3 V/5 V, 1 mW 2-/3-Channel 16-Bit, Sigma-Delta ADCs# AD7705BRU Technical Documentation
## 1. Application Scenarios
### Typical Use Cases
The AD7705BRU is a complete analog front-end for low-frequency measurement applications, primarily serving as a 16-bit sigma-delta ADC with programmable gain amplification. Key use cases include:
- Industrial Process Control : Directly interfaces with strain gauge bridges, pressure sensors, and temperature transducers
- Weigh Scale Systems : Provides high-resolution measurement for load cells with programmable gains up to 128
- Portable Instrumentation : Low power consumption (1 mW typical) enables battery-operated devices
- Medical Equipment : Suitable for ECG monitors, blood pressure monitors, and patient monitoring systems
- Smart Transmitters : 4-20 mA loop-powered field instruments with isolated measurement capabilities
### Industry Applications
Manufacturing & Automation
- PLC analog input modules
- Motor control feedback systems
- Position sensing and encoder interfaces
- Vibration monitoring equipment
Medical & Healthcare
- Portable diagnostic equipment
- Patient vital signs monitoring
- Laboratory analytical instruments
- Biomedical sensor interfaces
Consumer & Commercial
- Precision weighing scales
- Environmental monitoring systems
- Energy management systems
- Building automation sensors
### Practical Advantages and Limitations
Advantages:
- High Resolution : 16-bit no missing codes performance
- Programmable Gain : On-chip PGA with gains from 1 to 128
- Low Power : 1 mW typical power consumption at 3 V
- Flexible Supply : Operates from 2.7 V to 5.25 V single supply
- Integrated Solution : Complete analog front-end reduces external component count
- Digital Filtering : Programmable filter cutoff and output update rate
Limitations:
- Speed Constraint : Maximum output word rate of 500 Hz limits dynamic applications
- Channel Count : Only two fully differential input channels
- Interface Complexity : Requires careful SPI timing management
- Noise Performance : May require external filtering in high-noise environments
- Calibration Overhead : Regular calibration cycles needed for highest accuracy
## 2. Design Considerations
### Common Design Pitfalls and Solutions
Power Supply Decoupling
- Pitfall : Inadequate decoupling causing noise and accuracy degradation
- Solution : Use 10 μF tantalum and 100 nF ceramic capacitors close to supply pins
- Implementation : Place decoupling capacitors within 5 mm of VDD and AVDD pins
Reference Voltage Stability
- Pitfall : Using noisy or unstable reference voltage sources
- Solution : Employ low-noise references like AD780 or REF19x series
- Implementation : Buffer reference output if driving multiple ADCs
Digital Interface Timing
- Pitfall : SPI communication failures due to timing violations
- Solution : Implement proper CS, SCLK, and DIN/DOUT timing delays
- Implementation : Use microcontroller GPIO with configurable drive strength
### Compatibility Issues with Other Components
Microcontroller Interfaces
- SPI Compatibility : Works with standard 3-wire and 4-wire SPI interfaces
- Voltage Level Matching : Ensure logic level compatibility between ADC and host
- Clock Requirements : Internal or external clock operation available
Sensor Compatibility
- Bridge Sensors : Direct interface with 350Ω strain gauges and load cells
- Thermocouples : Requires cold junction compensation circuitry
- RTD Sensors : May need current excitation sources
Power Management
- Mixed Voltage Systems : Compatible with 3.3V and 5V systems
- Power Sequencing : No specific power-up sequence requirements
- Sleep Mode : Compatible with power-saving microcontroller modes
### PCB Layout Recommendations
Analog Section Layout
```
+-----------------------+
| Sensor Input → RC
