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 EMFerrors.
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
8502A
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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