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2.10 USING EXTERNAL
FEEDBACK
External feedback
provides
a means to extend the capabilities
of the Model 617 Electrometer
to such uses as logarithmic
cur-
rents,
non-decade
current
ranges,
as well as non-standard
coulombs
ranges. The following
paragraphs
discuss the basic
electrometer
input circuitry
and methods
to implement
these
functions.
2.10.1 Electrometer
Input Circuitry
A simplified
diagram
of the electrometer
input
in the amps
mode is shown in Figure
2-17.
An input current
applied to the
inverting
(-)
input of the op amp is nulled
by a current
fed
back through
the internal
feedback
network
made up of Rm
and CFB. Because
the output
of the op amp 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
external
feedback,
the
following
factors must be taken into account:
1. The maximum
current
value
that can be suppiied
by the
preamp
output
is 20mA in amps (1mA in V/R).
2. The input
impedance
in the external
feedback
mode
is
given by the relationship
21~ = Z@A",
where Zm is the
impedance
of the external feedback
network,
and A" is the
open-loop
gain of the electrometer
(typically
greater
than
106). Note that the input impedance
is ZIN = lOM0 II 2~
when zero check is enabled.
3.
The voltage
at the PREAMI'
OUT terrrtinal is given by the
formula:
V=
-IRFB,
where
Rm
is
the
value
of the feedback
resistance.
4. Any
feedback
elements
should
be housed
in a suitable
shielded
enclosure.
Insulators
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 impedance
and low input current
of the Model
617. If these insulators
become
contaminated,
they can be
scrubbed
with
methanol
and
then
dried
with
clean,
pressurized
air.
2.10.2 Shielded Fixture Construction
Since shielding
is so critical for proper
operation
of external
feedback,
it is recommended
that the shielded
fixture shown
Figure 2-17. Electrometer Input Circuitry (Amps
Mode)
in Figure 2-18 be used to house the feedback
element.
The fix-
ture
is constructed
of a Pomona
#2390
shielded
fixture
modified
with the standard
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 ru11 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.
2-22
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