Figure 2-2 TPS880x AMUX Buffer Schematics

The optimal photo amplifier and AMUX speeds depend on the LED pulse width and ADC sample rate. The photo amplifier time constant τ₁ is equal to the gain resistor R_PH times C_PH. τ₁ is adjusted by varying C_PH once R_PH is selected. The AMUX time constant τ₂ is equal to the AMUX buffer resistance R_AMUX times C_AMUX. τ₂ is adjusted by varying R_AMUX after setting C_AMUX between 330 pF and 1nF. Adjusting the amplifier speeds is an effective way to improve the SNR. This is demonstrated with configurations 3 and 4 in Table 2-1.

The highest SNR for a fixed pulse width is achieved with a fast amplifier speed and fast ADC sample rate, then taking multiple ADC samples over the width of the photo pulse and averaging them together. For example, configuration 5 in Table 2-1 has a higher SNR than configuration 4 because a faster amplifier speed and multiple ADC samples are taken. If the ADC sample rate is slow, or if only one top ADC sample is taken, then a slow amplifier speed achieves the highest SNR. Using higher and lower speeds than shown in Table 2-1 may improve the SNR further.