Touch Screen Digitizer# AD7843ARQ Technical Documentation
## 1. Application Scenarios
### Typical Use Cases
The AD7843ARQ is a 12-bit successive approximation analog-to-digital converter (ADC) with synchronous serial interface and low power operation, making it ideal for various precision measurement applications:
Primary Applications:
- Touch Screen Controllers : Four-wire resistive touch screen digitization with dedicated drivers
- Battery-Powered Instruments : Portable data loggers and handheld measurement devices
- Industrial Process Control : Sensor interface for temperature, pressure, and position monitoring
- Medical Equipment : Patient monitoring systems and portable diagnostic devices
- Automotive Systems : Dashboard instrumentation and sensor interfaces
### Industry Applications
Consumer Electronics
- Smartphones and tablets with resistive touch interfaces
- Gaming controllers and portable media players
- Home automation control panels
Industrial Automation
- PLC analog input modules
- Motor control feedback systems
- Process variable monitoring (4-20mA loops)
Medical Devices
- Portable blood glucose meters
- Patient vital signs monitors
- Medical imaging equipment interfaces
### Practical Advantages and Limitations
Advantages:
- Low Power Consumption : 400μA at 3V during normal operation, 1μA in power-down mode
- High Resolution : 12-bit resolution provides 4096 discrete levels
- Integrated Touch Screen Drivers : Built-in drivers simplify touch interface design
- Small Package : 16-pin QSOP package saves board space
- Wide Voltage Range : Operates from 2.7V to 5.25V single supply
Limitations:
- Moderate Speed : 125kSPS maximum sampling rate limits high-speed applications
- Single-Ended Inputs : Lacks true differential input capability
- Limited Channel Count : Only 2 auxiliary analog inputs beyond touch screen interface
- Temperature Range : Commercial temperature range (0°C to +70°C) limits industrial applications
## 2. Design Considerations
### Common Design Pitfalls and Solutions
Power Supply Decoupling
- Pitfall : Inadequate decoupling causing noise and reduced accuracy
- Solution : Use 10μF tantalum and 0.1μF ceramic capacitors close to VCC and GND pins
Reference Voltage Stability
- Pitfall : Unstable reference voltage affecting conversion accuracy
- Solution : Implement low-noise reference circuit with proper buffering
- Recommended : Use ADR431 or similar low-noise reference ICs
Digital Interface Noise
- Pitfall : Digital switching noise coupling into analog inputs
- Solution : Separate analog and digital ground planes with single-point connection
- Implementation : Use ferrite beads or resistors in digital signal lines
### Compatibility Issues
Microcontroller Interface
- SPI Compatibility : Requires SPI mode 0 or 3 (CPOL=0, CPHA=0)
- Voltage Level Matching : Ensure logic levels match between ADC and host controller
- Timing Constraints : Meet minimum CS to SCLK setup time (15ns typical)
Sensor Interface Considerations
- Input Impedance : 1MΩ typical input impedance may load high-impedance sensors
- Solution : Use operational amplifier buffers for high-impedance sources
- Recommended : AD8605 for low-power buffer applications
### PCB Layout Recommendations
Power Distribution
- Use star-point grounding for analog and digital sections
- Implement separate analog and digital power planes
- Place decoupling capacitors within 5mm of power pins
Signal Routing
- Route analog inputs away from digital signals and clock lines
- Use guard rings around sensitive analog inputs
- Keep serial interface traces short and matched in length
Component Placement
- Position reference components close to REF pins
- Place bulk storage capacitors near
