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Role of analogue input/output in closed-loop process

22 Dec 2014  | Gordon Lee

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Monotonicity in the DAC is very important. The output of a monotonic DAC always increases or, worst-case, remains the same as the DAC code increases. If a nonmonotonic DAC is used in a closed-loop system, the negative feedback could become a positive feedback. According to control theories, moreover, a system with positive feedback could be unstable. To determine whether the DAC is monotonic, look at its DNL plot. DNL error is the difference between an actual step height and the ideal value of 1 LSB. Step height means the difference in output voltage between two adjacent digital input codes. Figure 4 shows the DNL plot of the MAX5316 with amplifiers. For an analogue output to be monotonic, all the points in the DNL plot must be greater than -1 LSB.

 DNL plot

Figure 4: DNL for a -10V to +10V output range, with 20% overrange.

The total unadjusted error (TUE) plot shows the error of the actual output voltage compared to the ideal output voltage in per cent of the full-scale. Figure 5 shows the TUE plot of the MAX5316 with amplifier. In this case the maximum output error is 0.054% of the full-scale.

 TUE plot

Figure 5: Total unadjusted output error for a -10V to +10V output range, with 20% overrange.

Lastly, the settling time is the interval between the update DAC output command and the instant when the output reaches a value within a specified range. The settling time is affected by the slew rate of the DAC and the amplifier, and the overshoot and ringing of the amplifier. For example, with the MAXREFDES71 reference design, the voltage output settles to within 2 LSB in only 17µs.

Closing the loop
Two new sub-system reference designs are touted to meet the requirements of a "no compromises" closed-loop factory environment. The MAXREFDES71# and MAXREFDES32# sub-systems provide flexibility for low- and high-speed closed-loop systems using either voltage or current signals. They use dual ADCs (MAX11166) and DACs (MAX5316).

The 400ksps 16bit input channels, and high-speed 16bit output channels support ±10V and ±20mA signals plus 20% margin. Integrating several other components, their combined power and data isolation is 600VRMS. The MAXREFDES32 is exactly the same as the MAXREFDES71 except that it uses the flyback converter to generate the isolated supplies. Both designs connect to FMC-compatible field-programmable gate array (FPGA)/microcontroller development boards.

All the data shown here were generated on the MAXREFDES71# reference design board. Now designers can produce new closed-loop industrial systems with high-quality analogue input and output faster than before. And clearly, more reliable, more accurate closed-loop systems help ensure the efficiency and uptime of an Industry 4.0 factory.

About the author
Gordon Lee joined Maxim Integrated in 2005 and is currently a Senior Member of Technical Staff. His primary focus is on various types of sensors, signal processing, high-speed ADC applications, as well as FPGA, HDL, and embedded software design. He holds a M.S. in Electronics Engineering from San Jose State University.

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