2-36
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Chapter 2
Analog Input and Temperature Sensor Measurements
(2-7)
The accuracy of Equation 2-7 depends on the tolerances of the resistors that you use.
Caution
The SCB-68A is
not
designed for any input voltages
>30 V
rms
/42 V
pk
/60 VDC, even if a user-installed voltage divider reduces the voltage
to within the input range of the DAQ device. Input voltages >30 V
rms
/42 V
pk
/60 VDC
can damage the SCB-68A, any devices connected to it, and the host computer.
Overvoltage can also cause an electric shock hazard for the operator.
Selecting Components for Attenuating Voltage
To set up the resistors, complete the following steps.
1.
Select the value for
R
2
(10 k
Ω
is recommended).
2.
Use Equation 2-6 to calculate the value for
R
1
.
Base the
R
1
calculation on the following values:
•
Maximum
V
in
you expect from the transducer
•
Maximum voltage (<10 VDC) that you want to input to the DAQ device
Accuracy Considerations for Attenuating Voltage
For best results when attenuating voltage, choose a resistor that has the following characteristics:
•
Low wattage of approximately 0.125 W
•
Precision of at least 5%
•
Temperature stable
•
Tolerance of 5%
•
AXL package (suggested)
•
Carbon or metal film (suggested)
Verify that
R
1
and
R
2
drift together with respect to temperature; otherwise, the system may
consistently read incorrect values.
G
V
m
V
in
-------
R
2
R
1
R
2
+
------------------
=
=
