Layout of a system involving a 5-mm radial through-hole photodiode and LED is shown in Figure 179. The following guidelines should be followed to keep the crosstalk between transmitter and receiver low.
Use a four-layer board, so that all the analog and digital supplies can be well isolated from each other.
Place the photodiode and LED oriented orthogonal to each other .
Minimize the area of the transmitter current-carrying loop involving LED, VDD_LED-to-VSSL decoupling capacitor, and AFE.
Minimize the area of the receiver loop involving the photodiode, matching capacitor, and AFE.
Shield the receiver loop using AVSS ground in the top and bottom PCB layers. Also place a shielding ring around the photodiode and connect the shielding ring to AVSS. This shielding ring helps in reducing the electrical and optical crosstalk.
Shield the transmitter loop using IOVSS ground in all the PCB layers. Also place a shielding ring around the LED and connect the shielding ring to IOVSS.
LED terminals should not see the photodiode terminals directly. Any small amount of capacitive coupling between photodiode and LED terminals results in huge crosstalk. Grounded metal rings around photodiode and LED help in shielding.
Use vias around the transmitter and receiver loops in their respective ground planes to improve the shielding.
Connect the device thermal pad to AVSS.
Do not overlap different ground planes, keep them well isolated.
Figure 179. PCB Layout example
Figure 180. Ground Isolation Between AVSS and IOVSS in a Four-Layer PCB