© National Instruments
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4-9
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 4-3 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.
Adding Components for Attenuating Voltage on Digital
Inputs
To build a two-resistor circuit for attenuating voltages at the PFI 0 pin on the SCB-68A, refer to
the pad positions in Figure 4-13.
The SCB-68A ships with a surface mount 0
Ω
resistor in the SC10 position. You
must
remove
the resistor to use the position.
