To achieve rated accuracy when sourcing lkn or lOk0 to
an electrometer, zero must be used to cancel ZERO OFF-
SET and test lead resistance. When’zero ohms is sourced
to an electrometer, the resistance measurement will in-
clude these resistances. The electrometer can then sup-
press the reading so that it also reads zero.
The procedures to zero out offset and test lead resistance
are included in the sourcing techniques in paragraph 3.10.
3.7 GUARD
In general, the purpose of guarding is to eliminate leakage
resistance and capacitance that exist between output high
and output low. In the unguarded configuration (see
Figure 3-S), leakage occurs between the center conductor
(HI) and the inner shield (LO) of the output t&w cable.
This leakage (through R, and C,) may be enough to
adversely affect the output level and response time of the
source.
In the guarded configuration (see Figure 3-9), output low
is connected to common, allowing a guard potential (equal
to the potential at ouput high) to be connected to the in-
ner shield of the cable. With the voltage potential on out-
put high and the inner shield the same, no leakage cur-
rent can flow. The leakage from the guarded inner shield
to low (through R, and C,) is of no consequence since the
current is supplied from the guard source (V,) and not
from output high. More information on the principles of
guarding is contained in paragraph 3.11.3.
The Model 263 supplies the guard drive for AMPS and
COUL. However, the user must supply the guard drive
for AMPS V/R, COUL V/R, OHMS and VOLTS. When
GUARD is enabled with any of these functions selected,
the inner shield of the triax connector is floating unless
a guard drive is supplied by the user.
Guard Enable-To place the output of the Model 263 in
a guarded configuration press SHIFT GUARD. The yellow
GUARD light will turn on. Pressing SHIFT GUARD a se-
cond time will disable guard and return the output to the
unguarded configuration.
Whenever GUARD is enabled, source output low is
physically connected to the COMMON terminal.
Therefore, always use a separate cable to connect COM-
MON of the Model 263 to common of the device under
test (DUT).
Figure 3-8. Unguarded Circuit
3-10
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