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3330B
2
'
22
.
Guarded
f^easurements
2-23.
INTRODUCTION. When
using the
Model 3330B
significant errors
may
arise
out of undesired conversion
of
common
mode
signals
to
normal
mode.
This instrument
has
a
guard
shield that
functions to
minimize
common
mode-to-normal
mode
conversion.
The
following
para-
graphs
describe
errors that
may
arise
out of
common
mode
problems and
provide
a
means
to
reduce
them.
2-24.
COMMON MODE
REJECTION RATIO (CMRR)
In
non-guarded
applications, a
path
for
undesired
common
mode
signals
is
created
by
the
stray
capacitance
C
i
(Figure
2-5) representing the
circuitry-to-case
capacitance
in
the
instrument. This capacitance enables frequency
dependent
currents to
appear
in the
connecting
leads
between
the
in-
strument
and
load,
and
in
the instrument
circuitry
itself.
2-25.
The
guard
shield
provides a
means
of
making
the
ciicuitry-to-case
capacitance very
small,
thereby reducing
common
mode
currents.
This
is
done by
placing a
shield,
or guard,
between
the
circuitry
and
case.
Addition of
the
guard
results essentially in
two
things;
it
provides
a separate
path
for
common mode
signals,
and
it
effec-
tively
prevents
the introduction
of
common
mode
errors
in
the
circuitry
itself.
Where
common mode
signals
previously
had
a
path through
Cj
into
the
circuitry
and
connecting
leads
and through
R§, insertion
of
the guard
now
provides
a separate
path through
C
3
and
into
the guard
lead.
In addition, the
circuitry-to-case
capacitance
is
significantly
reduced,
and
the
CMRR
of
the
instrument
is
greatly
enhanced.
2-26.
SUGGESTIONS
FOR
USING
THE GUARD.
Under
most
applications
it is
satisfactory to strap
the front
panel
GUARD
to the
ground
terminal
of
the load. In applications
where
common
mode
signals are
of
no
concern, the
GUARD
may
be connected
to the
(ground)
terminal.
2-28.
The
VOLTAGE
TRIP
switch
and
VERNIER
con-
trols
provide protection
for
external
equipment
by
limiting
the
maximum
allowable
output
voltage to the external load.
The
range
of
voltage
limiting
is
selected
with the
VOLTAGE
TRIP
switches.
Refinement of
the value
of
the voltage to
be
limited
is
accomplished with
the
VERNIER
control.
a.
Without any
load connected
to
the
OUTPUT
ter-
minals, depress the
STDBY/RESET
switch
and
set
the
front-panel
controls to
the
following
positions:
NOT
GUARDED
Cl
Sirav
capHclTBiice, eirciiitryto
ofisB
C2
Strnv
capacitaiKe,
circuitry-to-guarcl
C3
Striiy
capncitimco, guard-io-casa
R;
ShiKid
resistance
Egrn
Common
mode
potential
Figure
2-5.
GUARDED
LOAD CONNECTIONS
MODE
Desired
voltage range
VOLTAGE
TRIP
To
the lowest voltage range
that
overlaps the desired
trip voltage.
CURRENT
LIMIT
As
desired
Decade
Dials
Desired
trip
voltage
b.
Depress the
OPR
switch.
c.
Slowly
slide
the
VERNIER
control to the
left
until
the
I
LIMIT/V TRIP
indicator illuminates
and
the
OPR
lamp
extinguishes.
The
voltage
trip
is
now
set
to
the value indicated
on
the
Decade
Dials,
and
the
instrument
is
tripped to the
STDBY
mode.
d.
To
reset
the instrument,
set
the
Decade
Dials
to a
value
less
than the
trip voltage,
depress the
STDBY/
RESET
switch,
and
then
depress the
OPR
switch.
2-30.
The
CURRENT
LIMITcontrol
provides
a
means
of
limiting the
amount
of output
current applied to the load.
Iflimiting
within the
output
current range of the
instrument
is
not
desirable,
set
the
CURRENT
LIMIT
control
to
HI.
Should
some
degree of
current limiting
be
desirable,
pro-
ceed
as
follows:
a.
Depress
the
STDBY
RESET
switch,
and
set
the
front
panel controls
to the
following
positions:
MODE
Desired current
(M A)
range
VOLTAGE
TRIP
As
desired.
However, must
be
>10V
for
fuU
scale
output
current.
To
set
a
precise
level,
use the voltage
trip
setting
procedure,
given eralier,and
set
the
Decade
Dials
to:
Rev.
4/75
2-5
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