By Cliff Ortmeyer, Global Head of Technical Marketing, Farnell
The digitalisation of society seems inevitable as the world moves toward ambient intelligence through the increased use of robotics and the Internet of Things (IoT). The data that streams between edge devices and the cloud, and the signals that inform robots on how best to move, are all handled in binary form. The physical world, however, remains resolutely analogue and it is this aspect of design that will often control how well systems that interact with the environment perform.
Analogue interfaces are needed more than ever because of the proliferation of sensors that are used to record and process environmental signals, as well as the actuators used to make changes. Power delivery, for example, relies on the ability to accurately predict and measure changes in electrical and magnetic fields in combination with electronics that switch the required currents at high speed. As a result, the interface between the digital and analogue domains is critical. To make the interface operate well often means taking the end application into account to ensure that subtleties in the analogue signals are not lost.
Components to support analogue and digital integration
As the interface between the analogue and digital world is so important and application-dependent, mixed-signal design remains a domain of intense specialisation. Though there may often seem to be an overwhelming array of products that can be used for a given design, the precise requirements of the end application need to be taken into account. Finding the right component for the job can be a challenge and designers must consider all options to make the most appropriate choice.
Previously it often made sense to choose a large number of discrete devices and integrate them on the PCB, however market demands mean the buyer needs highly integrated devices that can fit specific needs based on functionality, size, power consumption and accuracy. For example, in IoT-oriented designs the placement of sensors is as important to the system architecture as the performance of the front-end signal-conditioning and conversion circuitry.
There is generally a need to be able to place converters as close as possible to the input to avoid signal degradation caused by analogue parasitics. This can be seen in environmental monitoring applications, such as cold-chain logistics and data-centre monitoring, where there is a need for multiple temperature sensors that will be placed comparatively far apart. Using a compact converter and front-end with support for low-cost digital transmission to a hub is vital.
One such instance would be the use of a one-wire communication protocol. In the Maxim Integrated MAX31825 this provides the necessary digital transmission with the minimum cost even for large systems. The interface allows up to 64 sensors to be connected and addressed using a 64bit serial code. The temperature sensor provides the ability to read temperatures at up to 12-resolution over a range that can span -45°C to +145°C.
Enablement of sensors and machine learning
Other systems need to be able to handle a wide range of sensor inputs to reliably detect environmental conditions as well as the ability to process the information they provide in an intelligent fashion. If a sensor node can examine the data and only report significant changes, it greatly reduces the communications bandwidth required to report status to the systems around it. This, in turn, can ensure that energy consumption is kept to a minimum as RF transmission and reception accounts for a significant proportion of IoT node energy consumption. Keeping data processing inside the node, if performed efficiently, can make the battery last much longer. Now, even machine learning is being incorporated into mixed-signal devices.
The use of sensor fusion and machine learning is particularly important in systems that need to detect position and trajectory. The combination helps to deal with noisy signals caused by vibration and other sources of error. Although it is possible to combine high-end microcontrollers and data converters to achieve the necessary functionality, there are components that support both, such as the STMicroelectronics iNEMO. The system-in-package (SiP) products combines accelerometers, gyroscopes and magnetometer with a machine-learning code to reliably determine position in application as diverse as asset tracking and robotics. The integration helps keep power consumption to a minimum, by allowing a host MCU to be put to sleep for long periods, without sacrificing accuracy or always-on capability. The combination of functions in the iNEMO is a demonstration of how integration and experience from a specialist in its field boosts functionality and performance.
Importance of efficient power delivery
As well as a need for intelligent mixed-signal design the size and need for low-energy operation in IoT sensor nodes demonstrates the importance of efficient power delivery and integration. Specialists in power devices such as On Semiconductor have shown it is possible to pack a large amount of functionality into a small space. The FAN54120, for example, is the smallest battery charger currently on the market. Packaged in DFN6 or WLCSP6 form, the device was designed for ease of use. As a charger, it has no requirement for user interaction or active supervision from a host microcontroller. The charger handles single-cell lithium-ion or lithium-polymer batteries as well as conditions such as dead battery revival. This is in addition to the precharge, fast-charge and float-charge states that are required with these chemistries, all with an ultra-low battery discharge current of less than 120nA.
For systems that need to handle two cells in series and balance their voltages, MPS designed the MP2672. For flexibility, the device can be programmed using hardware pins or from software running on a host MCU using commands sent over the I2C bus. A narrow voltage DC power structure allows a system to continue operating while charging with a weak battery. This process regulates system output at the precharge threshold voltage while the battery charges.
Many systems, including sensor nodes, need a variety of supplies operating at different voltages. To drive specific circuitry without compromising size, weight and energy efficiency. Sporting a height of just 2mm, the MPM3695-10 made by MPS provides an ultra-thin DC/DC converter that can readily be mounted on the backside of the PCB but still provide the ability to deliver current up to 10A. To help manage its operation, the device has a full PMbus interface and offers excellent line and load regulation across a wide input voltage range.
A further example of the intense integration that is now happening in power design is Microchip Technology’s MCP19123, which combines high-efficiency power delivery with an on-chip microcontroller based on the 8bit PIC architecture. This enables the device carry out a wide range of supervisory and management functions within the system. The analogue section itself is highly configurable, using a programmable gain amplifier in the buck converter’s feedback path as we as an adjustable voltage reference. As well as power circuitry, the device includes timers and a temperature sensor to help monitor the health of the system in which it is used. The result is a PWM-based converter that delivers high efficiency and transient response but with the intelligence to respond to changing conditions.
Bringing it all together
As size, cost and integration pressures continue, analogue circuitry will continue to combine further with digital processing, leading to a huge amount of innovation in the supply chain. Though suitable parts will be offered by broad-based suppliers, the sheer variety of applications and vendors in the supply chain means the best fit for an application will often come from a more specialised supplier. Engineers need to look beyond the largest, most well-known analogue suppliers to source solutions for their next project. Experienced distributors with design-in support, such as Farnell, can analyse a customer’s needs and point them in the right direction.
