Guarding; Figure 2-23 Guarded Circuit - Keithley 7072 Instruction Manual

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OPERATION
2.7.7 Guarding
In a similar manner, guarding also reduces the effective
cable capacitance, resulting in much faster measurements
Guarding is important in high-impedance circuits where
on high-impedance circuits. Because any distributed
leakage resistance and capacitance could have degrading
capacitance is charged through the low impedance of the
effects on the measurement. Guarding consists of using
buffer amplifier rather than by the source, settling times
a shield surrounding a conductor that is carrying the high-
are shortened considerably by guarding.
impedance signal. This shield is driven by a low-
impedance amplifier to maintain the shield at signal poten-
tial. For triaxial cables, the inner shield is used as guard.
In order to use guarding effectively with the Model 7072,
the GUARD path of the matrix card should be connected
Guarding minimizes leakage resistance effects by driving
the cable shield with a unity gain amplifier, as shown in
Figure 2-23. Since the amplifier has a high input im-
pedance, it minimizes loading on the high-impedance
signal lead. Also, the low output impedance ensures that
the shield remains at signal potential, so that virtually no
leakage current flows through the leakage resistance, R,.
Leakage between inner and outer shields may be con-
siderable, but that leakage is of little consequence because
that current is supplied by the buffer amplifier rather than
the signal itself.
to the guard 'output of the sourcing or measuring instru-
ment. Figure 2-24 shows typical connections. Guard
should be properly carried through the inner shield to the
device under test to be completely effective. The shield-
ed, guarded test fixture arrangement shown in Figure 2-22
is recommended for safety purposes (guard voltage may
be hazardous with some instruments). With most in-
struments, special adapters or cables may be required to
connect guard to the inner shield, and at the same time
route signal Lo through a separate cable.
Inner Shield
Signal
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. - ,. Buffer ,...-. I
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Measuring
Instrument
Figure 2-23. Guarded Circuit