Simplifying design of industrial process-control systems with PLC evaluation boards (Part 2 of 2)

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Evaluation System Software and Evaluation Tools: The evaluation system is very versatile. Communication with the PC is achieved using LabView. The firmware for the microcontroller (ADuC7027) is written in C, which controls the low-level commands to and from the ADC and DAC channels.

Figure 9 shows the main screen interface. Pull-down menus on the left side allow the user to choose active ADC and DAC channels. Under each ADC and DAC menu there is a pull-down range menu, which is used to select the desired input and output ranges to be measured and controlled. The following input and output ranges are available: 4 mA to 20 mA, 0 mA to 20 mA, 0 mA to 24 mA, 0 V to 5 V, 0 V to 10 V, +/-5 V, and +/-10 V. Small signal input ranges can also be accommodated directly on the ADC by using its internal PGA.

Figure 9. Evaluation software main screen controller.
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The ADC Configure screen, shown in Figure 10, is used to set the ADC channel, update rate, and PGA gain; to enable or disable excitation currents; and for other general-purpose ADC settings. Each ADC channel is calibrated by connecting the corresponding DAC output channel to the ADC input terminal and adjusting each range. When using this method of calibration, therefore, the offset and gain errors of the AD5422 dictate the offset and gain of each channel. If these provide insufficient accuracy, ultrahigh-precision current and voltage sources can be used for calibration if desired.


Figure 10. ADC Configure screen.
(Click on image to enlarge)
After selecting the ADC's input channel, input range, and update rate, we can now use the ADC Stats screen, shown in Figure 11, to display some measured data. On this screen, the user chooses the number of data points to record; the software generates a histogram of the selected channel, calculates the peak-to-peak and rms noise, and displays the results. In the measurement shown here, the input is connected through the AD8220 to the AD7793: gain = 1, update rate = 16.7 Hz, number of samples = 512, input range = +/-10 V, input voltage = 2.5 V. The peak-to-peak resolution is 18.2 bits.


Figure 11. ADC Stats screen.
(Click on image to enlarge)