Integrating Analogue Devices Efficiently Into Digital Designs
George Paparrizos, Microchip Technology
The increasing complexity of designs over the last decade has produced many new types of digital devices - such as FPGAs, DSPs, FRAMs, and Flash microcontrollers. At the same time, requirements for more memory, intelligence and functionality, and the replacement of older discrete and mechanical components, have significantly increased the digital content of these systems. These developments have also created a demand for a growing range of high-performance analogue components, which must be incorporated into systems in an efficient manner.
There are many drivers behind this trend, the most obvious being the fact that real-world signals . such as pressure, temperature, motion, and others . need to be captured by an analogue component before they can be translated to a digital signal. A typical signal chain requires filtering, buffering, gain control and analogue-to-digital conversion (ADC) functions, which are all performed by analogue and mixed-signal devices (see Figure 1).
The design and manufacturing of new digital integrated circuits is based on a wide range of process technologies with a growing trend to smaller geometries. This has led to a wide variety of core voltage requirements for the many digital components on the board. In the past, these systems were dominated by 3.3V and 5.0V voltage rails. In today .s designs, 5.0V might still be required by a sensor, but the digital signal processors might operate at 1.5V, the microprocessor at 3.3V and the memory chip at 2.5V. This trend creates a need for a distributed power management scheme to convert the main voltage rail to the different voltage levels required by the various components. This can be implemented by DC/DC converter chips, such as linear regulators, switching regulators and charge pump converters. The choice from these three options is determined by the system .s requirements, such as noise sensitivity, battery life, board space, efficiency, cost and others (see Figure 2).
Another significant driver behind the higher analogue content in today.s electronic systems is the fact that the main processing unit is often required to focus on its main function to make best use of its computing power. This leads to many peripheral functions being placed externally, which also allows for increased redundancy and higher reliability.
The semiconductor supplier’s challenge
Taking all of these trends into account, it is clear that the biggest challenge for semiconductor suppliers is to offer system solutions that can assist the entire engineering community. To address this market requirement, suppliers. product development and marketing activities naturally go in one or more of the following directions:
• complement digital chips with an offering of digital-friendly analogue solutions
• put more emphasis on applications support and reference designs when there is no analogue capability
• integrate a wide range of analogue functional blocks with processing cores and memory. This results in mixed-signal chips that have digitally configurable analogue functionality, thereby providing ease-of-use to digital engineers.
Microchip Technology is one of the companies that is supporting all three directions today and therefore offers a high level of design flexibility.
Analogue functions in a typical microcontroller-based design
In a typical embedded-control application, a microcontroller or a microprocessor is the brain of the system. Its duty is to provide the main control, which is primarily defined by the firmware/ software configuration. Selecting the appropriate microcontroller or microprocessor device can be a difficult task. Speed, memory, calculation complexity, peripheral functions, and other requirements have to be taken into account. In many cases, the on-board memory of the processing unit is not sufficient, making the addition of a separate memory chip, such as EEPROM or Flash, inevitable.
A significant analogue attachment for a microcontroller is usually a system supervisor, also called a RESET IC. This device monitors the voltage level supplied to the microcontroller, making sure that it is sufficient for the microcontroller .s proper, error-free operation. Most of today.s microcontrollers can operate over a wide voltage range; however, they still need to be protected against brown-out or supply-glitching and ringing conditions. In addition, an external system-supervisor provides redundancy for many critical applications. The system supervisory IC typically holds the microcontroller in reset until the supply voltage reaches a predetermined value.
With the production of hand-held equipment on the rise, providing a constant voltage independent of the battery level is very important. This is the responsibility of the DC/DC converters, which accept the battery voltage as an input and convert it to a regulated output voltage that matches the microcontroller .s, or other logic IC.s requirements. The selection of the appropriate DC/DC converter scheme relies primarily on the design requirements; hence, there is not a single ideal solution for every design. Linear regulators are used when the power supply or bat- tery voltage is higher than the voltage necessary for the logic chip, or when a noisefree environment is required. Switching regulators, on the other hand, provide higher efficiency and can be used for step-up or step-down conversions. The third option is not commonly used . inductor-less DC/DC converters (charge pumps) are primarily used for inversion or doubling solutions when only a small load is present.
In signal conditioning applications, an analogue signal needs to go through different stages before it can be converted to the digital domain, read and processed by a microcontroller. These stages assure the preservation of the signal.s characteristics (buffering), the signal.s adequate magnitude for further processing (amplifying), and the rejection of unwanted frequencies (filtering). Again, the trick here is to know the system.s requirements for choosing the correct operational amplifier(s) to do the job. A very useful tool for this is Microchip.s FilterLab® active filter design tool. It allows the user to select different configurations, provides full schematic diagrams of the filter circuit with component values and displays the frequency response.
Typical microcontroller-based application with discrete analogue and mixed-signal components
Figure 3 shows a typical mixed-signal application. A system supervisor is monitoring the voltage rail to assure adequate supply voltage for the microcontroller, and to protect its operation during power-up and brown-out conditions. This RESET monitor also offers a manual reset capability that allows the user to initiate a reset cycle with a push-button. Since the power source is a 5V±10 percent supply rail, it is essential that this voltage be regulated to guarantee a stable, noise-free voltage level for the microcontroller.s reference input for the ADC. This is being achieved with the help of a small low-dropout linear regulator (LDO), which provides a high-accuracy 3.3V supply voltage to the microcontroller. As discussed earlier, in a typical signal chain design, sensor signals need to be filtered for eliminating aliasing problems before being brought to the digital domain. The operational amplifier shown in the circuit accomplishes this task.
Finally, with the help of a CAN transceiver, the system implements CAN-based communication for remote information exchange. In addition to all of the analogue and mixed-signal functions, more memory can be attached to the microcontroller if the integrated one is not sufficient.
As mentioned earlier, many microcontroller and microprocessor suppliers are increasingly integrating analogue peripheral functions into their product offerings. This allows for space and cost savings, but most importantly provides an easier solution to the digital engineer, since these functions can still be controlled by firmware- and digital-based commands.
The digital revolution we have experienced during the last decade has significantly increased the semiconductor content in today.s electronic equipment. A need for higher performance and more functionality has resulted in a greater number of both digital and analogue chips. At the same time, the interaction and interdependence between analogue and digital components has become inevitable in today.s system designs, making the engineer’s job more challenging than ever.
A handful of semiconductor suppliers are addressing this challenge by offering systembased solutions. Some can even provide the engineering community with the flexibility to choose from a pool of discrete, digital-friendly analogue components, as well as microcontrollers with integrated analogue and mixed-signal peripherals. This type of product offering, in combination with a variety of easy-to-use design tools, provides significant value by reducing design time, optimising system performance and minimising cost of ownership.
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