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Front Panel Operation
2.11 Using external feedback
The external feedback function provides a means to extend
the capabilities of the Model 6517A Electrometer to such
uses as logarithmic currents, non-decade current ranges, as
well as non-standard coulombs ranges. The following para-
graphs discuss the basic electrometer input circuitry and
methods to implement these functions.
2.11.1 Electrometer input circuitry
A simplified diagram of the electrometer input in the external
feedback mode is shown in Figure 2-42. An input current ap-
plied to the inverting (-) input of the op amp is nulled by a
current feedback through the internal feedback network
made up of R
and C
. Because the output of the op amp
FB
FB
appears at the PREAMP OUT, this internal network can be
replaced by an external network connected between the
preamp output and Input HI connections. When using exter-
nal feedback, the following factors must be taken into ac-
count:
100MΩ
HI
Input
LO
1Ω
Common
Preamp Out
(Chassis)
Figure 2-42
Electrometer input circuitry (external feedback mode)
2-54
C
FB
R
FB
Zero Check
Op Amp
To Ranging
Amplifier
S
4. The maximum current value that can be supplied by the
preamp output is 20mA in amps and ohms (1mA in
volts). The maximum voltage span in external feedback
is ±20V.
5. The input impedance in the external feedback mode is
given by the relationship Z
the impedance of the external feedback network, and A
is the open-loop gain of the electrometer (typically
6
greater than 55×10
). Note that the input impedance is
Z
= 10M Ω || Z
when zero check is enabled.
IN
FB
6. The voltage at the PREAMP OUT terminal is given by
the formula:
V = -IZ
7. Any feedback elements should be housed in a suitable
shielded enclosure (see paragraph 2.11.2 below). Insula-
tors connected to Input HI should be made of Teflon or
other high-quality insulating material and should be
thoroughly cleaned to maintain the high input imped-
ance and low input current of the Model 6517A. If these
insulators become contaminated, they can be cleaned
with methanol and then with clean, pressurized air.
2.11.2 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 2-43 be used to house the feedback
element. The fixture is constructed of a commercially avail-
able shielded fixture modified with the standard BNC con-
nectors 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
connection can be made by using a small solder lug secured
with a screw.
= Z
/A
, where: Z
IN
FB
V
FB
FB
is
V
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