Guarding; Matrix Expansion Effects On Card Specifications; Figure 2-20 Shielded And Guarded; Figure 2-21 Guarded Circuit - Keithley 7174A Instruction Manual

Operation
To
7174A
Figure 2-20
Shielded and guarded

2.7.7 Guarding

Guarding is important in high-impedance circuits where
leakage resistance and capacitance could have degrading
effects on the measurement. Guarding consists of using a
shield surrounding a conductor that is carrying the high
impedance signal. This shield is driven by a low-impedance
amplifier to maintain the shield at signal potential.
Guarding minimizes leakage resistance effects by driving the
cable shield with a unity gain amplifier, as shown in Figure
2-21. Since the amplifier has a high input impedance, it min-
imizes loading on the high-impedance signal lead. Also, the
low output impedance ensures that the shield remains at sig-
nal potential, so that virtually no leakage current flows
through the leakage resistance, R
and outer shields may be considerable, but that leakage is of
little consequence because that current is supplied by the
buffer amplifier rather than the signal itself.
In a similar manner, guarding also reduces the effective cable
capacitance, resulting in much faster measurements on high-
impedance circuits. Because any distributed capacitance is
charged through the low impedance of the buffer amplifier
rather than by the source, settling times are shortened consid-
erably by guarding.
In order to use guarding effectively with the Model 7174A,
the guard path of the matrix card should be connected to the
guard output of the sourcing or measuring instrument. Figure
2-20 shows typical connections. Guard should be properly
carried through the inner shield to the device under test to be
completely effective. The shielded, guarded test fixture
arrangement shown in Figure 2-20 is also recommended for
safety purposes (guard voltage may be hazardous with some
instruments).
2-24
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Source
Measure
Guard
Chassis
. Leakage between inner
L
Signal Source and Measure
(Probe or Contact Point)
DUT
Inner Shield
(Guard or LO)
(Chassis Ground)
Inner Shield
Source or Measure
Signal
R
L
R
s
E
M
E
s
DUT
Figure 2-21
Guarded circuit
2.7.8 Matrix expansion effects on card
specifications
Specifications such as those given for path isolation and off-
set current are with a single Model 7174A card installed in
the mainframe. Expanding the matrix by internally connect-
ing two or more Model 7174A cards together will degrade
system performance specifications (other types of cards do
not affect the specifications because they use different path-
ways in the mainframe backplane). The extent depends on
how many cards are used, as well as the amount of cabling
used to connect them together.
With internal pathway expansion, isolation among paths is
increased, and offset current is decreased, although the isola-
tor relays on the card do help to minimize these effects.
Outer Shield
X1
Buffer
Guard
Measuring
Instrument