TOUCH SCREEN CONTROLLER# ADS7845E Technical Documentation
## 1. Application Scenarios
### Typical Use Cases
The ADS7845E is a 12-bit sampling analog-to-digital converter (ADC) with synchronous serial interface and low-power operation, making it suitable for various precision measurement applications:
Primary Applications:
- Touch Screen Controllers : 4-wire resistive touch screen digitization with built-in drive and sense circuitry
- Battery Monitoring Systems : Precise voltage measurement in portable devices (0V to 5V input range)
- Industrial Process Control : Temperature, pressure, and flow measurement systems
- Medical Instrumentation : Patient monitoring equipment requiring low-power, high-accuracy conversion
- Portable Data Loggers : Environmental sensing with minimal power consumption
### Industry Applications
Consumer Electronics:
- Smartphones and tablets with resistive touch interfaces
- Portable gaming devices
- Digital cameras and camcorders
Industrial Automation:
- PLC analog input modules
- Sensor interface cards
- Process monitoring equipment
Automotive Systems:
- Climate control interfaces
- Infotainment touch panels
- Diagnostic equipment
Medical Devices:
- Portable patient monitors
- Medical test equipment
- Wearable health sensors
### Practical Advantages and Limitations
Advantages:
- Low Power Consumption : 750µW at 5V, 75µW in power-down mode
- High Accuracy : 12-bit resolution with ±1 LSB maximum differential nonlinearity
- Integrated Touch Screen Interface : Built-in drive and sense circuitry reduces external component count
- Small Package : 16-pin SSOP package saves board space
- Serial Interface : Simple 4-wire SPI-compatible interface reduces microcontroller I/O requirements
Limitations:
- Limited Input Channels : Only 2 differential/4 single-ended input channels
- Moderate Speed : 125kHz maximum sampling rate limits high-speed applications
- Single Supply Operation : 2.7V to 5V operation may require level shifting in mixed-voltage systems
- No Internal Reference : Requires external reference voltage source
## 2. Design Considerations
### Common Design Pitfalls and Solutions
Power Supply Decoupling:
- Problem : Inadequate decoupling causes noise and accuracy degradation
- Solution : Use 10µF tantalum capacitor at power input and 0.1µF ceramic capacitor close to VCC pin
Reference Voltage Stability:
- Problem : Unstable reference voltage introduces conversion errors
- Solution : Implement low-noise reference circuit with proper bypassing; use reference ICs like REF02 or similar
Digital Noise Coupling:
- Problem : Digital switching noise affects analog performance
- Solution : Separate analog and digital grounds, use ferrite beads in power lines, implement proper PCB partitioning
### Compatibility Issues with Other Components
Microcontroller Interface:
- SPI Timing : Verify microcontroller SPI clock polarity and phase settings (CPOL=0, CPHA=0)
- Voltage Levels : Ensure logic level compatibility between ADC and host microcontroller
- Interface Speed : Maximum SPI clock frequency of 2.1MHz at 5V supply
Sensor Compatibility:
- Input Range : 0V to VREF input range requires signal conditioning for sensors with different output ranges
- Impedance Matching : High-impedance sources may require buffer amplifiers
Reference Circuit Requirements:
- External reference voltage must be stable and low-noise
- Reference input impedance of 1kΩ typical
### PCB Layout Recommendations
Power Distribution:
- Use star-point grounding with separate analog and digital ground planes
- Connect analog and digital grounds at a single point near power supply
- Implement wide power traces with adequate current capacity
Signal Routing:
- Keep analog input traces short and away from digital signals
