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I am trying to understand the following schematic for a differential input pH probe.

here is the link

enter image description here

A1 and A2 role are unity gain opamp amplifying high impedance probe signal, while A3 makes the subtraction.

What does A4 really do? It subtracts tank ground potential to the differential pH signal? Circuit ground and tank ground are connected together?

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    \$\begingroup\$ It looks to me like the thermistor is there to correct for influence of the temperature on the ph reading - in which case the diagram is missing details and should be considered more of a block diagram and less of a schematic diagram. \$\endgroup\$ – JRE Aug 7 '19 at 12:56
  • \$\begingroup\$ agree with that, it's a block diagram, not a schematic. Doo you think tank ground and circuitry ground are bond together? \$\endgroup\$ – Vinlar Aug 7 '19 at 12:58
  • \$\begingroup\$ Tank ground and circuit ground are connected. \$\endgroup\$ – JRE Aug 7 '19 at 13:00
  • \$\begingroup\$ Find a proper schematic if you want proper answers. \$\endgroup\$ – Andy aka Aug 7 '19 at 13:26
  • \$\begingroup\$ by connecting the tank ground and circuit ground (=battery negative pole) together and if we have more than one system of this kind (aka many connected tanks) on a circuit at different points, this would produce several ground loops accross the tanks? \$\endgroup\$ – Vinlar Aug 7 '19 at 13:48

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