11-12 Digital I/O, Analog Outputs, and External Feedback
Shielded fixture construction
Since shielding is so critical for proper operation of external feedback, it is recommended that
a shielded fixture similar to the one shown in Figure 11-8 be used to house the feedback element.
The fixture is constructed of a commercially available shielded fixture modified with the stan-
dard BNC connectors replaced with triaxial female connectors. For convenience, a banana jack
can be mounted on the box to make the necessary preamp out connection.
Alternately, a wire could be run through a rubber grommet mounted in a hole in the side of
the box. Note that input low is connected to chassis ground within the shielded box. This con-
nection can be made by using a small solder lug secured with a screw.
Preamp Out
Input
(Chassis)
To Ranging
Amplifier
Common
+
-
Op Amp
Zero Check
<1Ω
HI
LO
S
C
FB
R
FB
100MΩ
Figure 11-7
Electrometer input
circuitry (external
eedback mode)
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