Every insulator, including PCB material, has a finite resistance that becomes a potential path for current to leak into the high-impedance traces. Copper guard traces are laid out around all the high-impedance input and feedback nodes to protect the signal path from current leaking through the board material. The guard presents a low-impedance path equipotential to the input traces. Any current leaking towards the input prefers the low-impedance guard path over the high-impedance input path. The guard and input are of equal potential; therefore, theoretically, no current flows between these nodes, and the input is protected.

The OPA928EVM PCB layout places the input traces and all sensitive feedback components within a perimeter of guard copper that is then enclosed by a grounded RF shield. The solder mask has been removed from the guard traces to reduce surface charge accumulation. Three-dimensional leakage protection is achieved through guard copper on internal PCB layers and guard-potential via fences surrounding sensitive nodes.

High-impedance, femtoampere-level circuits are highly sensitive to electromagnetic interference (EMI). To help reduce the effects of EMI in the OPA928EVM, three RF shields are connected to the board by surface-mounted shield clips. These shields fully enclose all exposed high-impedance traces and reduce external noise and EMI by shunting the noise to ground. Unless actively configuring or cleaning the EVM, keep these shields installed to reduce EMI and prevent contaminants from entering sensitive nodes. Some user configurations require the top shield to be removed during operation. For best EMI performance, operate the assembly within a shielded enclosure that is connected to ground.

There are three RF shields connected to the board by surface-mounted shield clips. These shields protect from external noise and EMI by shunting the noise to ground. The OPA928EVM is designed to fully enclose all exposed high-impedance traces within grounded RF shields during operation. The Hi-Z circuit has one shield on the bottom side of the EVM. The TIA circuit has two shields, one on the bottom of the EVM and one on the top. Keep these shields installed at all times unless actively configuring or cleaning the EVM.