AD5700ACPZRL7 HART-Certified Single-Chip Modem Drives Industrial IoT Innovation

^{November 10, 2025 — In the fields of industrial automation and smart instrumentation, reliable long-distance communication solutions are becoming pivotal for intelligent transformation. The AD5700ACPZ-RL7, a single-chip modem certified by the HART Communication Foundation, is redefining the communication architecture of industrial field devices with its exceptional integration and stable communication performance.}

 

 

 

I.Chip Introduction
 

 

^{The AD5700ACPZ-RL7 is a high-performance HART modem chip specifically designed for industrial automation applications. Featuring a compact package, it integrates complete HART protocol physical layer functionality. With rigorous industrial-grade certification, this chip meets communication requirements in harsh industrial environments, providing reliable communication solutions for smart transmitters and process control equipment.}

 

 

 

^{1.Core Technical Features}

^{Complete HART Protocol Support}

^{Integrated 1200Hz/2200Hz FSK modem}

^{Supports full HART physical layer protocol stack}

^{Built-in programmable gain amplifier and filters}

^{Automatic carrier detection and signal quality monitoring}

 

^{High-Performance Analog Front-End}

^{16-bit high-precision ADC and DAC}

^{Integrated precision voltage reference}

^{Supports ±60V overvoltage protection}

^{Excellent anti-interference performance}

 

^{Industrial-Grade Reliability Design}

^{Operating temperature range: -40℃ to +125℃}

^{Single power supply: 3.3V/5V}

^{Low-power architecture with standby current <10μA}

 

^{2.Design Value and Advantages}

^{System Design Simplification}

^{Single-chip implementation of complete HART communication functionality}

^{Significantly reduces the number of external components}

^{Lowers system complexity and overall cost}

 

^{Flexible Configuration Capabilities}

^{Standard UART interface for host communication}

^{Programmable communication parameters}

^{Supports multiple operating modes}

 

^{Reliable Communication Performance}

^{Powerful noise suppression capability}

^{Automatic signal gain control}

^{Stable long-distance transmission}

 

 

 

II. Functional Block Diagram Analysis

 

 

^{1. HART Modem Core}

^{FSK MODULATOR: Frequency Shift Keying modulator that converts digital signals into HART protocol-specified frequency signals (1200Hz and 2200Hz).}

^{FSK DEMODULATOR: Demodulator that extracts digital data from HART signals.}

^{BAND-PASS FILTER AND BIASING: Band-pass filtering and biasing circuit used for signal conditioning and noise suppression.}

 

 

^{2. Clock and Oscillator}

^{OSC: Internal oscillator, working with external crystals (e.g., XIAL1, XIAL2) to provide clock signals.}

^{CLKOUT: Clock output pin, capable of providing clock signals to external devices.}

^{CLK_CFG0 / CLK_CFG1: Clock configuration pins used to set clock modes or frequencies.}

 

^{3. Communication Interface
UART Interface:}

^{TXD / RXD: Serial data transmission/reception.}

^{RTS / CTS: Request to Send/Clear to Send, used for hardware flow control.}

^{CD: Carrier Detect, indicating the presence of HART signals.}

^{DUPLEX: Full-duplex control, used to switch between transmission and reception modes.}

 

^{4. Power and Reference Voltage}

^{IOVcc: I/O power supply voltage}

^{REG_CAP: Regulator capacitor pin for internal voltage regulator}

^{VOLTAGE REFERENCE: Internal voltage reference source}

^{REF_REF_IN: External reference voltage input}

 

^{5. Analog Signal Processing}

^{BUFFER: Buffer amplifier for enhanced drive capability}

^{ADC_IP: ADC input pin for analog signal acquisition}

^{HART_IN/HART_OUT: HART signal input and output pins}

^{HART_VO: HART voltage output}

 

^{6. Control and Configuration}

^{HART_SEL: HART mode selection pin}

^{ABC: Automatic Bias Control}

 

 

Application Scenarios

 

^{This chip is commonly used in:}

^{Industrial Process Automation (e.g., HART communication in PLC and DCS systems)}

^{Smart Transmitters (field instruments supporting HART protocol)}

^{HART Multiplexers}

^{Analog I/O Modules (e.g., used with DACs like AD5421 and AD5422)}

 

^{Pin Functions and Signal Flow}

^{Input Pins: HART_IN (receives external HART signals), RXD (receives serial data), CLK_CFG0/1 (clock configuration), RESET (reset), etc.}

^{Output Pins: HART_OUT (transmits HART signals), TXD (transmits serial data), CLKOUT (outputs clock signals), etc.}

^{Power and Ground: Vcc (positive power supply), AGND (analog ground), DGND (digital ground), IOVcc (I/O power supply), ensuring stable chip power supply and signal reference ground.}

 

^{Summary
The AD5700/AD5700-1 is a highly integrated HART modem chip that combines modulation/demodulation, filtering, clock management, and UART interface functionalities. It is suitable for reliable digital communication between industrial field instruments and control systems. Its low power consumption and high level of integration make it an ideal choice for HART device design.}

 

 

 

 

III. Schematic Diagram of Frequency Shift Keying (FSK) Encoder Principle

 

 

 

1. Technical Principle Analysis: DDS Engine as the Core

 

^{The AD5700 utilizes a Direct Digital Synthesis (DDS) engine to implement FSK modulation. DDS technology generates signals through digital accumulators and waveform lookup tables, with its core advantage being inherent phase continuity. During frequency switching, no phase jumps occur, resulting in an FSK output with a pure spectrum and strong anti-interference capability, significantly enhancing communication reliability.}

 

 

^{2. Functional Mapping: From Schematic to Chip Internals}

^{Figure 20 "DDS-based FSK Encoder," though a simplified model, directly maps to the internal functional modules of the AD5700:}

 

^{1."Frequency Control Word" Register: Corresponds to the configuration registers of the AD5700. Users write parameters via the SPI interface to set the carrier frequencies (e.g., 1200Hz and 2200Hz).}

 

^{2."DDS Engine and DAC": This is the core of the AD5700—it integrates complete DDS logic and a high-precision DAC. When digital signals ('0' and '1') from the HART protocol control the switching of the frequency control word, this module instantaneously and seamlessly generates the corresponding pure analog sine waves.}

 

^{3.FSK Output: The final output from the MOD_OUT pin of the AD5700 is a phase-continuous FSK analog signal, precisely filtered and processed, ready to be sent to subsequent driver circuits and coupled to the 4-20mA current loop.}

 

^{3.Industrial Application Value
The DDS-based architecture delivers three core advantages for industrial applications:}

^{1.High Precision: Digital control ensures absolute frequency accuracy, guaranteeing compatibility with all HART devices.}

^{2.High Reliability: Continuous phase characteristics reduce bit error rates, making it suitable for harsh industrial environments.}

^{3.Simplified Design: A single chip replaces complex discrete circuits, significantly reducing PCB area and calibration costs.}

^{This design makes the AD5700 an ideal solution for HART communication in process control applications.}

 

 

 

IV. External Circuit Configuration

 

 

Analysis of HART_OUT Circuit Design with Resistive Load

 

^{1.Core Function
This circuit is responsible for coupling the FSK signal generated by the AD5700 compliantly to the HART bus.}

 

^{2.Component Function Analysis}

^{2.2 μF series capacitor: Serves as a DC-blocking capacitor, preventing DC bias from entering the bus while allowing AC FSK signals to pass without attenuation.}

^{22 nF capacitor to ground: Functions as a high-frequency filter and decoupling capacitor, removing high-frequency noise from the output signal and enhancing operational amplifier stability.}

 

^{R_LOAD (230 Ω to 600 Ω): Represents the standard impedance of the HART network. The total load impedance must be ensured within this range to guarantee signal strength and avoid reflections.}

 

^{3.Design Value
This structure is a validated standard output interface, ensuring proper communication between the device and any standard HART master device.}

 

 

^{Analysis of External Filter Circuit Design for ADC_IP}

 

^{1.Core Function
This circuit filters and attenuates input signals, providing a clean and appropriately scaled signal to the chip's internal carrier detection circuit.}

 

^{2.Component Function Analysis}

^{1.2 MΩ resistor & 300 pF capacitor: Form an RC low-pass filter to suppress high-frequency noise and transient interference, preventing false triggering.}

 

^{Resistor network (1.2 MΩ, 150 kΩ): Significantly attenuates the signal to ensure the input amplitude remains within a safe range and matches the detection level requirements.}

 

^{3.Design Value
This filter circuit is critical for achieving stable and reliable carrier detection in harsh industrial environments, effectively preventing false alarms or missed detections.}

 

 

 

V. Typical Application Circuit in Industrial HART Communication Systems

 

Analysis of HART Circuit in Current Input Module
 

^{This diagram depicts the AD5700 serving as a HART slave device, integrated into a typical interface circuit of a 4-20mA current input module (e.g., located in the AI card channel of a control system).}

 

^{1.Core Function Positioning}

^{This circuit is designed to receive FSK commands from field HART transmitters and send FSK response data without interrupting or interfering with the primary 4-20mA analog signal.}

 

 

 

^{2.Key Circuit Module Analysis}

^{Signal Extraction and Injection Network:}

^{150Ω Resistor: This is the core impedance matching resistor for HART communication. Together with the line capacitance, it forms an approximately 500Ω load to meet HART physical layer specifications, ensuring signals can be properly coupled and extracted.}

^{75kΩ & 22kΩ Voltage Divider Resistors: Used to establish a 0.75V DC bias voltage on the field side of the FSK output switch, providing the correct operating point for the output stage switching transistor.}

 

^{Transient Voltage Protection:}

^{150Ω Resistor and TVS Diode together form the front-end protection circuit. The 150Ω resistor serves for current limiting, while the TVS diode clamps high-voltage transient pulses, protecting the backend AD5700 chip from damage caused by field-induced surges and electrostatic discharge.}

 

^{AD5700 Interface:}

^{HART_OUT: Injects FSK signals into the bus through a network composed of a 75kΩ resistor and 150pF capacitor.}

^{ADC_IP: Receives FSK signals from the bus via a high-impedance RC low-pass filter formed by a 1.2MΩ resistor and 300pF capacitor, and performs carrier detection.}

 

^{3.Design Value}

^{This is a complete, industrially robust HART input channel solution that enables the coexistence of analog signals and digital communication while incorporating essential protection measures.}

 

^{Secondary HART Device Circuit Analysis
This diagram illustrates the simplified connection method of the AD5700 as a secondary HART device (such as a handheld HART communicator or other intelligent devices in the system that do not participate in 4-20mA control).}

 

 

Secondary HART Device Circuit Analysis

 

^{This diagram illustrates the simplified connection method of the AD5700 as a secondary HART device (such as a handheld HART communicator or other intelligent devices in the system that do not participate in 4-20mA control).}

 

 

 

^{1.Core Function Positioning}

^{This circuit enables the device to be connected in parallel to the HART bus for monitoring or communicating with primary HART master/slave devices without being directly connected in series in the 4-20mA current loop.}

 

^{2.Key Circuit Module Analysis}

^{High-Impedance Parallel Interface:}

^{1.2MΩ Resistor: This high-value resistor ensures the secondary device has minimal loading effect on the 4-20mA DC main loop, hardly affecting normal analog signal transmission.}

^{300pF Capacitor: Forms a low-pass filter with the resistor to suppress high-frequency noise while allowing HART FSK signals to pass through.}

^{Signal Path:
Both receiving and transmitting paths are coupled to the bus through this high-impedance network. Due to the parallel connection, its transmitted and received signals are superimposed on the main line.}

 

^{3.Design Value}

^{Provides a standard, non-intrusive method for connecting HART devices. This design is simple, cost-effective, and ensures that the original control system's normal operation remains unaffected when debugging or monitoring devices are connected.}

 

 

^{HART Circuit for Current Input Module}

^{Positioning: Integrated as a HART slave device in 4-20mA analog input channels
Core Design: Utilizes 150Ω impedance matching resistor to ensure signal coupling, establishes 0.75V DC bias through 75kΩ/22kΩ resistor network
Protection Mechanism: Equipped with TVS diodes and current-limiting resistors for transient voltage protection
Interface Characteristics: Implements signal transmission and reception through precision RC networks, meeting industrial environment reliability requirements}

 

^{Secondary HART Device Circuit}

^{Positioning: Connected as a parallel device (e.g., handheld configurator) to HART networks
Core Design: Implements non-intrusive connection using 1.2MΩ high-value resistors
Signal Coupling: Forms AC path through 300pF capacitors without affecting DC signal transmission
Application Advantage: Simple structure with low cost, suitable for temporarily connected diagnostic and configuration equipment}

 

^{These two circuit types define the standard connection methods for the AD5700 as either a core slave device or an auxiliary device in HART networks, providing standardized solutions for different application scenarios.}

 

 

 

 

VI. Typical Connection Circuit Diagram

 

 

^{Digital Interface Circuit Between AD5700 and Microcontroller
This diagram defines the communication and control connections between the AD5700 and the system's main MCU, serving as the "digital brain" of the entire system.}

 

 

^{1.Core Function:
Establishes a data exchange and control channel between the MCU and the AD5700.}

 

^{2.Key Connection Analysis:}

^{Serial Communication Interface:}

^{TXD: Receives data to be sent from the MCU. The MCU transmits HART commands and data through this pin to the AD5700 for FSK modulation.}

^{RXD: Outputs received data to the MCU. The AD5700 sends demodulated HART data to the MCU through this pin.}

 

^{Hardware Flow Control:}

^{RTS: Request to Send, controlled by the MCU. The MCU pulls this pin low to notify the AD5700 to prepare for data reception, and the AD5700 will make corresponding preparations based on this signal.}

 

^{Carrier Detection:}

^{CD: Carrier Detect output, driven by the AD5700. When the chip detects a valid FSK signal on the HART bus, it notifies the MCU through this pin, indicating "data is being received."}

 

^{Reset and Enable:}

^{RESET: Global reset pin used to restore the chip to its power-on initial state.}

^{RET_EN: Reserved or specific function enable pin. Its specific functionality should be configured according to the datasheet.}

 

^{3.Design Value:}

^{This set of connections forms a standard asynchronous serial interface, enabling the AD5700 to be easily controlled by the MCU like most serial peripherals, significantly simplifying software driver development.}

 

 

 

^{Power and Analog Input/Output Interface Circuit of AD5700
This diagram defines the chip's power supply, reference voltage, and analog signal input/output paths, which are critical for ensuring HART signal quality.}

 

 

 

 

^{1.Core Functions:}

^{Provides clean power supply and precise voltage reference for the chip}

^{Handles HART analog signal input and output processing}

 

^{2.Critical Circuit Analysis:}

^{Power Management:}

^{Wide Voltage Supply: VCC supports a broad voltage range of 1.7V to 5.5V, facilitating compatibility with various low-power MCU systems.}

^{Decoupling Capacitors: A combination of 10µF and 100nF capacitors is used for power decoupling, filtering out low-frequency and high-frequency noise respectively to ensure power quality, which is crucial for the stable operation of analog circuits.}

^{REG_CAP: This pin for the external capacitor of the internal voltage regulator must be connected to a capacitor of the specified value as per the datasheet to ensure the stability of the internal power supply.}

 

^{3.Design Value:}

^{This circuit section serves as the foundation for ensuring the performance of the HART communication physical layer. Meticulous power and reference designs guarantee signal generation accuracy, while proper analog interface configuration ensures signal robustness in harsh industrial environments.}

 

^Summary

^{By combining these two diagrams, a complete HART slave device communication subsystem is formed:}

 

^{Diagram 1 (Digital Interface) handles protocol and data processing, enabling the MCU to control and interact with the AD5700.}

^{Diagram 2 (Power and Analog Interface) is responsible for signal generation, conditioning, and integrity, converting digital commands into high-quality HART analog signals and reliably extracting signals from the bus.}

 

^{By following these reference designs, engineers can efficiently and reliably integrate HART communication functionality into their 4-20mA transmitters, actuators, or control systems.}

 

 

VII. Reference Demonstration Circuit Block Diagram for HART-Enabled Smart Transmitters

 

^{System Architecture Overview: A Typical HART Smart Transmitter}

^{This block diagram illustrates an intelligent transmitter solution based on the ADuCM360 microcontroller and AD5421 digital-to-analog converter. Its workflow can be summarized as follows:}

 

 

 

 

 

^{1.Sensing: Physical signals are collected through pressure sensors and temperature sensors (e.g., PT100).
2.Processing: The microcontroller ADuCM360 performs calculations, linearization, and compensation.
3.Output and Communication:}

• ^{Processed data is converted into a standard 4-20mA analog current signal output via the AD5421.}
• ^{Simultaneously, the AD5700 superimposes digital configuration or diagnostic data onto the 4-20mA loop in the form of HART FSK signals, enabling bidirectional communication.}

 

^{Core Role and Connection Analysis of AD5700
In the system, the AD5700 serves the critical role of a HART modem, and its connectivity clearly defines its functional positioning:}

 

^{1.Command and Data Interface with Host MCU}

^{Connection Target: UART of ADuCM360 microcontroller
Function: This serves as the "brain" connection for the AD5700. The MCU transmits HART data (e.g., device model, range, diagnostic information) to the AD5700 via UART and receives HART commands from the host station (e.g., querying parameters, modifying settings) from the AD5700. Signals such as TXD, RXD, RTS, and CD interact here to achieve precise communication timing control.}

 

^{2.Analog Signal Integration with DAC}

^{Connection Target: AD5421 current output DAC.}

^{Function: This is the mixing point for HART signals and 4-20mA analog signals. The FSK signal generated by the AD5700 is output from the HART_OUT pin and coupled to the output stage of the AD5421, precisely superimposed on the 4-20mA DC signal. This design ensures that HART communication does not interfere with the critical analog main signal, enabling signal coexistence on the same pair of wires.}

 

^{3.Carrier Detection and System Linkage}

^{Connection Target: Test connector (T1: CD).}

^{Function: The CD pin of the AD5700 is not only connected to the MCU but may also be routed to test points. This facilitates engineers in monitoring HART communication activity on the bus during debugging, serving as a critical window for system diagnostics and maintenance.}

 

 

^{Application Scenarios and Design Value}

^{This reference design reveals the core value of AD5700 in the Industrial Internet of Things:}

 

^{Enabling Intelligent and Digital Upgrades: It transforms traditional 4-20mA transmitters from purely analog devices into intelligent devices capable of remote configuration, calibration, diagnostics, and fault prediction. Engineers can manage and maintain devices without being on-site, using HART handheld devices or control systems.}

 

^{Ensuring Communication Reliability: In an industrial-grade environment composed of "HART input filters" and "loop protection" circuits, the AD5700 guarantees stable and reliable HART communication even in noisy industrial settings.}

 

^{Providing a Complete Solution: This block diagram showcases Analog Devices' end-to-end chip solution, spanning sensing, processing, output, and communication. As a dedicated communication component, the AD5700 achieves optimal synergy with other company chips like microcontrollers and DACs, significantly simplifying design complexity and shortening time-to-market.}

 

^{Summary
In this HART smart transmitter demonstration circuit, the AD5700ACPZ-RL7 plays an indispensable role as a "communication officer." It efficiently and reliably encodes digital information and modulates it onto the analog current loop, serving as a key enabling component for achieving intelligent and networked process automation equipment.}

 

 

 

VIII. Loop-Powered Transmitter Circuit Block Diagram

 

 

^{This diagram illustrates the core application circuit of the AD5700/AD5700-1 in a typical loop-powered (two-wire) transmitter. This architecture represents one of the most classic and challenging designs in the process control field, with the AD5700 playing a critical role in it.}

^{The following analysis clarifies the key functions of the AD5700 in the loop-powered system:}

 

 

 

^{System Core Challenge: Balancing Power and Communication}

^{The fundamental constraint of loop-powered transmitters lies in the fact that all power consumption of the entire device must be drawn from the 4-20mA current loop, with total power consumption not exceeding approximately 3.5mA (at 4mA). Exceeding this limit would disrupt the analog signal's zero point. It is within this context that the value of the AD5700 becomes prominent.}

 

^{Analysis of the AD5700's Key Role and Connections in the System}

^{Under these demanding conditions, the AD5700 serves as an ultra-low power HART modem.}

 

^{1.Ultimate Power Management}

^{Power Source: The diagram shows the AD5700's VCC is supplied by the system's internal REGOUT voltage regulator. The input energy for this regulator is entirely derived from the loop.}

 

^{Core Value: The AD5700's inherently ultra-low power consumption is a prerequisite for its application in loop-powered systems. It operates efficiently within minimal current budgets, ensuring that HART signal transmission and reception do not increase total loop consumption to a level that affects the 4mA baseline.}

 

^{2.HART Signal Injection and Extraction}

^{Signal Transmission: The FSK signal generated by the AD5700's HART_OUT pin is coupled into the loop through a high-impedance network composed of a 1.2MΩ resistor and a 150pF capacitor. This high-impedance design ensures that only negligible additional DC current is consumed when injecting AC HART signals.}

 

^{Signal Reception: HART signals from the loop are fed into the ADC_IP pin for demodulation through another high-impedance RC filter network, consisting of a 1.2MΩ resistor, a 300pF capacitor, and a 150kΩ resistor. This network similarly minimizes current draw from the loop.}

 

^{3.Collaboration with the System Microcontroller}

^{Serial Interface: Connected to the main MCU via TXD, RXD, RTS, and CD for data exchange and communication timing control, consistent with its implementation in other applications.}

 

 

^{Application Scenarios and Design Value}

 

^{This diagram presents a complete and feasible loop-powered smart transmitter solution, where the AD5700 serves as the key enabler of its "intelligence":}

^{1.Enables True Two-Wire Smart Transmitters: It allows sensors to output 4-20mA analog signals while supporting bidirectional HART digital communication without requiring external power. This represents the ultimate goal of field instrument design.}

 

^{2.​​​​​Resolves Core Design Contradictions: With its ultra-low power consumption and high-impedance interface design, the AD5700 perfectly resolves the fundamental conflict between "communication functionality" and "strict power budget" in loop-powered systems.}

 

^{3.Ensures Communication Reliability: Even under extremely low power conditions, the DDS-based architecture guarantees the quality of HART signal generation, while the front-end filtering network ensures communication robustness in noisy industrial environments.}

 

^{4.Simplifies Compliance Design: This reference circuit provides a validated topology, helping engineers quickly pass HART physical layer compliance tests, particularly the stringent requirements for signal amplitude, waveform, and power consumption.}

 

^{Summary
In this loop-powered transmitter block diagram, the AD5700ACPZ-RL7 plays a dual role as both an "Energy Efficiency Master" and a "Communication Enabler." It not only functions as a HART protocol modem but is also a specialized chip optimized for extremely constrained power environments. Its existence makes it possible to develop high-performance, low-power, loop-powered HART smart transmitters, serving as a core driving force for industrial sensors advancing toward higher intelligence and integration.}