SLVAEX3 October 2020 TPS8802 , TPS8804
A smoke concentration measurement follows this general procedure:
The SNR of the smoke concentration measurement depends on many parameters in the system, including amplifier speeds, LED current, LED pulse width, ADC sample rate and digital filtering. These parameters can be adjusted to improve the SNR. The LED and photodiode parameters, such as quantum efficiency and half-angle, and the photo chamber geometry also have an effect on the SNR but are beyond the scope of this report. Table 2-1 lists various measurement configurations and the SNR at 1 nA of photodiode current. The SNR at 1 nA is reported here as a unitless ratio and can be converted to decibels by taking the base-10 logarithm and multiplying by 20. Table 2-1 provides a reference for users to design a smoke detector using the TPS880x AFE. The parameters in the table are discussed in Section 2.3 and Section 2.4. The configurations in Table 2-1 are selected from measurements in Section 4. Section 4 also outlines the measurement process.
| Config. | tLED (μs) | τ1 (μs) | τ2 (μs) | NBASE | NTOP | tSAMP (μs) | tTOP (μs) | Filter | SNR at 1 nA |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 50 | 15 | 15 | 5 | 1 | 20 | 68 | - | 13.0 |
| 2 | 50 | 15 | 15 | 5 | 2 | 20 | 53 | Average | 15.6 |
| 3 | 100 | 15 | 15 | 10 | 1 | 20 | 91 | - | 18.9 |
| 4 | 100 | 59 | 60 | 10 | 1 | 20 | 129 | - | 25.7 |
| 5 | 100 | 15 | 15 | 10 | 5 | 20 | 48 | Average | 31.1 |
| 6 | 100 | 15 | 15 | 10 | 7 | 20 | 8 | Matched | 32.3 |
| 7 | 100 | 15 | 15 | 20 | 8 | 10 | 68 | Average | 33.1 |
| 8 | 200 | 59 | 60 | 20 | 1 | 20 | 216 | - | 42.5 |