Introduction
The raw output from a pyroelectric sensor is a tiny signal, typically millivolts or less, requiring significant amplification before it can be processed. This guide covers the design of a complete analog front-end.
Signal Characteristics
- Amplitude: 1-50 mV peak-to-peak (depending on target distance and size).
- Frequency range: 0.1-10 Hz (human motion).
- Source impedance: Very high (JFET output, but needs load resistor).
- Noise floor: Thermal and 1/f noise dominate.
Amplification Stages
Stage 1: Impedance Conversion and Biasing
The sensor’s JFET output requires a load resistor (10-100 kΩ) to ground. This stage provides a low-impedance signal for further amplification. A coupling capacitor (1-10 µF) removes the DC bias (typically 0.5-1.5V).
Stage 2: First-Stage Amplification
Use a low-noise op-amp (e.g., TLV9002, OPA333, MCP6001) in non-inverting configuration with gain of 100-1000 (40-60 dB). Include a high-pass filter (C with feedback R) to set lower cutoff around 0.1 Hz.
Gain = 1 + Rf/Rg
Lower cutoff f_l = 1/(2π × Rf × C) where C is the feedback capacitor (optional)
Stage 3: Band-Limiting Filter
A second-order low-pass filter with cutoff around 10 Hz removes high-frequency noise. Sallen-Key topology with gain of 1 is common.
Stage 4: Second-Stage Amplification (Optional)
If more gain is needed, add another non-inverting stage. Total gain typically 60-80 dB (1000-10,000×).
Stage 5: Comparator / Threshold Detector
Compare the amplified signal to a reference voltage. Use a comparator with hysteresis (e.g., LMV7235) to prevent oscillation. Hysteresis of 50-100 mV is typical.
Component Selection Guidelines
| Component | Requirements | Recommended |
|---|---|---|
| Op-amps | Low noise, low input bias current, rail-to-rail | OPA333, TLV9002, MCP6001 |
| Resistors | Low noise, 1% tolerance | Metal film |
| Capacitors | Low leakage for coupling, C0G/NP0 for filters | Film or C0G |
| Comparator | Built-in hysteresis, push-pull output | LMV7235, MAX9025 |
Noise Analysis
The dominant noise sources are:
- Sensor’s own noise (specified as NEP).
- Op-amp voltage noise (1/f and broadband).
- Resistor thermal noise.
Total input-referred noise should be below the minimum expected signal (e.g., < 1 mV).
Power Supply Considerations
- Use low-noise LDO regulators.
- Decouple each op-amp with 0.1 µF and 10 µF capacitors.
- Keep analog and digital grounds separate.
Example Circuit
A complete design using OPA333:
Sensor out → 47k load → 10µF coupling → 1k to non-inverting input of OPA333
Gain = 1 + 1M/10k = 101
Feedback capacitor 1.5µF in parallel with 1M sets lower cutoff ≈ 0.1 Hz
Output → 2nd order Sallen-Key low-pass (fc=10Hz) → comparator with hysteresis
PCB Layout Tips
- Keep sensor connections short.
- Guard ring around high-impedance nodes.
- Use ground plane.
- Separate analog and digital sections.
Conclusion
A well-designed analog front-end is critical for PIR sensor performance. Careful component selection and layout ensure maximum sensitivity with minimum noise.