LC2MOS 20-Bit A/D Converter# AD7703BR Technical Documentation
## 1. Application Scenarios
### Typical Use Cases
The AD7703BR 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:
- Precision Weigh Scales : Industrial and laboratory scales requiring 16-20 bit resolution
- Temperature Measurement : High-accuracy thermocouple and RTD measurements
- Pressure Transducers : Industrial process control and monitoring systems
- Medical Instrumentation : Patient monitoring equipment and diagnostic devices
- Strain Gauge Systems : Structural monitoring and material testing equipment
### Industry Applications
Industrial Automation
- Process control systems requiring 0.0015% accuracy
- 4-20mA loop monitoring with high precision
- Motor control feedback systems
Medical Devices
- Portable medical monitoring equipment
- Blood pressure monitors
- Patient weighing systems
Test and Measurement
- Laboratory-grade measurement instruments
- Data acquisition systems
- Calibration equipment
### Practical Advantages and Limitations
Advantages:
- High Resolution : 20-bit no missing codes ensures precise measurements
- Low Power : 40mW typical power consumption suitable for portable devices
- Integrated Features : On-chip digital filtering eliminates need for external components
- Wide Dynamic Range : 115dB typical for accurate signal capture
- Flexible Input Ranges : Programmable gain amplifier (1-128) accommodates various sensor outputs
Limitations:
- Limited Speed : Maximum output word rate of 500Hz restricts high-speed applications
- Complex Calibration : Requires careful calibration procedures for optimal performance
- Sensitivity to Noise : High resolution makes the device susceptible to power supply and digital noise
- Temperature Drift : Requires temperature compensation in precision applications
## 2. Design Considerations
### Common Design Pitfalls and Solutions
Power Supply Issues
- Pitfall : Inadequate power supply decoupling causing noise in measurements
- Solution : Use 10μF tantalum capacitor in parallel with 0.1μF ceramic capacitor at each power pin
Clock Configuration
- Pitfall : Incorrect clock frequency selection affecting conversion accuracy
- Solution : Maintain clock frequency between 100kHz and 2.5MHz as specified in datasheet
Grounding Problems
- Pitfall : Mixed analog and digital grounds creating ground loops
- Solution : Implement star grounding with separate analog and digital ground planes
### Compatibility Issues with Other Components
Microcontroller Interface
- SPI Compatibility : Requires 3-wire or 4-wire SPI interface with microcontroller
- Voltage Level Matching : Ensure logic level compatibility between AD7703BR and host processor
- Timing Constraints : Meet setup and hold times for serial interface communication
Sensor Compatibility
- Input Range Matching : Verify sensor output ranges match ADC input capabilities
- Impedance Matching : Ensure source impedance doesn't exceed 1kΩ for accurate measurements
### PCB Layout Recommendations
Component Placement
- Place decoupling capacitors within 5mm of power pins
- Keep crystal or clock source close to CLKIN pin
- Separate analog and digital components physically
Routing Guidelines
- Use separate analog and digital ground planes connected at single point
- Route analog signals away from digital traces and clock lines
- Implement guard rings around sensitive analog inputs
Power Distribution
- Use wide traces for power supply routing
- Implement multiple vias for ground connections
- Consider separate regulators for analog and digital supplies
## 3. Technical Specifications
### Key Parameter Explanations
Resolution : 20-bit no missing codes
- Ensures monotonic transfer function across entire input range
- Provides 1,048,576 discrete output codes
Input Voltage Range :
