LMP7721: Why isn't solder mask covered inside the guard trace?

Hi Ryusuke,

A "Guard" is a driven trace or shield that physically surrounds the input trace and feedback circuitry that is held at a potential equal to the average input signal potential. Since the input circuitry and the guard are kept at the same potential, the leakage current between the two nodes is practically zero. The guard is a low-impedance node, so any external leakages will "leak" into the guard and not into the protected input. This is also the reason why solder mask should not cover the input and the guard area including guard traces on either side of the PCB as the residue left from the solder mask could cause instantaneous current leakage between the guard and the input signal.