LC2MOS 20-Bit A/D Converter# AD7703BN Technical Documentation
## 1. Application Scenarios
### Typical Use Cases
The AD7703BN is a 20-bit sigma-delta analog-to-digital converter (ADC) specifically designed for low-frequency measurement applications requiring high resolution and accuracy.
Primary Applications:
- Industrial Process Control : Temperature, pressure, and flow measurement systems
- Weigh Scale Systems : Precision weighing applications requiring 16-20 bit resolution
- Medical Instrumentation : Patient monitoring equipment, portable medical devices
- Portable Instrumentation : Battery-powered measurement systems
- Smart Transmitters : 4-20mA loop-powered field instruments
### Industry Applications
Industrial Automation
- PLC analog input modules
- Process variable transmitters
- Data acquisition systems
- Environmental monitoring equipment
Medical Devices
- Portable blood pressure monitors
- Patient vital signs monitoring
- Laboratory analytical instruments
- Biomedical sensor interfaces
Consumer Electronics
- High-end digital multimeters
- Precision measurement equipment
- Scientific instruments
- Automotive test systems
### Practical Advantages and Limitations
Advantages:
- High Resolution : 20-bit no missing codes performance
- Low Power : Typically 40mW power consumption
- Integrated Features : On-chip digital filtering and calibration
- Flexible Input : Accepts differential or pseudo-differential inputs
- Self-Calibration : Automatic calibration modes reduce design complexity
Limitations:
- Limited Speed : Maximum conversion rate of 4kHz limits dynamic applications
- External Components : Requires external crystal or clock source
- Noise Sensitivity : Sigma-delta architecture susceptible to high-frequency noise
- Complex Interface : Requires careful timing control for serial communication
## 2. Design Considerations
### Common Design Pitfalls and Solutions
Power Supply Decoupling
- Pitfall : Inadequate decoupling causing noise and accuracy degradation
- Solution : Use 10μF tantalum and 100nF ceramic capacitors close to power pins
- Implementation : Place decoupling capacitors within 10mm of VDD and VSS pins
Clock Source Stability
- Pitfall : Unstable clock source affecting conversion accuracy
- Solution : Use crystal oscillator with 10ppm stability or better
- Implementation : Follow manufacturer's crystal loading recommendations precisely
Grounding Issues
- Pitfall : Poor ground layout introducing digital noise into analog signals
- Solution : Implement star grounding with separate analog and digital grounds
- Implementation : Connect analog and digital grounds at single point near power supply
### Compatibility Issues with Other Components
Microcontroller Interface
- Issue : Timing compatibility with various microcontroller SPI interfaces
- Solution : Verify timing margins and use appropriate clock polarities
- Compatible MCUs : Most modern microcontrollers with hardware SPI (ARM, PIC, AVR)
Sensor Compatibility
- Issue : Input voltage range matching with various sensors
- Solution : Use appropriate signal conditioning for sensor output levels
- Typical Sensors : RTDs, thermocouples, strain gauges, pressure sensors
Reference Voltage
- Issue : Reference voltage stability and temperature coefficient
- Solution : Use precision references like AD780, REF19x series
- Requirements : Low noise, high stability (<3ppm/°C) references recommended
### PCB Layout Recommendations
Analog Section Layout
- Keep analog input traces short and away from digital signals
- Use guard rings around analog input pins for high-impedance sources
- Implement proper shielding for sensitive analog inputs
Power Distribution
- Use separate power planes for analog and digital sections
- Implement star-point grounding near the ADC
- Ensure low-impedance power distribution paths
Component Placement
- Place bypass capacitors as close as possible to power pins
- Position crystal/crystal oscillator near CLKIN pin
