Varying the LED pulse length t_LED has a significant effect on the SNR. When scaling t_LED, τ₁ and τ₂ by two, the SNR is expected to improve by the square root of two. With one ADC sample, the SNR/nA increases from 13.0 to 23.3 to 42.5 when t_LED is increased from 50 μs to 100 μs to 200 μs. However, this also increases the power consumption, since the LED is enabled for more time. If the LED current is scaled to keep the power consumption constant, assuming that the photodiode current increases proportionally to the LED current and the LED supply voltage is constant, the 50 µs pulse has the highest SNR as shown in Figure 4-16. In general, increasing the LED pulse length and amplifier time constants is an effective way to increase the SNR when power consumption is flexible.

tSAMP=20 µs

Figure 4-15 SNR at 1 nA Using Optimal Averaging Filter Varying LED Pulse Width

tSAMP=20 µs

Figure 4-16 SNR at 1nA Using Optimal Averaging Filter Varying LED Pulse Width and LED Current