Power Supply Design for Precision Analog Circuits
Clean power supplies are essential for achieving the full performance of precision analog circuits. This application note covers power supply design considerations for Rorebai high-resolution ADCs, precision op-amps, and RF transceivers.
Noise Requirements: High-resolution ADCs are sensitive to power supply noise. A 16-bit ADC with 5V full-scale has an LSB size of 76µV. Power supply ripple should be well below this level - typically less than 1mVpp for precision applications. Use low-noise LDOs rather than switching regulators for analog supplies when possible.
LDO Selection: Select LDOs with low noise specifications (<10µVRMS) and good power supply rejection ratio (PSRR) at relevant frequencies. The LDO should have adequate PSRR (>60dB) at the ADC sampling frequency to prevent clock noise from coupling into the analog supply.
Isolation: Separate analog and digital power supplies to prevent digital switching noise from corrupting analog circuits. Use ferrite beads or small resistors between supplies to provide isolation while maintaining DC continuity. Consider separate regulators for analog and digital sections.
Sequencing: Some devices have specific power-up sequencing requirements. Ensure that digital I/O pins are not driven before the device power supply is stable. Follow datasheet recommendations for power supply sequencing and ramp rates.
Monitoring: Consider implementing power supply monitoring for critical applications. Monitor both voltage levels and current consumption to detect faults early. Some Rorebai devices include built-in supply monitoring features.
Thermal Considerations: Power dissipation in regulators affects system reliability. Calculate total power dissipation and ensure adequate heatsinking. Consider thermal shutdown features in regulator selection for safety-critical applications.
💡 FAE Insights
Professional Insight
Power supply quality directly impacts analog circuit performance.
⚠️ Common Pitfalls
- ✗ Using noisy switching regulators
- ✗ Insufficient decoupling
- ✗ Poor isolation between domains
- ✗ Ignoring PSRR specifications
📋 Customer Cases
Medical Device Manufacturer
Medical
Challenge
ADC readings showed periodic noise spikes
Solution
Replaced with low-noise LDO and added ferrite isolation
Results
Noise eliminated, achieved expected resolution
Frequently Asked Questions
1. Can I use a switching regulator for analog supplies?
Switching regulators for analog: (1) Generally not recommended - noise too high for precision; (2) If necessary - use post-regulation with LDO; (3) Synchronization - sync to ADC clock to avoid beat frequencies; (4) Filtering - extensive LC filtering required; (5) Placement - keep far from analog circuits; (6) Monitoring - check for interference with spectrum analyzer. Best practice: use dedicated LDOs for analog supplies.
2. What PSRR do I need for my ADC?
PSRR requirements: (1) Calculate - PSRR > 20log(Vsupply/Vlsb) + margin; (2) 16-bit example - need >80dB PSRR for 5V supply; (3) Frequency - check PSRR at ADC sampling rate; (4) LDO selection - choose with >60dB PSRR at 1MHz; (5) Decoupling - improves effective PSRR at high frequencies; (6) Measurement - verify with injected noise. Higher resolution ADCs need better PSRR.