Analog Devices Inc. AD698APZ

AD698APZ LVDT Signal Conditioner from Analog Devices Product Summary

The Analog Devices AD698APZ is a highly integrated signal conditioning subsystem designed specifically for Linear Variable Differential Transformers (LVDTs), as well as for compatibility with half-bridge LVDTs and Rotary Variable Differential Transformers (RVDTs). Packaged in a compact 28-pin PLCC, the AD698APZ provides an all-in-one solution for converting mechanical position changes detected by LVDTs into linear DC voltage outputs—either unipolar or bipolar—with high accuracy and long-term repeatability. Particularly suited for industrial automation, precision measurement, and electromechanical servo systems, the AD698APZ represents a robust and consistent approach to LVDT interfacing, reducing design complexity and setup time for engineers.

Main Advantages and Features of the AD698APZ

The AD698APZ offers a unique combination of advanced signal processing capabilities and user-friendly characteristics that make it stand out in the realm of LVDT signal conditioners. Most notably:

Integrated solution: Incorporates oscillator, voltage reference, power amplifier, synchronous demodulators, ratiometric computational circuitry, and output driver—requiring only passive external components to complete the application circuit.

Wide compatibility: Accepts half-bridge, 4-wire (series-opposed) LVDTs, and RVDTs, making it extremely versatile for various sensor types and configurations.

Simplified adjustment: No significant factory or field adjustments are required due to the internal ratiometric computation, which minimizes drift and scale factor errors associated with excitation variations or temperature changes.

Flexible excitation: User-selectable LVDT excitation frequencies from 20 Hz to 20 kHz and excitation amplitudes from 2.1 V to 24 V rms, supporting a broad range of transducer requirements.

Robust output: Provides unipolar or bipolar DC voltage output, with output voltage swing up to ±11 V and a typical output current up to 11 mA.

Precision performance: Linearity of 0.05% FSR, gain drift of 20 ppm/°C, and exceptional temperature stability due to ratiometric correction.

Power: Operable from both single and dual supply rails, with wide supply voltage range (13–36 V) and low current consumption.

These features make the AD698APZ a strong candidate for engineers seeking to streamline LVDT signal chain design, whether retrofitting legacy systems or developing new applications.

How the AD698APZ Works Functional Block Diagram and Operation

At the core of the AD698APZ is a highly integrated analog front-end and computational block, optimized to interface directly with LVDTs and RVDTs. The internal block diagram reveals several major functions:

Oscillator and drive amplifier: Generates a low-distortion sine wave for LVDT excitation—frequency and amplitude set by external passive components.

Dual synchronous demodulation channels: Inputs for both primary (drive reference) and secondary (position signal) windings of the LVDT are processed using synchronous demodulators, enabling phase-sensitive detection and noise rejection.

Ratiometric computational core: To minimize scaling errors due to drift or variations in excitation amplitude, the AD698APZ digitally computes the ratio of the secondary to primary signals (A/B), providing immunity to excitation fluctuations.

Output signal processing: Filter stages and output amplifiers scale the result according to user-defined full-scale output and mechanical displacement.

Versatile reference: On-board filtered reference voltages ensure accuracy and stability in output signals.

This architecture ensures that the output voltage precisely tracks actual core movement, regardless of supply drift or environmental variations—addressing common challenges in traditional LVDT signal conditioning circuits.

Typical Technical Specifications of the AD698APZ

The robust analog signal chain of the AD698APZ yields impressive specification highlights at ambient temperature (+25°C) when powered with ±15 V:

Output voltage range: ±11 V typical

Output current: Up to 11 mA continuous, 20 mA short circuit

Output nonlinearity: Typically 75 ppm of full-scale, max 500 ppm across the temperature range

Gain error: ±1.0% max, gain drift 20 ppm/°C typical

Offset error: ±1% max, offset drift 5 ppm/°C typical

Excitation voltage: Configurable range from 2.1 V to 24 V rms, supporting varied LVDT loads

Excitation output current: Up to 50 mA rms, thermally protected

Sine wave distortion: Typically −50 dB total harmonic distortion

Signal input: High input impedance (200 kΩ), usable with input signals as low as 100 mV rms

Such parameters ensure the AD698APZ meets the precision and consistency requirements for high-resolution position measurement applications.

AD698APZ Electrical, Thermal, and Mechanical Specifications

The AD698APZ offers robust environmental and mechanical resilience, aligning with industrial demands:

Power supply range: ±13 V to ±18 V for dual supply, 17.5 V for single supply

Operating temperature: −40°C to +85°C (AD698APZ), with a more rugged −55°C to +125°C version available (AD698SQ)

Package: 28-pin PLCC (dimensions 11.51 × 11.51 mm), SMD for flexible design integration

RoHS and REACH compliance, MSL 3 (168 hours)

Junction-to-case thermal resistance: 30°C/W (PLCC package)

Junction-to-ambient thermal resistance: 60°C/W (PLCC package)

Storage temperature: −65°C to +150°C

These characteristics make the device suitable for environments subject to industrial or automotive temperature extremes and moisture constraints, while compact SMD packaging aids miniaturization and reliability.

Important Application Guidelines Implementing the AD698APZ in Practical Designs

When incorporating the AD698APZ into engineering solutions, several key aspects should be addressed:

LVDT compatibility: The AD698APZ supports both half-bridge and 4-wire (series-opposed) LVDTs as well as RVDTs. Configurations differ slightly in external circuitry, mainly with respect to phase compensation and reference selection.

Excitation configuration: Selecting the correct drive frequency and amplitude per LVDT manufacturer’s specifications maximizes measurement dynamic range and linearity. The external capacitor, C1, sets the excitation frequency (20 Hz to 20 kHz).

Ratiometric output: Thanks to internal ratio-metric computation, scale factor drift due to excitation amplitude fluctuations or temperature is virtually eliminated, improving system reliability over time and reducing system calibration demands.

Multiple transducer drive: The excitation output is powerful enough (up to 24 V rms and 50 mA) to drive multiple LVDTs in parallel, as long as total power dissipation and thermal limits are respected.

Output flexibility: Offset and gain settings are user-adjustable via external resistors, permitting simple adaptation to required system voltage output swings (e.g., ±10 V, 0 to +10 V, etc.).

Supply configuration: Works with both single and dual supply rails, increasing design flexibility (critical for retrofits or platforms with limited rail resources).

Design Approach with the AD698APZ Single- and Dual-Supply Options

The AD698APZ is designed for straightforward circuit integration, with only a handful of passive components needed to set frequency, gain, filtering, and offset. The general design flow for dual and single supply operation is as follows:

Define system bandwidth and required excitation frequency; typically, excitation frequency should be 10x greater than the measurement bandwidth.

Select the LVDT and ascertain its sensitivity and mechanical stroke; calculate the expected voltage transformation ratio (VTR).

Choose excitation amplitude and adjust with R1 (see typical curves) to ensure both A and B channels operate within the 1.0 V to 3.5 V rms optimal input range.

Use external passive components (R2 for gain, C2–C4 for filtering/bandwidth, R3/R4 for offset) to set the system output scaling and signal conditioning as per mechanical displacement range.

For single-supply operation, additional power rail referencing and output offset components (R5, R6, and C5) are specified based on the required output voltage swing.

Output performance—including gain, offset, and filtering—can be fine-tuned without complex trimming or calibration, due to internal ratiometric compensation.

Reference connection diagrams provided by Analog Devices can be followed closely for a range of typical LVDT and supply topologies.

Alternative or Substitute Models for the AD698APZ

As the AD698APZ is marked obsolete, identifying alternative solutions is critical for ongoing designs and support. Potential equivalent or replacement models include:

AD598 (Analog Devices): Earlier-generation LVDT signal conditioner with similar basic function but different ratiometric architecture—note that the summing of LVDT secondaries (A+B) must remain constant.

Other Analog Devices LVDT conditioners: Reviewing the Analog Devices portfolio for current-generation products in the LVDT signal conditioning family is recommended as the company periodically updates offerings.

Texas Instruments PGA970: A modern LVDT/RVDT signal conditioner with programmable features and digital output capabilities, suitable for upgrades.

Control Concepts, Measurement Specialties (TE), and other instrumentation vendors: These companies offer both ASIC-based and modular LVDT/RVDT signal conditioners which may fit similar performance, pinout, or analog integration requirements.

When choosing an alternative, key evaluation criteria should include pin compatibility, LVDT excitation flexibility, bandwidth, linearity, supply voltage range, and long-term availability.

Conclusion

The Analog Devices AD698APZ LVDT signal conditioner stands out as a precise, reliable, and integrated solution for converting mechanical movement into robust analog voltage outputs, suitable for a range of industrial and instrumentation applications. Its advanced ratiometric architecture and flexible configuration significantly reduce design complexity and total system error, making it a reference design for engineers handling position measurement challenges. However, given its status as an obsolete product, design-in engineers and procurement professionals should evaluate recommended replacements or newer technology alternatives during product lifecycle management. In any case, understanding the AD698APZ's operation, key parameters, and integration strategies remains highly valuable for supporting legacy equipment and ensuring optimal performance in precision measurement systems.