Universal LVDT Signal Conditioner# AD698AP - LVDT Signal Conditioner Technical Documentation
*Manufacturer: Analog Devices Inc. (ADI)*
## 1. Application Scenarios
### Typical Use Cases
The AD698AP is a complete, monolithic Linear Variable Differential Transformer (LVDT) signal conditioning subsystem that provides significant advantages in precision position sensing applications. Typical use cases include:
Industrial Position Sensing
- Machine tool positioning systems requiring ±0.1% linearity
- Robotic arm joint angle measurement with resolution down to 0.01°
- Valve position feedback in process control systems
- Actuator stroke measurement in automotive and aerospace applications
Precision Measurement Systems
- Material testing equipment for strain and displacement measurement
- Coordinate measuring machines (CMM) requiring micron-level accuracy
- Vibration monitoring systems with DC to 20kHz frequency response
- Thickness gauging in manufacturing processes
### Industry Applications
Aerospace & Defense
- Aircraft flight control surface position monitoring
- Landing gear position indication systems
- Missile guidance system component positioning
- *Advantage*: Operates reliably in harsh environments with wide temperature ranges (-40°C to +85°C)
- *Limitation*: Requires external protection for extreme vibration conditions
Automotive Manufacturing
- Engine component positioning (throttle valves, EGR systems)
- Transmission shift position sensing
- Suspension height measurement systems
- *Advantage*: Excellent EMI rejection in electrically noisy environments
- *Limitation*: Limited to DC excitation frequencies (up to 20kHz)
Industrial Automation
- CNC machine tool feedback systems
- Hydraulic cylinder position control
- Packaging machinery linear position measurement
- *Advantage*: Single-chip solution reduces component count and board space
- *Limitation*: Requires careful LVDT selection for optimal performance
### Practical Advantages and Limitations
Advantages:
- Complete signal conditioning solution in single package
- Ratometric operation eliminates excitation amplitude sensitivity
- Wide operating temperature range (-40°C to +85°C)
- Low power consumption (typically 6mA supply current)
- Excellent long-term stability and repeatability
Limitations:
- Limited to DC excitation operation (20kHz maximum)
- Requires external components for complete system (LVDT, resistors, capacitors)
- Sensitivity to PCB layout and grounding practices
- Fixed excitation frequency determined by external components
## 2. Design Considerations
### Common Design Pitfalls and Solutions
Pitfall 1: Incorrect Excitation Frequency Selection
- *Problem*: Choosing inappropriate RC values for oscillator frequency
- *Solution*: Use formula f = 1/(C1 × R1) with R1 = 500kΩ recommended
- *Example*: For 2.5kHz operation, use C1 = 800pF
Pitfall 2: Poor LVDT Interface Design
- *Problem*: Inadequate drive capability for long cable runs
- *Solution*: Use buffer amplifiers for cable lengths > 3 meters
- *Implementation*: Add unity-gain buffers between AD698 and LVDT
Pitfall 3: Incorrect Filter Component Selection
- *Problem*: Improper bandwidth setting causing noise or slow response
- *Solution*: Calculate filter components based on required bandwidth
- *Guideline*: Use C2 = 1μF and R2 = 100kΩ for 1.6Hz bandwidth
### Compatibility Issues with Other Components
Power Supply Requirements
- Compatible with ±12V to ±15V dual supplies
- Ensure clean, well-regulated power with <10mV ripple
- Use 0.1μF decoupling capacitors close to power pins
LVDT Selection Compatibility
- Works with 4-wire LVDTs having secondary windings in series-opposing configuration
