Explaining Analog Front Ends (AFEs) for Industrial Applications (Japanese blog)
0. Table of Contents
1. What is an Analog Front-End (AFE)?
1.1 Various AFEs
1.2 AFE for Industrial Applications
1.3 The concept of universal input
2. NAFE13388 family
2.1 Signal Input / Excitation Voltage/Current Source / Built-in Reference Voltage
2.1.1 Input Circuit
2.1.2 ADC
2.1.3 Excitation voltage/current source
2.1.4 Example of input rearrangement
2.2 Software Interface
2.2.1 Logical Channels
2.2.2 Measurement Commands
2.3 Self-diagnosis function
2.3.1 Monitoring of power supply voltage, internal reference voltage, and excitation voltage
2.3.2 Input Monitoring
2.3.3 Monitoring the Clock Source
2.3.4 Temperature monitoring
2.4 Factory Calibration and User Calibration
2.5 Evaluation Environment
2.5.1 Hardware
2.5.2 Software
2.6 Application Notes
3. Summary
4. Reference materials
1. What is an Analog Front-End (AFE)?
" Analog front end " (hereafter referred to as AFE ) refers to the part that bridges the gap between analog and digital in a system that uses a microcontroller or processor.
Some AFEs only input analog signals, some only output analog signals, and some do both.
1.1 Various AFEs
1. In an audio system, this part (commonly realized by a chip called a codec) converts analog audio signals from a microphone into digital signals that can be processed and recorded, or that can be converted into analog signals that can be played through speakers or earphones.
2. In a battery management system (BMS), a microcontroller or processor controls charging and discharging, but there is also an AFE part that measures analog values such as battery voltage and current.
Each product uses a dedicated AFE tailored to its application.
Its main functions include analog-to-digital converter ( ADC ) and digital-to-analog converter ( DAC ), as well as pre- and post-stage filters and amplifiers that match the input and output signal characteristics.
Many microcontrollers also have built-in ADCs/DACs for analog input and output. The analog input and output of these microcontrollers range from 0 to 3.3V, and they can perform analog-to-digital conversion with a resolution of around 10 to 14 bits.
However, for high-precision measurements required for industrial applications, with larger/smaller positive and negative voltage ranges and 16-bit or 24-bit output , the AFE introduced here is required.
1.2 AFE for Industrial Applications
In general, industrial applications require high-precision measurement of analog signals for factory lines and process management. Analog signals may also need to be exchanged for purposes such as valve control.
The analog inputs of industrial AFEs are connected to various sensors, such as temperature sensors like thermocouples and resistance temperature detectors (RTDs), force and pressure sensors like load cells, etc. Analog inputs are also used to measure pure voltage and current.
A circuit tailored to the characteristics is required:
For example, thermocouples used for temperature measurement output a very small voltage, so a low-noise amplifier is required for high-precision measurements.
An RTD is a device whose resistance changes with temperature. It measures the voltage when a constant current (called the excitation current) is passed through it. However, if a large excitation current is passed through it, the RTD will heat up and accurate temperature measurement will be impossible, so only a weak excitation current can be passed through it. Since a weak excitation current can only generate a small voltage , a high-precision, low-noise amplifier is required along with a precise constant current source for excitation .
Load cells that measure force (weight or pressure) detect small changes (voltage changes using a Wheatstone bridge) in an element to which an excitation voltage is applied. This requires a high-precision, low-noise amplifier .
When measuring voltage or current, the input range is adjusted to match the object being measured.
Because the characteristics of these sensors and the output they measure are different, previous industrial AFEs have dealt with this by incorporating individual circuits in the front stage that are tailored to each sensor's characteristics.
1.3 The concept of universal input
It is inefficient to prepare equipment with an input circuit for each measurement target. Dedicated hardware must be developed and a product lineup created for each measurement target.
If these could be standardized, the above problems would be solved at once.
NXP's AFE allows you to "universalize" the input. Specifically, it has up to eight single-ended inputs of +/-12.5V (four differential inputs of +/-25V) and an internal PGA (Programmable Gain Adjustment). Gain Amp can be set from 0.2 to 16 times. In addition, the internal excitation voltage and current source can be output from any input pin.
By using such a chip, it is possible to prepare a board with only the minimum external circuitry and simply switch between them using software depending on the measurement target. NXP's AFE (NFAE series) also has input protection functions built into the chip, so external components can be kept to a minimum and there is no need to add protection diodes, which would affect measurement accuracy.
2. NAFE13388 family
As a concrete example, let's take a look at the features of NXP's AFE, the NAFE13388 . The NAFE13388 is an AFE with eight universal analog inputs (single-ended or four differential) and ten GPIOs.
The interface with the microcontroller is SPI (maximum frequency 32MHz).
This product family includes the high-speed NAFE73388 with a high sampling frequency, a 4-input model with fewer inputs, and models with or without excitation voltage and current sources, factory calibration, and 16- or 24-bit output.
2.1 Signal Input / Excitation Voltage/Current Source / Built-in Reference Voltage
2.1.1 Input Circuit
The input is single-ended and has pins that can accommodate up to eight inputs, and the built-in multiplexer can be configured to accept single-ended or differential inputs, as well as excitation voltage and current outputs.
The figure shows part of the circuit diagram for the NAFE13388-UIM (NAFE13388-Universal Input Module) evaluation board (from the two terminal blocks on the right to the AFE input section on the left). As you can see, the AFE has a very simple configuration, with only a simple passive filter and a resistor to limit current when overvoltage is applied in front of it.
Various sensors can be connected to the terminal block as they are.
The board has built-in protection circuits to protect against incorrect power supply connections to the input terminals and ESD, so there is no need to place external protection elements. However, the leakage current of the diodes used as protection elements can affect the measurement results, so if an external element is required, it must be selected carefully. The protection element built into the NAFE13388 is an extremely low leakage diode that does not affect the measurement results.
There is no need for voltage dividers to input higher voltages or bias circuits to adjust offsets. The NAFE13388 can directly input signals of +/-12.5V, so there is no need to worry about the accuracy or heat generation of these circuits.
2.1.2 ADC
The AD converter consists of a third-order ΣΔ modulator and a SINC filter with various settings.
In AD conversion using a ΣΔ modulator, the required resolution is obtained by downsampling data converted to 1 bit at a high sampling frequency. The resolution obtained after this downsampling is called the "effective number of bits ( ENOB )." The larger the downsampling ratio, the higher the ENOB.
To get 17 bits of ENOB with the NAFE13388, you need to set the sample rate to 72 kHz. With the faster NAFE73388, you can get the same ENOB at a sample rate of 144 kHz. To get 24 bits of precision, you need a sample rate of 30 Hz (NAFE13388) or 60 Hz (NAFE73388).
The NAFE13388 has a built-in high-precision reference voltage source required for AD conversion, so no external reference voltage is required. A temperature compensation loop achieves %精度(% FS (full temperature range) within the operating temperature range. A PGA that can be set from 0.2x to 16x is located in front of the AD converter. The full-scale range for single-ended input is +/-12.5V (PGA gain: 0.2x) to +/-0.15625V (PGA gain: 16x).
The following graph is taken from the NAFE13388 datasheet, and shows the change in temperature on the horizontal axis versus the change in reading when 10V is applied to the input. The temperature compensation loop keeps the measurement error very small.
2.1.3 Excitation voltage/current source
The excitation voltage and current can be set in 12 steps within the ranges of +/-6mV to +/-12V and +/-977nA to +/-2mA, respectively, and can be output from any input pin.
This means that elements such as RTDs and load cells can be handled simply by rearranging the software without the need for additional circuits.
2.1.4 Example of input rearrangement
The simplest example of single-ended voltage measurement is shown below. In this example, the voltage of a signal source connected between the AI3N and AICOM terminals is measured.
In the following example, a load cell is connected. The internal excitation voltage source is output from the AI3P terminal and the voltage is applied to the bridge circuit. The result is measured as the differential voltage between the AI2P and AI2N terminals.
The following example shows how to measure an RTD using a 4-wire connection. In this example, a current from the internal excitation current source is output from the AI4P terminal, and the voltage across the RTD is measured as a differential voltage between the AI1P and AI1N terminals to determine the resistance value.
The last example is measuring the power supply voltage supplied to the AFE chip. The power supply voltage connected to the internal multiplexer can be measured with an ADC.
2.2 Software Interface
2.2.1 Logical Channels
In addition to the input pin selection (specifying any pin as single-ended or differential input), excitation voltage/current and output pin selection as described above, settings such as PGA gain , sampling frequency / post-SINC filter , and temperature compensation loop ON/OFF can be managed collectively. This management unit is called a logical channel , and up to 16 logical channels can be set. Each of the 16 logical channels can be managed, enabled, and disabled individually.
Once a logical channel is set, it is retained inside the AFE. The logical channel can be enabled or disabled simply by setting or clearing a bit in a 16-bit register. For example, settings such as single-ended voltage, load cell measurement, RTD measurement, and power supply voltage, as in the example in the previous section, can be quickly recombined using logical channels.
It is possible to have multiple logical channels enabled at the same time. In this case, the measurement commands described in the next section can be used to measure each logical channel individually or all logical channels together as a sequence.
2.2.2 Measurement Commands
A/D conversion is performed for each logical channel. This can be performed on an individual channel by software command, or on a single sequence of all enabled logical channels, or in a continuous loop. Conversion can also be triggered by an external clock signal. This feature can also be used to synchronize multiple AFE chips.
Measurement automation is possible by executing multiple logical channel conversion sequences in a loop. The converted data is stored in the registers of each logical channel, so it can be read out at any time. A signal notifying the completion of conversion can be output for each channel or as a series of sequences. This allows data to be read out synchronously with the conversion.
2.3 Self-diagnosis function
2.3.1 Monitoring of power supply voltage, internal reference voltage, and excitation voltage
The input selection multiplexer can switch not only the input pins, but also the excitation voltage, current source, two reference voltages, and the power supply voltage supplied to the chip.
By configuring such inputs as logical channels and incorporating them into the AD conversion sequence, you can continuously monitor your own operating status. If the chip contains factory calibration data, the reference voltage at the time of chip shipment is written to the non-volatile memory area. This can be used to monitor problems during product assembly and changes over time.
2.3.2 Input Monitoring
A signal input range can be set for the logical channel. If a voltage outside this range is input, an alert will be generated.
There is a possibility of wire breakage between the various sensors and the inputs. A 65nA excitation current source is provided to monitor this. By enabling this and monitoring the voltage, you can check for abnormalities. Alternatively, if you suspect cable deterioration, even if it does not go as far as a break, you can monitor by incorporating impedance change measurement.
2.3.3 Monitoring the Clock Source
The NAFE13388 checks for an external clock input at startup, and if none is present, attempts to oscillate using a crystal oscillator. If that is not present, the clock is generated using the built-in RC oscillator. If a clock is supplied from an external clock or crystal oscillator, it is compared with the clock generated by the internal RC oscillator, and an alert is issued if there is an abnormality such as the clock stopping or a large frequency deviation.
2.3.4 Temperature monitoring
The NAFE13388 also has a built-in temperature sensor. This is used to correct drift in measurements, and also provides an alert at a specified temperature to protect the chip itself, as well as a shutdown protection function when the temperature exceeds a certain limit.
2.4 Factory Calibration and User Calibration
The gain and offset errors for each PGA gain setting are corrected in the digital domain after AD conversion. There are 16 sets of registers to store these correction coefficients, and one set can be specified for each logical channel. If the NAFE13388 is factory calibrated , correction coefficients for each gain setting are pre-set, and by using these, an accuracy of +/-0.06%FS (typical value, room temperature) can be achieved without user calibration .
If the accuracy required for the final product is within this range, it will eliminate the time-consuming and laborious calibration process before shipping, resulting in significant cost savings. This correction coefficient can also be adjusted by the user. User calibration can improve accuracy to +/-0.002%FS . The calibrated coefficient is automatically corrected by setting it in the register specified by the logical channel, so there is no need to post-process the read value. It is also possible to intentionally manipulate the correction coefficient to adjust the measurement value to any range.
2.5 Evaluation Environment
2.5.1 Hardware
This high-performance AFE can be easily tested with the Arduino shield evaluation board: NAFE13388-UIM . The Arduino shield connector can be used to connect to various microcontroller evaluation boards.
2.5.2 Software
NXP Official Software
First, to check the basic operation, you can use the NXP microcontroller board: FRDM-MCXN947 and a Windows PC to evaluate the basic functions by operating from the GUI.
Furthermore, various evaluation and demo codes are available on the NXP Application Code Hub for dynamic evaluation/demo and as examples of driver layer code written in C. For the NAFE13388, code that runs on the same microcontroller board as above, FRDM-MCXN947, has been released, making it possible to evaluate everything from basic operation to data output using USB, CAN, and Ethernet.
Open Source Code
In addition to this, the code released as open source includes samples that run on NXP's microcontroller boards: FRDM-MCXN947 , FRDM-MCXN236 , FRDM -MCXA156 , and FRDM-MCXA153 , which can also be used to verify operation.
In addition, the microcontroller evaluation board IMXRT1050-EVKB , which is equipped with NXP's high-performance microcontroller i.MX RT1050, provides drivers and application examples using MicroPython .
The NAFE13388-UIM can be tested in combination with an Arduino microcontroller board as an "Arduino shield." Drivers for the various FRDM-MCX series listed above that have been ported to the Arduino SDK environment have been made public, as well as sample code (sketches) that run on the Arduino UNO R3 , Arduino UNO R4 Minima , and Arduino UNO R4 WiFi .
2.6 Application Notes
Various application notes are also available for the NAFE series. "Industrial Application Measurement Using NXP AFE" AN14102_JA (Japanese application note) introduces specific examples of connecting sensors and register settings for self-diagnosis, voltage measurement, current measurement, temperature measurement using 4-wire/3-wire/2-wire RTDs and thermocouples, and weight measurement using load cells.
Application notes are also available that provide more detailed information on various measurement methods and demo code. Please access these from the "Application Notes" section on the following page: https://www.nxp.jp/products/NAFEx3388
3. Summary
The NAFE13388 is a single-chip, high-precision analog front-end chip with eight software-configurable inputs.
By preparing a board with a simple input circuit equipped with this chip, you can use software to configure it to support a variety of measurement targets. Single-ended voltages of +/-12.5V (differential +/-25V) can be input directly, and the necessary protection circuits are also built into the chip, minimizing the number of external components and minimizing factors that affect cost and measurement error.
Furthermore, it is equipped with a wide range of self-diagnostic functions, making it fully capable of meeting the ever-increasing demand for functional safety. Built-in functions can be used to check the connection status of the object being measured, the status of installation, and the AFE operation.
Despite being such a high-performance chip, it's very easy to start evaluating it. You can check its operation using the GUI software on your PC with an Arduino shield-type evaluation board and an NXP microcontroller. Dynamic evaluation is also possible using sample code that is closer to real applications. What's more, the open-source code supports a wide range of microcontrollers.
For measurements using analog sensors, we recommend NXP's AFE: NAFE13388, which offers flexibility with simple hardware and software and enables high-precision measurements!
4. Reference materials
Product Page: Highly Configurable 8-Channel ±25 V Universal Input Analog Front End with Excitation
Evaluation Board: NAFE13388-UIM 8-Channel Universal Input AFE Arduino Shield Board
Related information: SPI Bus Overview (Japanese blog)
Related information: Installing MCUXpresso IDE and SDK (Japanese blog)
Change history: 2025-10-29: First Edition 2026-02-05: Evaluation Environment → Software → Open Source Code section: Added supported Arduino board types. Updated photos. 2026-06-21: Added table of contents and section numbers.
=========================
We are currently unable to respond to comments in the "Comment" section of this post. We apologize for the inconvenience, but when you contact us, please use the " NXP Technical Questions - How to contact us ( Japanese blog ) " (If you are already an NXP distributor or have a relationship with NXP, you may ask your question directly to the person in charge.) General-purpose AD converters built into microcontrollers cannot be used to measure the various sensors and voltages and currents with high precision required for industrial applications. Although various companies have supplied products specifically designed for "industrial use," they are still far from integrating all the necessary functions. In particular, the input circuitry requires discrete circuits, making it difficult to ensure accuracy and miniaturize the device. In addition, addressing functional safety, which will become essential in the future, has also been a major challenge. NXP's industrial analog front-end, NAFE13388, solves these problems simply and reduces costs by simplifying the product lineup and streamlining the manufacturing process.
introduction Sensor Japanese blog
查看全文
