AVIEW
12 8
29 GT
81
SH+LBL
88
Calculate
value
of unknown
Fi *OVERLOAB-
resistor
3? AVIEW
Display
if overload
33 GTO
at
34 J ENB.
49