Crucial role of analog front ends, creating bridge to digital world
Delve into the design of many embedded systems and you may find a jumble of components between external sensors, actuators and interfaces before signals disappear into the digital domain.
These parts are typically analog ICs, passives and sometimes modules, which form the crucial analog front end (AFE). We use AFEs in almost all applications, from Industrial Internet of Things (IIoT) to automotive and wearables to deep-space wireless communications. Our natural world is analog, with hundreds of parameters measuring everything from environmental conditions to mechanical forces. We convert them into the digital domain for the convenience of performing rapid calculations and transferring the resulting data from one place to another. The digital world also allows systems to control an analog process quickly and easily, for example, regulating an industrial cooker's temperature.
Pointing to an AFE on a circuit diagram can be a challenge. Passive components are commonplace, so the odd capacitor here or there might be for other purposes. Also, because an AFE is usually application-dependent and there are hundreds of use cases, it might be difficult to identify on a schematic.
Diversity of analog front ends
Here are three examples of the many potential AFEs you might find in use.
Industrial sensors: Connecting analog environmental sensors to an industrial automation system provides the essential data required to control a manufacturing process. However, any industrial workplace tends to be electrically noisy due to switchgear, motors and other high-power sources.
Short-duration, high-voltage electrical transients (dV/dt) will induce spurious voltage spikes on analog signal lines, which then enter the digital domain through conversion, potentially causing erratic system operation. These spikes may also be sufficiently large enough to damage sensitive electronic components. For this example, an AFE must filter out the transients and protect the analog-to-digital conversion components from harm.
Wireless data link receiver: Wireless communication is ubiquitous and has an equally diverse set of use cases, from short-range analog voice to long-range, high-bandwidth data communication. Examples include the popular protocols of Wi-Fi, Bluetooth, NB-IoT and LoRa.
The available radio frequency (RF) spectrum for data communication is typically narrow and becomes overcrowded quickly. RF receivers need high selectivity — the ability to detect the required signal from others on adjacent frequencies. Receiver sensitivity is also essential, handling signals with a high dynamic range. A receiver's AFE should help improve selectivity and either attenuate or increase the input signal to avoid overloading the receiver's input.
Wearable sports watch: Wearable smartwatches have become extremely popular in recent years, most monitoring vital signals such as the person's heart rate. The most popular heart rate detection method involves optical sensors and spectrum-specific LEDs to detect capillary movement. The detected analog signal is then processed to determine the heart rate.
Unfortunately, body movements from exercise routines, talking and walking create multiple noise artifacts superimposed on the required heart pulse signals. For this use case, an AFE must filter out the unwanted high and low-frequency noise leaving only the heart's pulses for onward processing.
Key functions of an analog front end
The principal functions of an analog front end are:
- Interfacing
- Signal conditioning
- Conversion
An example AFE for an industrial process control loop is shown below. The output from a remote analog temperature sensor is connected via an industrial-standard 4-20 mA current loop interface to a host microcontroller in the control room.
Analog front ends in an industrial process control loop might be used to read a sensor, control a heater and to implement the 4-20mA communication channel.
AFEs take many forms. For some applications, discrete components or ICs provide the necessary functions. However, a complete AFE may be integrated into a single IC package. The resulting output is available over a digital interface, with I2C, SPI and UART being the most popular.
AFE interfacing
The input to an AFE may come from an antenna, a sensor element or a switch. Each has a particular set of interfacing requirements. For example, a simple analog temperature sensor element typically outputs a small voltage proportional to the ambient temperature. The design engineer will review the sensor's datasheet to determine the output range and how the rest of the signal chain needs to be architected.
Some analog sensors are incorporated into IC packages with supporting circuitry to manage the output voltage. These require a power supply. A simple thermocouple, for example, changes its resistance characteristic according to the temperature, so the next step is to condition the signal so that the ADC has a voltage to measure rather than resistance.
The interface must also buffer and protect the AFE stages from damage, possibly requiring current or voltage-limiting components. Galvanic isolation between the analog input and the signal conditioning and conversion components might also be an application-specific requirement.
Why we use signal conditioning
The purpose of signal conditioning is to prepare the analog signal for accurate and repeatable conversion into the digital domain. The integrity of the input signal needs to be maintained across the AFE. Depending on the type of analog signal involved, operations may include level changing, attenuation, amplification, filtering, impedance matching, and linearization. Isolation, level limiting and circuit protection might also occur within this AFE stage.
The diversity of AFE use cases will dictate the degree of signal conditioning required. The priority is removing any unwanted noise or artifacts from causing a poor or unreliable conversion. The same applies to signal conversion from the digital domain into the analog world. A DAC's output levels might not be suitable for controlling an actuator, for example, or wireless transmission.
The final stage of an AFE is signal conversion. ADCs perform this function and are usually available as an IC, with the critical attributes the sampling frequency, conversion resolution and dynamic input range. There are several popular conversion architectures, each suiting particular use cases. ADCs and DACs are just some of the forms of conversion devices. IQ modulators and associated circuitry transfer digital data into a radio frequency signal for wireless communication.
What are the building blocks of an AFE?
The above section highlighted many of the functions an AFE may need to perform. Each process can be achieved using discrete components, modules or ICs. Some functions, such as implementing a low-pass filter, may use passive components (capacitors, inductors and resistors) or active integrated circuits. The filter's design criteria, such as the cut-off frequency, bandwidth and insertion loss, will guide the designer on which approach to use.
A complete signal chain incorporating multiple sensors and actuators, including the AFE, bridging the analog and digital domains, may look like this.
The signal chain of this example application links the analog and digital domains together. (Source: Avnet)
Building block components include:
ADC/DACs: These essential ICs perform the heavy lifting of an AFE but can only perform effective conversion if their operating conditions are ideal. This doesn’t just apply to the input (output) signals. Stable, low ripple power supplies and electromagnetic interference (EMI) mitigation measures are also needed. This article explains more about ADCs and DACs.
Op-amps: An operational amplifier (op-amp) is a highly versatile and flexible component used to design programmable gain amplifiers, buffers, active filters and comparators. A buffer ensures that the output signal from a high-impedance analog sensor does not suffer signal integrity loss when loaded with a lower impedance presented by the next stage. Op-amps are also ideal for creating a variety of active filters, with device bandwidths into the GHz RF spectrum for high-speed applications. Many of Avnet’s suppliers offer op-amp filter design tools that accommodate a wide range of applications.
A comparator functions differently from an op-amp, with typically a much wider dynamic range of input voltages. A comparator provides a binary output indicating if an input voltage is above or below a reference source.
The functional architecture of an ST LPS22HH high-performance absolute pressure sensor (Source: STMicroelectronics)
Passive components: The role of passive components in an analog front end should not be overlooked. This is particularly the case for RF applications requiring bandpass filters or impedance matching an antenna to a radio transceiver system-on-chip. At high frequencies, surface-mount capacitors and inductors, together with a PCB-printed antenna, achieve a compact footprint. Bandpass filters help reduce receiver de-sensitization from other local RF signals. Miniature, integrated, surface-mount RF bandpass filters are also widely available.
Integrated monolithic AFE ICs: The electronics industry trend of achieving high levels of integration has led to the development of many fully integrated AFE devices aimed at specific applications. Some include the sensor element, the AFE, a microcontroller, and one or more digital interfaces for communicating with the application host.
The image below illustrates the internal architecture of the LPS22HH absolute pressure sensor from STMicroelectronics. The IC incorporates a MEMS-based pressure sensor and temperature sensor and connects to a host microcontroller via I2C or SPI. Pressure and temperature information is placed in output registers accessed over the I2C/SPI serial bus.
The functional block diagram of the NXP N-AFE series of flexible and configurable multi-input analog front-end ICs is for use in industrial applications. (Source: NXP)
For industrial automation applications, the NXP N-AFE series offers a comprehensive set of interfacing, conditioning and conversion options for up to eight software-configurable input channels. AFE functions include multiplexers, a programmable gain amplifier, a sigma-delta ADC, an SPI interface and associated control logic.
The role of an AFE
An AFE performs a crucial function within a design, bridging the analog and digital worlds. Without one, data conversion performance would not be as precise and communication links would be less reliable. For the design engineer, understanding the analog environment and the constraints of the host application is essential to architecting a suitable AFE. Unlike many circuit functions, few general-purpose AFE ICs are available. Instead, the usual options are either architecting an AFE using discrete components or using an application-specific AFE IC.
Because board space is a scarce, silicon vendors continue to release AFE ICs that meet the specific use case requirements. Market sectors driving the most significant demand include wearables, industrial automation, automotive and cellular infrastructure.
