[SOLVED] Confusing "Black box" resistance determination

[Zaphappy]

I have an instrument with (3) 2 terminal inputs. Each labeled U- U+ (3 channels)

All inputs measure 4 megohms and any terminal measures 4 megohms between.

With meter across chan 1 (U- U+) and I short chan 1 U- to chan 2 U- resistance drops to 3 megohms.

When I additionally short all U- inputs together I get 2.67 megohms.

If I measure across any channel and short across another the resistance remains 4 meg.

How the heck to I figure out the actual circuit ?

This is a 3 phase power analyzer with "isolated" inputs. I need the equivalent schematic so I can adapt input dividers.

I have attached the manufacturer schematic. Still confused

 

[Zaphappy]

Ok, not so confused anymore... They must tie together via the supply GND of the difference amps. I'm guessing a classic diff amp with 4 resistors on front end acting as a divider to the diff inputs. These would all tie to op amp grounds via the non feedback resistors (if that makes sense)...

When found, solution will be simple...