In a photoelectric smoke sensor, an LED periodically emits light into a chamber. The light scatters off smoke particles and enters a photodiode. The photodiode outputs a current proportional to the incident light, which is proportional to the smoke concentration. The LED current must be pulsed and regulated, and the photodiode current must be amplified with low noise and high linearity. Therefore, smoke detectors require a LED driver and photodiode amplifier to obtain an accurate signal from the sensor. With the signal amplified, an analog-to-digital converter (ADC) on the MCU captures samples of the signals and the MCU calculates the smoke concentration.

The TPS880x AFE integrates the regulators, drivers and amplifiers to interface the smoke detector’s power supply, sensors, and microcontroller unit (MCU). The TPS880x has adjustable LED driver current, LED pulse width, amplifier bandwidth, amplifier gain, and output filter bandwidth. Each of these parameters, along with the ADC timing, affects the SNR. When all of these parameters are adjusted, reliable smoke sensing with only nanoamps of photodiode current is achieved.

This report analyzes how each of the system parameters affects the SNR. Modeling the system provides valuable insight on how to optimize the SNR. Measurements verify the trends in the model and provide configurations for users to start their designs.

SNR is commonly defined as the ratio of signal power to noise power. In a smoke detector, the signal of interest is the smoke concentration, calculated from measurements of the photodiode current. Amplifying the photodiode current adds noise which makes the smoke concentration measurement fluctuate even if the actual smoke concentration is constant. Therefore, having high SNR is essential to quickly and accurately decide if the smoke concentration is at a dangerous level.

In this report, SNR is presented as the smoke measurement amplitude divided by the smoke measurement standard deviation. This metric provides the designer with the statistics of the smoke measurement. Because the smoke measurement amplitude is proportional to the smoke concentration, SNR increases with the smoke concentration. Therefore, the SNR in this report is specified at 1 nA photodiode current. The conversion from smoke concentration to photodiode current depends on the LED, photodiode, and chamber geometry. In general, the photodiode current is proportional to the LED current. To calculate the SNR at a different current, multiply the SNR by the new current and divide by 1 nA.