HP 3455A - Page 25

Model 34SSA
Section ill
3-19.
Math
Featura.
3-20. The math
feature of th 3455A
allows
the
measure-
ment value to be
offset and/or
scaled by
known
values
or to be
expressed in
percent of a
reference
value.
3-21. Scala
Mode. The scale mode of
the math
feature is
described
by the
formula: result
=
where x is the
y
measurement
value,
z
is the offset
value, and
y
is the
scale factor.
This mode
allows the
measurement
value
to
be
modified by the
addition, subtraction,
multiplication
or division
of a
known value. Addition
and subtraction
are
performed by
entering the
number to be
added or
subtracted in "z”
and entering I
in
“y”-
The scale
for-
mula
then
becomes: result
«
x
-
(A
z)
s
x
-
(±
z).
1
Division is performed by
entering 0 in
*'z’’ and
the
divisor value
in
“y.”
The
scale
formula then
becomes:
result
=
x
-
0
=
x.
Multiplication
is perform-
y y
performed by
dividing the
measurement value by the in-
verse of the
multiplier
value; that is,
multiplication is
performed by
dividing by a
fraction. The
scale formula
becomes:
result
=
.
x
-
0
=
xy. As
an example: to
1/y
multiply by 10,
divide by the
inverse of 10
which is 1/10
or
.1. Various
examples using the
scale mode
are as
follows:
a. Current
Measurement: Accurate
current
measurements can be
made
by
using a
low value resistor
shunting the 34S5A’s
input terminals.
The value of the
resistor is
then entered in the
“y”
register (see
Paragraph 3-22), and zero is
entered in the "z”
register
With the
resistor connected at the input
terminal and the
instrument set in the
voltage mode, current
measurements can
now be made. You can do
this by
connecting the
input across the resistor
and measuring
the voltage drop
across the resistor. This
voltage drop is
proportional to
the current through the
resistor. By
switching the
34SSA to the scale mode,
the reading
becomes an accurate current
reading in milliamps. Since
the
resistor
value
is
in kilo ohms (R) and stored
in
*‘y”.
and since
zero is stored in
*‘z", the scale
equation
becomes:
x-y
_
current in milliamps
y
R R
where R
=
Resistor across the input
terminals
V
=
Voltage drop across
the resistor
b.
Temperature Measurement: A
temperature mea-
surement can
be made by using a
line or
resistive
temperature sensor.
Assume
that the sensor has a
resistance of I kilohm at
25“C and
changes 5900
ppm/*C. At 0®C the
sensor
would have a resistance of 852. S
ohm
(1
kilohm
-
[5.9
ohms] 25). This
number is
divided
by
1000 since the
34S5A
measurement results are expressed in kilohm
and
is
entered in the “z”
register
to
remove the offset at
0”C. The
measurement result of (he 34S5A is scaled to
read
directly in degrees centigrade by
solving the equa-
tion for the value of
“y”.
This is done
where the results
of
the equation are equal to
25”
C
since the sensor
resistance is specified at that
temperature. The scale
equation becomes:
25
=
1 K-.8525 K
,
.1475 K
y y
y
solving for
y:y
=
^
=
.0059
with this number
25
entered
in
the
“y”
register, the 3455A
measurement
result
will be presented directly in ”C.
c.
Accurate 2 Wire Ohm
Measurement: When trying
to
make an accurate 2
wire
ohm
measurement, the input
lead resistance
and the internal resistance of
the
3455A
should be
subtracted out from the
reading. This is done
by
setting the instrument to the desired
range and short
the
input leads at the
measuring
point.
Store a 1 in
"y"
and
store the input lead resistance
reading in "z”. Open
the input leads and
connect the unknown resistor to the
leads.
With the 3455A set in the Scale
mode, the value
of
the
unknown
resistor is
displayed without the input
lead resistance.
Since a 1 is stored in
"y"
and
the lead
resistance (R) is stored in “z”, the
scale equation
becomes:
^~y
=
=
unknown
resistance in
ohms
Y 1
where
x
=
total measured resistance
including R
R
=
lead resistance
3-22. Error Mode.
The error
mode of the math
feature is described by
the formula: result in
g
x-y
x
y
100,
where “x” is the present measurement
value
and
“y”
is the reference
value. An application of this
feature might be an inspection test of resist rs.
This
nominal resistor
value would
be
entered in the
“y”
register in kilohm
(3455A) resistance measurements are
presented in kilohm). As an
example, assume the test is
made on a group
of 750 ohm resistors with a tolerance
of
5^9
.
The
nominal resistor value (750 ohms) is entered
in the
“y”
register as .750. The error
equation
becomes: result in
•?#
=
X-.750
x
100.
A resistor
with
-m
an actual
value
of
790
ohms
would give
a
measurement
result of: error
= .790-750
x 100
*
5. 333331^0,
750
indicating the resistor is out of tolerance by .33333*7o.
d. Limit Testing:
The
Scale
mode of the 3455A can
also be used to do
Limit Testing. This can be ac-
complished since the largest number which can be
3-3