AD7705BRZ-REEL

^{September 20, 2025 News — With increasing demands for signal processing accuracy in automotive electronics and portable devices, high-resolution analog-to-digital converters (ADCs) are becoming core components of precision measurement systems. The AD7705BRZ-REEL from Analog Devices Inc., with its 16-bit no-missing-code accuracy, low power consumption, and high integration, provides highly reliable data acquisition solutions for in-vehicle sensor monitoring, industrial process control, and portable instruments.}

^{I. Chip Introduction: AD7705BRZ-REEL
The AD7705BRZ-REEL is a complete 16-bit, low-power Σ-Δ ADC specifically designed for measuring low-frequency analog signals. It directly digitizes weak signals from sensors without requiring complex external conditioning circuits, significantly simplifying system design and reducing overall costs.}

II. Functional Block Diagram Analysis

Core Functional Module Details

^{1.Analog Front End}

^{Multiplexer (MUX): Responsible for selecting the input channel. The AD7705 has two fully differential channels. The MUX selects which input pair (AIN1+/AIN1- or AIN2+/AIN2-) to route to the subsequent circuit based on the configuration register.}

^{Programmable Gain Amplifier (PGA): Amplifies the selected weak analog signal (e.g., from thermocouples or load cells). The gain can be programmed from 1 to 128 to accommodate input signals of varying magnitudes.}

^{Σ-Δ Modulator: This is the core of the high-precision ADC. It converts the amplified analog voltage into a high-speed bit stream (1-Bit Stream) composed of 1s and 0s. The density of "1"s in this stream is proportional to the average value of the input voltage.}

Digital Processing Section

^{Digital Filter: Converts 1-bit stream to 16-bit digital code, controls output rate and power-line rejection
Control Logic: Receives MCU commands via serial port, configures operating modes and stores register settings
Clock Circuit: Requires external crystal to provide master clock}

^{Interface and Control}

^{Serial Interface (SPI-compatible): Includes chip select (CS), serial clock (SCLK), data input (DIN), and data output (DOUT) pins for MCU communication. Also features a data ready (DRDY) output pin to indicate when new conversion data is available for reading.}

^{Reset (RESET): Hardware reset pin used to restore the chip to its power-on default state.}

^{Signal Flow Summary}

^{Analog Signal Path: External sensor signal → MUX → PGA → Σ-Δ modulator (converted to 1-bit digital stream)}

^{Digital Signal Path: 1-bit stream → Digital filter (converted to 16-bit data) → Control logic → Output to MCU via serial interface}

^{III. Pin Configuration and Functional Description}

^{Important Notes:}

^{1.Power Pins: VDD (positive supply, +3V to +5.25V) and GND are typically located on the side or bottom of the package and are not directly labeled in the 2D top view, but must be correctly connected during PCB layout.}

^{2.Serial Data Pins: DIN (data input), DOUT (data output), and DRDY (data ready, output) are critical communication pins. These are also not directly visible in the top view and require reference to the complete pinout diagram in the datasheet. In the actual 16-pin SOIC package, these pins are located on the opposite side.}

^{3.Reference Voltage: The quality of the reference voltage on the REF IN(+) and REF IN(-) pins directly determines the ADC's conversion accuracy and must use a stable, low-noise reference source.}

IV. Basic Connection Diagram Description

^{The following diagram clearly illustrates the typical application circuit connections for the 16-bit low-power Σ-Δ ADC—AD7705BRZ-REEL. This circuit represents the minimum system configuration required for proper chip operation, including essential external components such as power supply, reference voltage, clock source, analog inputs, and digital interface.}

Core Connection Description:

^{1.Power Supply and Decoupling}

^{Uses a +5V analog power supply with parallel 10μF and 0.1μF capacitors for decoupling to ensure clean and stable power.}

^{2.Reference Voltage}

^{Relies on an external high-precision reference source (e.g., AD780) to provide the reference voltage, whose performance directly determines the absolute accuracy of the conversion results.}

^{3.Clock Source}

^{An external crystal connected to the MCLK IN and MCLK OUT pins provides a stable master clock for the converter.}

^{4.Analog Input}

^{Supports differential input connections to effectively suppress common-mode noise, making it suitable for connecting various sensors.}

^{5.Digital Interface}

^{Uses an SPI-compatible interface (SCLK, CS, DIN, DOUT) to communicate with the microcontroller. The DRDY pin serves as a status signal, efficiently indicating data readiness.RESET: Hardware reset pin used to restore the chip to its power-on default state.}

^{Summary:
This basic connection diagram highlights the core advantage of the AD7705BRZ-REEL as a highly integrated ADC: minimal external circuitry. Designers need only provide a stable power supply, reference voltage, and clock to build a high-precision data acquisition system capable of directly processing weak sensor signals. Correct connections and high-quality external components are essential for fully realizing its 16-bit performance.}

V. High-Precision PT100 RTD Temperature Measurement Application (4-Wire Configuration)

^{This diagram illustrates a typical application of the AD7705 in a high-precision, 4-wire PT100 RTD temperature measurement circuit. The design utilizes the ratiometric measurement principle and 4-wire connection method to effectively eliminate the influence of lead resistance, achieving extremely high measurement accuracy.}

^{Core Design Principle: Ratiometric Measurement}

^{Excitation Source: Uses an external 400μA precision constant current source to excite the PT100 RTD}

^{Reference Voltage: The same current source flows through a low-temperature-coefficient precision 6.25kΩ resistor to generate the reference voltage VREF​.}

^{Signal Voltage: The voltage drop VRTD​ generated by the current source across the PT100 serves as the analog input signal.}

Ratiometric Advantage: Since both the input voltage VRTD​ and reference voltage VREF are generated by the same current source, the ADC output code depends solely on the resistance ratio between the PT100 and the 6.25 kΩ resistor:

Code∝VREF​VRTD​​=I×RREF​I×RRTD​​=6.25kΩRRTD​​
 

Therefore, any minor fluctuations in the excitation current are simultaneously canceled out in both the numerator and denominator, fundamentally eliminating the impact of current source accuracy and drift on the measurement results.

^{Four-Wire Connection and Lead Resistance Elimination}
 

^{Force Line and Sense Line Separation:}

^{RL1 and RL4 are excitation leads that carry a 400μA current, which will produce a voltage drop. However, this drop manifests as a common-mode voltage.}

^{RL2 and RL3 are sense leads connected directly to the high-impedance inputs of the AD7705. Since the input current of the ADC is extremely low (typically in the nanoampere range), the voltage drop across RL2 and RL3 is negligible.}

^{Result: The AD7705 accurately measures the true voltage across the RTD itself (V_RTD) through its differential input channel, which remains completely unaffected by the lead resistances RL1 to RL4.}

^{Key Design Parameters and Considerations}

^{Reference Resistor:
The 6.25 kΩ resistor must be a precision resistor with a low temperature coefficient (such as 5 ppm/°C or better), as its stability directly determines the stability of the reference voltage (V_REF) and is critical to system accuracy.}

^{Buffer Usage:
Due to the low source impedance of the RTD, the internal input buffer of the AD7705 typically does not need to be enabled. If enabling the buffer is necessary (e.g., to enhance RF immunity), a resistor must be connected between the RTD sense point and the analog ground of the AD7705 to set the correct common-mode voltage range.}

^{Common-Mode Voltage:
The design must ensure that the common-mode voltage generated by V_RTD and V_REF remains within the specified operating range of the AD7705.}

^{Advantages Offered by the AD7705
This application fully leverages the strengths of the AD7705: its high input impedance ensures accurate sensing, the high-resolution PGA directly amplifies minute signals, and its exceptional digital filtering suppresses environmental noise. Combined with the four-wire ratio measurement method, it forms an extremely stable and reliable temperature measurement solution that requires no complex calibration, making it ideally suited for industrial control and laboratory instrumentation.}

VI.Smart Transmitter Application Circuit Diagram

^{This diagram illustrates the AD7705 serving as the core ADC in a classic 4-20mA smart transmitter system for industrial applications. The system is divided into field (sensor) side and control side by an isolation barrier to ensure safety.}

^{System Architecture and Signal Flow}

^{Field-Side Acquisition & Processing
A high-precision ADC directly interfaces with the sensor, converting analog signals into digital data. A microcontroller receives this data, performs calculation and calibration, and integrates the HART protocol to enable smart communication.}

^{Isolated Transmission for Safety
Signals are electrically isolated via an isolation barrier, ensuring safe separation between the field side and the control side. Digital signals are transmitted across the barrier through isolation components.}

^{Control-Side Output & Power Supply
A dedicated loop-powered DAC converts the processed digital signal into a 4–20 mA current output. It also utilizes the loop voltage to power the front-end circuitry, forming a complete two-wire system.}

^{System Architecture and Core Functions}

^{The system consists of a field side (including the AD7705 and an MCU) and a control side (including the AD421), with an isolation barrier in between to provide electrical isolation and ensure safety. The AD7705 is responsible for converting sensor signals into high-precision digital data. The MCU performs intelligent processing (such as PID calculations and HART protocol communication), and the AD421 converts the digital results into a standard 4-20mA loop current output while also supplying power to the front-end circuitry.}

^{Core Role in the Design}

^{1.High Precision:
16-bit no-missing-code performance ensures measurement accuracy.}

^{2.Low Power Consumption:
Extremely low power draw makes it ideal for applications with strict power budgets, such as 4-20mA loop-powered systems.}

^{3.Integrated PGA:
Amplifies small sensor signals directly, simplifying front-end design.}

^{Summary: This circuit represents a classic design in the field of industrial measurement. The AD7705 handles high-precision analog-to-digital conversion, the MCU enables intelligent processing, and the AD421 performs digital-to-current conversion with loop power supply. Together, they form a reliable, intelligent, and safely isolated two-wire transmitter solution.}