Take these steps to maximize performance of analog peripherals
For hardware design engineers, moving analog information in and out of the digital domain is a large part of project design. By comparison, once analog information becomes digital variables, the design emphasis switches to software development. With analog peripherals now readily available alongside the processing core, it is important to know if integrated analog peripherals are up to the job. And if engineers can only access the analog features through software, is this as flexible as discrete design?
What are the most common analog circuit functions?
Analog signal chains and analog front ends typically comprise op-amps, comparators, filters, analog-to-digital converters, and digital-to-analog converters. This discrete nature is the basis of an analog building-block approach to design.
Every analog signal chain is designed for a specific application. As board sizes shrink, designers must squeeze more features into a smaller footprint. Using more analog peripherals is part of the solution. The number and type of integrated analog peripherals vary.
- Op-amps are versatile differential input devices. With a handful of passive components, they can be configured as active filters, amplifiers or attenuators. They can perform addition, subtraction, integration, or differentiation on analog signals. They can operate across a broad frequency spectrum from DC to hundreds of MHz.
- Comparators, a more specialized form of an op-amp, are used for precise comparison of input signals, often using an accurate reference source.
- Analog-to-digital converters (ADC) and digital-to-analog converters (DAC) are, perhaps, the workhorse IC components of any analog signal chain. When selecting a discrete ADC or DAC, many parameters require attention, from resolution to accuracy and precision.
MCU vendors publish the specifications of integrated analog peripherals. These can be compared with discrete counterparts showing how closely they meet your application’s requirements.
Op-amps and programmable gain amplifiers
An integrated op-amp might feature multiplexed differential inputs, allowing it to be used in more than one way. This is good news as it can further reduce board space and BOM, assuming both options are not needed concurrently.
In most cases, integration means you only need to provide one supply to the MCU for both digital and analog functions. This can also help simplify the circuit and reduce the BOM.
Key parameters to compare include:
- The op-amp's gain bandwidth product (GBP)
- Its input offset voltage
- The slew rate
- The noise level
Integrated analog features like op-amps may also be subject to the MCU's sleep modes. This can benefit battery-powered applications in a way discrete analog functions cannot easily provide. This may simplify a design, but if the op-amp provides an interrupt to wake the MCU it may still need to function continuously.
Rail-to-rail inputs and outputs might be a specific requirement, so check if the integrated op-amp can achieve this. Also, additional passives might be required, so does the integrated op-amp provide extra pins? Multiple op-amps can further extend possible use cases.
Multiple op-amps can be configured to create various analog circuits
The configuration possibilities when using MCUs with multiple integrated op-amps increase the usefulness of the device and can reduce external analog components significantly.
Integrated programmable gain amplifiers (PGA) typically use software-switched resistors to adjust the op-amp's gain characteristic. Switching resistors can be used to program the op-amp's gain, for example.
Integrated analog components using switched resistors
An MCU with integrated analog components, such as op-amps, may also integrate resistor arrays that can be switched in and out to adjust parameters, like gain
ADCs and DACs
Integrated ADCs and DACs benefit from multiplexed inputs and outputs and a firmware-configurable analog signal chain. A firmware-driven approach yields greater design flexibility and the opportunity for reconfigurability, which is atypical when using discrete analog ICs. Some MCUs support configurable analog signal chains featuring op-amps, multiplexers and a successive approximation register (SAR) ADC.
In common with other integrated analog peripherals, the functional specifications require investigation. A discrete approach may be necessary if the analog signal chain stipulates high integrity, high-resolution accuracy and precision. Integrating analog functions on the same die as the MCU is a compromise subject to the process technology being used. Digital signal processing techniques such as averaging, filtering or oversampling can compensate for some conversion shortcomings, but these burden the MCU's resources and add firmware development overhead.
Noise, geometry and analog coexistence considerations
Thermal noise and 1/f noise impact the performance of op-amps, ADCs and DACs. Most modern general-purpose MCUs benefit from being fabricated on small process nodes such as 28 nm, allowing higher levels of integration. Unfortunately, the smaller geometries introduce leakage, non-linearities, and increases in 1/f noise. Most discrete high-accuracy ADCs and DACs employ larger geometries up to 180 nm to overcome such limitations.
Two other aspects of integrating most of the analog signal chains in the MCU are isolation and EMC. By connecting the analog inputs directly to the MCU, there is a risk that high voltage dV/dt transients might cause damage or unreliable operation. Additional external signal conditioning circuitry such as isolation, protection, biasing and level-limiting components may be necessary, adding BOM cost and count.
Also, performing the analog or digital conversion process on the same die as the MCU’s clocks, processor core and high-speed digital interfaces introduces the likelihood of electromagnetic compatibility (EMC) issues. Temperature-related non-linearities may also impact conversion accuracy due to the ADC or DAC proximity to the CPU core and associated logic. Various digital techniques reduce the impact of electromagnetic interference (EMI) by halting system clocks and, therefore, core operation during conversion. If the application requires the precise conversion of sensitive analog signals, using a shielded and standalone ADC away from the MCU may be desirable.
Software development tools for integrated analog peripherals
Working with integrated analog peripherals requires a slightly different approach from the software development perspective. Rather than communicating with the registers of a standalone ADC or DAC over I2C or SPI, the integrated method uses registers internal to the MCU. This simplifies software development to some extent, yielding complete control of the analog and digital domains through the MCU and its development environment.
In common with most microcontroller toolchains, analog peripheral functions become abstracted at a hardware level (HAL) to simplify software development. Semiconductor vendors provide tools to aid the routing of analog functions
When to integrate and when to separate analog functions
Selecting an MCU with integrated analog functions is a viable, space- and cost-saving alternative to using standalone analog ICs. The decision on which route to take will depend on the design criteria of the analog circuitry. A separate analog front end is desirable if your application demands ultra-high accuracy and sensitivity. However, many applications, particularly those that are space-constrained and require minimal BOM cost, will benefit significantly from an integrated MCU. Application-optimized integrated MCUs provide the best balance of analog and CPU performance.