Bias Circuit

PA.7 Discrete Input Voltage from the Circuit Board
• For this measurment, we need a high-resolution output voltage source. If we
are willing to give up voltage range, this can be accomplished with a simple
resistor voltage divider. In the example here, the circuit forms an apporximate
100:1 divider (270- and 2.7-kΩ resistors). This provides a source with a
resolution of 49 µV (bipolar).
• Discrete.vi will be used to investigate the discrete nature of the input
channels. Open the VI and set high limit = 0.1 (with low limit = –0.1), for a
receiving resolution that is also about 49 µV. Use Max Vout = 1 V (1000 mV).
The output voltage sweep range is then Max Vout/100 = 10 mV with the
voltage divider. The programmed output voltage step size is Max Vout/100 =
10 mV. Thus, after the voltage divider, this is 100 µV. The resolution of the
output and input functions is similar.
• Run Discrete.vi. The plot is essentially a straight line with similar output and
input resolution. Note that the voltage sent out (X-axis) is the actual channel
voltage and that the voltage received (Y-axis) is after the voltage divider.
Note also that the scatter of the input channel samples (right graph) reflects
the discrete nature of the receiving function and the points are separated by
about 49 µV.
• Set high limit = 10 V. Run the VI with the same sweep range (example
below). The output voltage (after the voltage divider) has a much higher
resolution (better) than the receiving function. The steps in the sweep plot
have a magnitude equal to the receiving resolution. Also, the sample points
are scattered by the amount of the resolution of about 4.9 mV. An example of
this case is shown in Fig. A.18 (Unit A). To check the value of the differences
between scattered points, run the VI on SLOW and halt the execution in a
transition region. Then step through the Chan0_in Array(mV) Indicator.