A keep-out can be used for many reasons, some (but not all) of them being:
- Forcing an automatic pour to keep away from fixed copper art.
- A small bit of high voltage, for which the designer wanted a nice area of larger distance. (With clearance rules you sometimes get ghastly little tips and nicks)
- In a place where a complex shape creates many hard strange shapes that may drive up the cost of production in an automated check.
- To force the designer to not put any components there, or no comonents of a type (you set it up early in the design so that you can't accidentally do something stupid).
- To force the designer to not route anything there, or not route specific signals.
- To force a warning about objects above the component - keep-out's can also operate in the space above the copper.
- To create an area where ground isn't, because that's required for characteristic impedances.
etc etc etc.
Which one it is? I don't know, because you have cleverly not given me anything to go by in that regard.
The ref to VCM must be Common Mode Voltage used for high impedance guarding to reduce signal capacitance and common mode noise. The entire VIN(-) and feedback path is a keep out zone from ground capacitance ( bottom layer)
Judging by the two series feedback R's, my guess is two large standard values (eg 10M) to increase gain at the expensive of high impedance and great sensitivity to stray capacitance of a couple pF to (perhaps floating) ground noise.
Hence the reason why ground is used every around except under these parts.
Short answer is keep out GROUND, guessing to reduce patient noise, Not their complaints ;)
Pin 4 being low Z (Vee) does not matter for keepout.
Gain is controlled by R between Pin 1&8 yet only pin 1 is used with R between Vin(-)pin2 and pin 1, resulting in basically just common mode buffer and no differential gain.
It might be used for EKG or EEG such as as Right LEG driver.
... Subject to change without notice... :)