**AD421BR: A Comprehensive Guide to the 4-20mA Loop-Powered HART®-Compatible DAC**

In the world of industrial process control and automation, the **4-20mA current loop** has long been the undisputed standard for transmitting analog signals. Its resilience to noise and ability to power field instruments remotely make it indispensable. The **AD421BR** from Analog Devices stands as a pivotal component in this domain, integrating a precision **Digital-to-Analog Converter (DAC)**, a loop power regulator, and HART® modem functionality into a single chip. This guide explores its architecture, key features, and application advantages.

**Architecture and Operating Principle**

The AD421BR is a complete, loop-powered, 16-bit DAC designed to function as the core of a smart field transmitter. Its operation is elegantly simple yet powerful. The entire device is powered by the **4-20mA loop current itself**, eliminating the need for a separate local power source. It draws a minimal quiescent current, ensuring the entire 4-20mA range is available for the output signal.

Internally, it comprises several key blocks:

* A **16-bit sigma-delta DAC** with a voltage output that is incredibly stable and accurate.

* A low-dropout (LDO) voltage regulator that provides a stable, programmable output voltage (from 3.3V to 5V) to power the associated sensor, microcontroller, and other circuitry.

* A **HART® FSK modem** interface. This block allows the microcontroller to superimpose digital HART communication signals onto the 4-20mA analog signal without any interference.

**Key Features and Advantages**

1. **High Resolution and Accuracy:** The 16-bit DAC ensures exceptional resolution, enabling the precise representation of process variables like pressure, temperature, or flow. Its low integral nonlinearity (INL) guarantees high accuracy across the entire operating range.

2. **Inherent HART® Compatibility:** This is a defining feature. The integrated modem interface allows the AD421BR to seamlessly support **bidirectional HART communication**. This enables device configuration, calibration, diagnostics, and the transmission of additional digital data alongside the primary analog value, all over the same two wires.

3. **Complete Loop-Powered Solution:** By generating a regulated supply voltage for its host system, the AD421BR drastically simplifies design. It reduces component count, board space, and overall system cost while enhancing reliability.

4. **Flexibility and Programmability:** The output range can be configured for standard 4-20mA operation or a 0-20mA span. The LDO output voltage is settable with external resistors, offering design flexibility for different microcontroller and sensor voltage requirements.

5. **Robust Performance:** Designed for the harsh industrial environment, it offers excellent noise rejection and operates reliably over an extended industrial temperature range.

**Typical Application Circuit**

In a standard smart transmitter design, the 4-20mA loop supply is connected across the AD421BR and the loop sense resistor. The host microcontroller, powered by the AD421BR's LDO, reads the sensor data via its ADC. It then sends the digital value to the AD421BR's DAC via a serial interface (SPI). The microcontroller also modulates/demodulates HART signals through the AD421BR's modem pins, enabling digital communication with a HART master system.

**Conclusion**

The AD421BR is far more than just a DAC; it is a **complete signal conditioning and communication solution** for intelligent, loop-powered field instruments. Its integration of a high-performance DAC, voltage regulator, and HART modem into a single package makes it an unparalleled choice for designers seeking to build reliable, accurate, and feature-rich transmitters for modern industrial automation systems.

**ICGOODFIND:** The AD421BR successfully integrates a high-precision 16-bit DAC, a loop power regulator, and HART modem functionality, providing a compact, efficient, and highly reliable total solution for 4-20mA loop-powered smart transmitters, significantly simplifying design complexity and enhancing system performance.

**Keywords:**

**4-20mA Current Loop**

**HART® Communication**

**Digital-to-Analog Converter (DAC)**

**Loop-Powered**

**Smart Transmitter**