A brief discussion on high-performance AO technology - AD5755 solution analysis

As the global industrial intelligence process continues to advance, the industrial automation industry market has also been attracting much attention. This time, through the technical distributor Shijian, we invited their customer, Mr. Li, a hardware engineer currently working for a domestic first-tier industrial automation manufacturer, to discuss high-performance AO technology with us.

The first-tier industrial automation manufacturer where Mr. Li works is oriented to autonomous control, intelligent management, safety and reliability, providing intelligent manufacturing products and solutions with automation control systems as the core, covering industrial software, automation instruments and operation and maintenance services. Mr. Li has 7 years of experience in electronic system design and serves as a control system IO technical architect. He has rich experience in the design of analog-digital hybrid circuits, signal chain circuits, etc.

Abstract: Based on the actual needs of the industrial automation field, this article analyzes the basic characteristics of high-performance AO technology in an easy-to-understand manner, and proposes a simple and easy solution based on AD5755.

1. Why do we need high-performance AO technology?
With the development of the automation industry, industrial control systems are constantly upgrading towards stronger performance, higher reliability, and smarter applicability. As the interface between the control system and industrial equipment, the IO system also needs to continuously upgrade its performance to meet the higher operating requirements of the control system and industrial equipment.

AO (Analog Output) devices in industrial control systems are important components of IO systems. As the "hands" of the control system to operate industrial equipment, AO devices determine the operating status of field equipment by outputting voltage and current signals. Therefore, the performance of AO devices determines the execution level of the control results of the control system on industrial equipment. Currently, mainstream AO devices on the market support current and voltage signals, with signal accuracy ranging from approximately 0.2% to 0.5%, and signal refresh speeds ranging from a dozen milliseconds to several dozen milliseconds.

The driving objects of AO devices include not only various valve actuators, but also motor equipment, electromagnetic equipment, lighting equipment, etc. The requirements for control accuracy and speed in some high-end applications have exceeded the mainstream AO performance on the market. In some applications, additional signal isolation or conversion equipment is required between AO devices and actuators, which leads to a decrease in AO output performance. In order to adapt to high-precision and high-speed actuators, or to reduce the impact of signal isolation or conversion equipment on system performance, the output performance of AO devices needs to be further improved.

2. What kind of AO technology do we need?
On the surface, we need to improve the signal accuracy and refresh speed of AO equipment, but after in-depth analysis, this is an extremely complex issue.

The signal types of AO devices are commonly voltage signals and current signals. Standard industrial voltage signals are 0V~5V, 1V~5V, and standard industrial current signals are 0mA~10mA, 4mA~20mA. In addition, there are some non-standard signal ranges. In terms of port characteristics, voltage signal output requires low output impedance of the port , and current signal output requires high output impedance of the port. In actual applications, due to the requirements of driving capability and the limitations of feedback structure, the design of port output impedance must take into account both voltage and current, which brings certain design complexity.

Control systems used in industrial sites often face complex environmental conditions, especially installation space, ventilation conditions, and ambient temperature. The appearance design of AO equipment needs to be as compact as possible, which requires the size of the AO circuit to be as small as possible. To maintain high accuracy under small size and wide temperature conditions, the circuit components need to have low temperature drift, or the temperature rise of the AO circuit needs to be reduced as much as possible.

Figure 1 is a common AO output solution for converting voltage to current. In order to make the load current adjustable, the voltage drop of the output stage transistor needs to vary over a wide range. Under certain load conditions, the heat generated by the transistor will be very high. If the heat dissipation capacity of the AO device is not enough to offset the heat, the DAC, op amp and other components will have large accuracy errors due to large temperature drift.

Figure 1

AO components in PLC and DCS equipment often have more than one AO ​​output. When a single AO component accommodates multiple AO outputs, the circuit design complexity, circuit scale and space conflicts, heat generation and other issues will become more serious. This leads to the difficulty of improving the performance of AO equipment and achieving both reliability and ease of use.

In short, high-performance AO technology needs to further optimize port compatibility, improve circuit integration, and reduce heat generation and the impact of heat generation on the basis of improving signal accuracy and refresh speed. Traditional signal chain circuit solutions based on discrete device design are difficult to take into account all requirements.

3. AD5755 Solution
The AD5755 launched by ADI perfectly solves the above problems. This DAC encapsulates 4 independent voltage and current output circuits, each of which can independently output voltage or current signals. AD5755 supports a maximum signal range of ±10V and 0mA~24mA, which basically covers all signal types in industrial equipment applications.

AD5755 has designed a BOOST controller for each AO unit, which is used to automatically adjust the output voltage of the AO signal power supply according to the signal output and load conditions of the current channel. From the example in Figure 1 above, it can be seen that when the voltage value of the AO output power supply can be adjusted according to the output feedback, the tube voltage drop of the output stage driver tube can be limited to a preset value, such as around 2.4V. At this time, the thermal power of the driver tube will become very low, and the largest heat source in the entire AO circuit will be greatly limited. Solving the problem from the source of heat generation will greatly improve the heating and heat dissipation of the entire AO component.

Figure 2

The AD5755 solution integrates four sets of DACs, operational amplifiers, power amplifiers and other traditional signal chain structures, as well as a self-regulating power supply controller, into one chip, replacing the huge circuit scale and design workload when using discrete components in the past. The chip area is only 9mm*9mm, which greatly reduces the circuit area and the volume of the AO device. At the same time, the self-regulating power supply controller is used in conjunction with the AO unit to effectively solve the temperature drift and aging problems caused by circuit heating. It further improves the environmental adaptability of signal accuracy and the reliability of the circuit.

According to ADI's official manual, AD5755 can achieve a basic accuracy of 0.05%, and its dynamic response time is as low as a dozen to tens of microseconds, which is much higher than the mainstream products in the existing market in terms of accuracy and speed indicators. It is an ideal solution for the next generation of high-performance AO systems.

In addition to important indicators such as accuracy and speed, AD5755 also has HART signal modulation input function for intelligent instrumentation needs. We only need to add an independent HART Modem and input the HART signal into the CHART pin of AD5755, and then we can directly output the current signal with HART communication at the Iout end.

When designing AO equipment using AD5755, we only need to focus on protecting the chip. We can add necessary protection circuits such as RC, LC, magnetic beads, TVS, self-resetting fuses, etc. to the port to greatly improve the system's tolerance. The figure below shows a more general protection solution for readers' reference only.

Figure 3

By comparing the traditional solution with the AD5755 solution, we can easily find that the traditional process method based on electronic components + PCB has its own bottlenecks. The bottleneck that is difficult to break through under the traditional process framework can be easily solved in the face of advanced process solutions (such as MEMS or Chip, etc.). We expect that more highly integrated chip solutions or module solutions will emerge in the market in the future to provide us with product design resources with stronger performance, higher reliability, and more intelligent applicability.