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.
The example here illustrates the transition range of voltage between steps; that is, the transition is not abrupt. This is due to noise and the bit uncertainty that exists in the transition regions. Again, the bit-uncertainty effect may be observed during the sweep with Plot Speed set on SLOW.
For comparing the two cases with the two limit values and with smoothing of the data with averaging, use AvgDiscrete.vi. This VI runs the subVI Discrete.vi sequentially with the high limit set to 01 and 10 (with the low limit of –0.1 and –10. The two plots appear together in a single graph in the Front Panel of AvgDiscrete.vi.
Open AvgDiscrete.vi and Discrete.vi. In AvgDiscrete.vi, verify that Max Out = 1000 mV and that Number Avgs is set at 5 to 10. Run AvgDiscrete.vi to compare the two plots with data smoothing.
