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8502A
NOTE
Errors
due
to
thermal
EMF's
should be
consid-
ered
when
making
low
level,
high resolution
measurements. Thermal
EMF's
(voltages pro-
duced by
temperature
differences
between
con-
tacts
of two
dissimilar
metals
or
by
temper-
ature gradients
along a
length
of
material)
may
cause
differences
in
potential
of
several
micro-
volts.
Since
the
8502A
utilizes
gold-plated
input
terminals, the
use
of
low
EMF,
shielded
cables
with
gold-plated spade
lugs will
minim-
ize
thermal
EMF
errors.
2-32.
Front
Panel Controls
2-33.
Most
front panel
controls
are
momentary
contact
switches.
If
held
in
continuously, such switches
will
be read
only
one
time. In addition,
some
of
these
switches
will
hold
a particular
reading
in the
8502A's
display for
as
long
as
they
are
held depressed. Front panel switches
may
have
1
,
2,
or
3 functions;
switch sequencing
will
determine
wliich
function
is
in
effect for multi-function switches.
Refer
to
"OPERATING
DIRECTIONS"
later
in
this
section for
detailed front pane! control procedures.
2-34.
Function
2-35.
The
DC
volts
function
is
standard
with
this instru-
ment;
optional
modules must
be included
for
all
other
functions.
Complete
information
for
any
options included
with
your
8502A
can be
found
in
Section
6 of
this
manual.
If
a
function
is
selected for
which
the
appropriate
module
is
not
installed,
"Error
9"
will
appear
in
the
display.
2-30.
Range
2-37.
DC
volts
can be measured on
five
successive
ranges
from 100
mV
to 1(X)0V. Respective
resolutions vary
from
1
uV
to
1
mV.
Input impedance on
the
lOOV
and
lOOOV
ranges
is
10
Mohms.
On
the
lower
three ranges
it is
greater
than
10,000
Mohms.
Overrange
capabilities,
DC
voltage
accuracies,
and
overload protection conditions
are
detailed
in
Section
1
of
this
manual.
2-38.
Modifiers
2-39.
FILTER
2-40,
The
8502A
has
five
filter
modes.
1
.
F
—
slow
filter,
no
time
out
—
FILTER
LED
on.
2.
FO
—
fast
filter,
no
time
out.
3.
FI
—
filter
bypass.
4.
F2
—
slow
filter,
with
time out
(550
msec,
settling
time)
-
FILTER
LED
on.
5.
F3
-
fast
filter,
with time out (50
msec,
settling
time).
("Fast"
and "slow"
refer
to the
respective
filter's
settling
time.)
2-41.
The
slow
filter
will
provide
better
line
frequency
rejection;
the
fast
filter
discriminates
against
higher
fre-
quency and
harmonically
related
line
noise.
Time
outs
are
always
inserted
before the
first
reading
is
taken
in
a
newly
selected
function or
range.
With
F2
or
F3
selected,
time
outs
will
always be
inserted
before
each subsequent
reading
in that
function
or range.
2-42.
SAMPLE
2-43.
Samples taken
in
the
8502
A
can be synchronous
or
asynchronous
to the
line
frequency. Line frequency
noise rejection
can be increased
by
averaging
more
samples
per displayed
reading
(refer
to
NOISE REJECTION
specif-
ications in
Table
1-3
of
this
manual.)
Provision
is
therefore
made
to
change
the
number
of
samples taken
for
each
reading (display update). Setting the
samples
per reading
will
yield a specific
reading time. Additional time per
read-
ing
will
be necessary
whenever
memoo'
operations
(Offset,
Scaling, etc.) are
involved.
Ohms
measurements
will
also
require
more
time
due
to the multiple
readings involved.
2-44.
For example,
if
rate
7
is
selected,
128 samples
will
be taken
and
averaged per
reading.
With no
memory
oper-
ations involved, digitizing
time
for a
dc
volts
reading
will
therefore take
533 ms. Sample
rate designations
(such
as
7)
are actually
the
number
of samples per
reading expressed
as
exponents of
2; rate
1
-
~ 128
samples
per reading.
2-45.
EXTERNAL REFERENCE
2-46.
An
external reference
dc
voltage
can be used
to
divide the
inputs
in
any
function.
A
true
ratio
measurement
is
then
computed by
the Controller.
Input
Reading =
dc
Ext. Ref. Input
2-47.
A
pair
of
external
reference terminals
will
be
found
either
separately
on
the
rear
panel or together
in
the
optional
rear
input connector.
]
.
The
applied external reference voltage
may
be
a
maximum
of
±20V
at either
external reference
HI
or
LO
or
40V
between HI and LO,
with
respect to
SENSE
LO
(or to
Ohms
Guard
in the rear
input
connector).
2-6
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