Necessary protection ring for weak current detection

Analyze the following circuit PCB Layout . One Transimpedance amplifier , With a light that works in photovoltaic mode Electric diode interface （ The photodiode is zero biased ）. Produced by photodiodes The output current is proportional to the light level . The amplifier will signal current IPD Convert to output voltage . Formula for ： VOUT = IPD × RF

 pcb The layout is as follows ：

 Ipd Current path and 5v When the power supply line is relatively close , There must be a resistance , such as RL Yes 5GΩ, So from 5v The current flowing into the amplifier here has 1nA, This current is not acceptable in photovoltaic mode .

Ways to improve ： stay Ipd The outer layer of this weak current circuit is wrapped with a ground protection ring , In this way, the current flows directly to the ground , The weak current circuit is protected .