Analog Devices Inc. AD698SQ

Overview

AD698SQ from Analog Devices is a monolithic LVDT signal conditioner IC that converts mechanical core position into a precise DC output voltage via ratiometric synchronous demodulation. It integrates an oscillator, voltage reference, dual demodulator channels, and computational circuitry - delivering ±11 V output, 0.05% linearity, and operation across –55°C to +125°C in a 24-lead CERDIP package for aerospace and high-reliability industrial position sensing.

For engineers reviewing the AD698SQ datasheet, AD698SQ pinout, AD698SQ application, or AD698SQ equivalent, key selection criteria include its ratiometric A/B transfer function, excitation frequency range (20 Hz–20 kHz), unipolar/bipolar output configurability, ±15 V dual-supply operation, and compatibility with half-bridge and 4-wire series-opposed LVDTs.

Technical Context

The AD698SQ implements a duty-cycle-based divider architecture that computes the ratio of secondary (A) to primary (B) LVDT voltages, eliminating gain errors from oscillator amplitude drift. Its internal multivibrator-driven sine shaper delivers low-distortion excitation (–50 dB THD) with frequency set by a single capacitor and amplitude set by a resistor.

Two independent synchronous demodulation channels process A (LVDT secondary) and B (LVDT primary) inputs; phase compensation pins (+ACOMP/–ACOMP) allow correction of LVDT-induced phase shifts. The output amplifier scales a 500 µA reference current using external R2 to produce a final voltage per V_OUT = 500 µA × (A/B) × R2.

Key Specifications

Pinout & Package

AD698SQ is housed in a hermetically sealed 24-lead CERDIP package (Q-24-2), rated for –55°C to +125°C operation and compatible with high-reliability soldering processes including MIL-STD-202 Method 210.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar LVDT signal conditioning applications.

Compared with AD598SD, AD698SQ eliminates A+B constancy requirement and adds phase compensation - improving transducer interchangeability and tolerance to LVDT aging. Compared with PGA900QPWR, AD698SQ provides direct analog output without firmware dependency, reducing system complexity for deterministic real-time control loops.

Availability

AD698SQ is available at Aetrix Electronics and suitable for aerospace actuation, downhole instrumentation, and military vehicle suspension systems requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for AD698SQ includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Analog Devices, Inc. is a global semiconductor leader specializing in high-performance analog, mixed-signal, and digital signal processing technologies for precision measurement and control.

The AD698 product line was engineered specifically for high-reliability LVDT/RVDT signal conditioning in harsh environments, emphasizing ratiometric accuracy, wide temperature operation, and minimal external component count.

FAQ

What is the maximum excitation voltage the AD698SQ can deliver to an LVDT primary winding?

The AD698SQ delivers up to 24 V rms differential excitation across EXC1 and EXC2 pins, configurable via resistor R1 between LEV1 and LEV2. At R1 = 487 Ω, output reaches 14–21.2 V rms; at open R1, it drops to 1.2–2.15 V rms. This range supports LVDTs with varying drive requirements while maintaining low THD (–50 dB typical).

Does the AD698SQ require external calibration components like trimmer potentiometers?

No, the AD698SQ requires no external adjustments. Its ratiometric A/B architecture, factory-trimmed gain and offset, and integrated oscillator eliminate manual calibration. External passive components (R1, C1, R2, C2–C4) set operational parameters only - not calibration - making AD698SQ ideal for high-volume, low-maintenance deployments.

Can the AD698SQ interface with both half-bridge and 4-wire LVDTs without hardware changes?

Yes, the AD698SQ natively supports both half-bridge and 4-wire series-opposed LVDT configurations. Pin assignments for AIN/BIN inputs and excitation outputs remain identical; only external wiring differs. No jumper settings, mode pins, or configuration registers are needed - simplifying design reuse across multiple transducer types.

What is the purpose of the +ACOMP and –ACOMP pins on the AD698SQ?

The +ACOMP and –ACOMP pins provide access to Channel A's comparator inputs, enabling external phase compensation networks. These pins allow insertion of RC networks to add phase lead or lag to the A-channel signal, correcting LVDT primary-to-secondary phase shifts - especially critical for high-frequency or long-cable installations where phase error degrades linearity.

How does the AD698SQ maintain accuracy when LVDT excitation amplitude drifts with temperature?

The AD698SQ maintains accuracy by computing the ratio of secondary voltage (A) to primary voltage (B). Since both signals experience identical amplitude drift from oscillator variation, the ratio remains invariant. This ratiometric method reduces gain drift to 20 ppm/°C - far superior to non-ratiometric conditioners where excitation drift directly translates to output error.