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I am putting together an analog PCB. I am reading an audio signal and using large gain to clip it (effectively a crude digital signal 0-5V) using a differential amplifier. It is a 2-layer PCB with the bottom layer being my ground plane. The datasheet for the op amp recommends surrounding the input stage resistors with a grounded polygon for shielding. The main trace digital output trace has to run back nearby the input stage unfortunately given design constraints.

My question is: should this shielding polygon (via-stitched) extend to connect to local analog ground pins? Or should my stitched polygon simply "float" and then only use vias to connect nearby ground pins directly to the bottom layer ground plane?

Not shown: bottom layer is solid ground plane. SGNL is the large-amplitude signal. I do not have a power plane, simply routed power traces. My concern is that connecting local ground pins to the polygon will allow a careless ground loop somewhere.

Perhaps the below image will clarify: enter image description here

or

enter image description here

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  • \$\begingroup\$ This is called a "guard ring". You can look them up. Either way, it should definitely not just float. \$\endgroup\$
    – DKNguyen
    Feb 9, 2021 at 21:32
  • \$\begingroup\$ For some reason I had not previously heard of that term, helpful to google results. I have not been able to find results though: does a guard ring (should a guard ring) be exposed copper? I know soldermask has some dielectric properties, but what exactly is the reason for many guard rings to be exposed copper/gold? My signal is in the auditory kHz range (not MHz) but still imagine it is wise to implement. \$\endgroup\$
    – Matt001
    Feb 10, 2021 at 14:53
  • \$\begingroup\$ No mask. It provides leakage current a way to flow around the ring. \$\endgroup\$
    – DKNguyen
    Feb 10, 2021 at 14:57

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