SECTION 2. OPERATING INSTRUCTIONS
1-l. 1NPUT CONNECTIONS.
a.
“inding Pose Terminals.
L. DMMMode. Uinding post terminals are pro-
vided o” the front panel for connections EO input
HI (Red), input LO (Black), and chassis ground
(ccee”). For voltmeter, ameter, and ohmmeter
measurements, the source should be conneceed be-
tween the lower set of red and black termi”als.
Tile green rennin=1 may be connected to input LO
by use of a shorting link when grounded operation
is desired. see paragraph 2-11 for a discussion
Of fl”aZi”g operation.
‘. Xilliobmecer Mode. A separate pair of red
and black terminals is provided for use when making
four-rermina1 reSiSLa”ce nleaeurements. The rnilli-
ohm ct~rrent source provides a positive dc current
at ihe iHI (Red) terminal and is used in conjuncrio”
wietl ci,e DMMterminals. See paragraph 2-9 for a
discussion of milliohmeeer operation. The Model
1641 Kelvin Test Lead Set (furnished with the Model
L64) simplifies connections co the resistance to be
measured. The Test Lead provides clip-o” connec-
tions for both voltage and current leads without
the need for separate cables.
b. Noise Considerations. The limit of resolution
in voltage and current measurements is derermined
largely hy rhe noise generated in the source. Stray
low-level noise is present in some form in nearly al1
electrical circuits. The instrument does not distin-
guish between stray and signal voltages since it mea-
sures the net voltage. When using the 1 mV and 10 mV
ranges, consider the presence of low-level elecrrical
phenomena such a~ thermocouples (thermoelectric ef-
fect,, flexing of coaxial cables (triboelectric ef-
fect), apparent residual charges on capacitors (die-
lectric absorption,, and battery actFan of two dFf-
ferenr contact materials (galvanic action).
1. Thermal Ems. Thermal ‘auf.9 (thermoelectric
porentiale) are generated by thermal differences
between two junctions of dissimilar metals. To
minimize ehe drift caused by thermal emfs, use cop-
per leads to connect ehe circuit to the i”s~rume”t.
The front or rear panel ZERO control cm, be used
to buck out thermal offsets if aecessary.
The KeiChley accessory Model 1483 Low Thermal Con-
nection Kit confains a11 necessary materials for
making very low thermal cop,xr crimp connections
for minimizing thermal effecrs.
2. A-C Electric Fields. The presence of elec-
tric fields
generaced
by power lines or other
s”“rceB can have an effect on instrument operation.
A-C voltages which are very large with respect to
the full-scale range sensieivicy could drive the
a-c amplifier into saturafian, thus producing a”
err”“e”“s d-c oq.tpue.
4
NOTE
The inscrumen~ provides attenuation of line
frequency noise superimposed on an input
signal. The a-c rejection is specified as
f0110!.W:
WXR: Greater rha” 80 dB above one digit
for a voltage of line frequency or rwice
line frequency an the nw5t sensitive range?.
decreasing to 60 dB on the 100-millivolt
and higher ranges. 1000 volts peak-to-peak
mCXXim”m.
CMRR: Greater than 120 dB above one digit
at dc and 40 to 100 Hz ac. 500 volts dc,
100 volts peak-to-peak ac maximum
Proper shielding as described in paragraph 2-1,~
can minimize noise pick-up when rhe insrrumenc is
in the presence of excessive a-c Eiclds or when very
sensitive measurements are being made.
3. Magnetic Fields. ‘The presence Of strong mag-
“etic fields can be a potential source of a--c noise.
Magnetic flux lines which cut a conductor can pro-
duce large a-c noise especially at power line fre-
quencies. The voltage induced due t” magnetic flux
is proportional to the area enclosed by the circuit
as well as the race of change of maflnetic flux. 2%”
effective wag co minimize magnetic pickup is LO ,’
=rra”ge all wiring SO Chaf fhe loop area enclosed
is as small as possible (such as twisting input leads).
h Second way t” minimize mnR”etic pickup is to use
shielding as described in paragraph 2-1,~.
C. Shielding.
1. Electric Fields. Shielding is usually “eces-
say, when the instrument is in the presence of very
large a-c fields or when very sensitive measureme”ts
are being made. The shields of the measurement cir-
cuit and leads should be connected together t” ground
at only one point. This provides a “tree” config-
uration, which minimizes ground loops.
2. Magnetic Fields. Magnetic shielding is use-
ful where very large magnetic fields are present.
Shielding. which is available in the form of plates,
foil or cables, ca” be used to shield the measuring
circuit, the lead wires, or the instrument itself.
3. other Considerations.
a) Voltmeter Measurements. “se shielded in-
put leads when so”rcr resistances are greater
than lmegohm or when long input cables are used.
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