LC2MOS 20-Bit A/D Converter# AD7703AN Technical Documentation
*Manufacturer: Analog Devices Inc. (ADI)*
## 1. Application Scenarios
### Typical Use Cases
The AD7703AN is a complete 20-bit sigma-delta analog-to-digital converter (ADC) designed for low-frequency measurement applications. Its primary use cases include:
Direct Sensor Interface Applications
- Strain Gauge Measurements : Direct connection to bridge sensors with programmable gain amplifier (PGA) handling low-level signals
- Thermocouple Interfaces : High resolution enables precise temperature measurements with cold-junction compensation
- RTD (Resistance Temperature Detector) Systems : 20-bit resolution provides accurate temperature readings across wide ranges
- Pressure Transducers : Suitable for industrial pressure monitoring with built-in signal conditioning
Process Control Systems
- 4-20mA Loop Monitoring : Current-to-voltage conversion with high precision measurement
- Weigh Scale Systems : Excellent for precision weighing applications requiring 16-20 bit effective resolution
- Portable Instrumentation : Low power consumption (typically 40mW) enables battery-operated designs
### Industry Applications
Industrial Automation
- PLC analog input modules
- Motor control feedback systems
- Process variable monitoring (temperature, pressure, flow)
- Factory automation sensors
Medical Equipment
- Patient monitoring systems
- Portable medical instruments
- Diagnostic equipment requiring high precision
Test and Measurement
- Laboratory instruments
- Data acquisition systems
- Calibration equipment
Consumer Electronics
- High-end audio equipment
- Precision measurement tools
- Environmental monitoring devices
### Practical Advantages and Limitations
Advantages:
- High Resolution : 20-bit no missing codes provides excellent measurement precision
- Integrated Solution : Contains PGA, modulator, and digital filter in single package
- Low Noise Performance : Typical noise of 150nV RMS at gain of 128
- Flexible Power Supply : Operates from ±5V supplies
- Self-Calibration : On-chip calibration eliminates gain and offset errors
- Serial Interface : Simple 3-wire interface reduces microcontroller I/O requirements
Limitations:
- Limited Speed : Maximum output word rate of 500Hz restricts dynamic applications
- Input Range : Bipolar input range of ±VREF/Gain may require external conditioning
- Clock Dependency : Performance depends on stable external clock source
- Noise Sensitivity : High-impedance inputs require careful layout for noise immunity
## 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 at each power pin, placed close to the device
Reference Voltage Stability
- Pitfall : Using noisy or unstable reference voltage sources
- Solution : Employ low-noise references like AD780 or REF19x series with proper bypassing
Clock Signal Integrity
- Pitfall : Clock jitter affecting conversion accuracy
- Solution : Use crystal oscillators or dedicated clock generators; avoid microcontroller clock outputs
Input Signal Conditioning
- Pitfall : Direct sensor connection without proper filtering
- Solution : Implement anti-aliasing filters with cutoff frequency below 1/2 the sampling rate
### Compatibility Issues with Other Components
Microcontroller Interfaces
- Issue : Timing compatibility with different microcontroller families
- Resolution : Ensure microcontroller SPI interface supports 3-wire mode and adequate timing margins
Sensor Compatibility
- Issue : Mismatch between sensor output range and ADC input requirements
- Resolution : Use external amplifiers or attenuators to match signal levels
Mixed-Signal Systems
- Issue : Digital noise coupling into analog sections
- Resolution : Implement proper ground separation and filtering techniques
### PCB Layout Recommendations
