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I have an AC voltage sensing circuit which uses an op-amp circuit with a LOC110 linear optical isolator.

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The left-side op-amp is powered by a supply voltage derived from a different AC source (an inverter):

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The power rails for the left-side op-amp will be VCC/2 and GNDA.

Should I connect the neutral line of the AC supply in the op-amp circuit to GNDA, or does this happen inside the operational amplifier anyway? If it doesn't, does that affect circuit operation?

I have another one of these AC sensing circuits on the same board, which uses a different AC source (the inverter) Both sources have separate live and neutral lines. I was wondering if supplying the same VCC/2 and GNDA to the left-side op-amp of both the circuits would effectively connect their neutral lines together (inside the op-amp?), and if that would be a problem.

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    \$\begingroup\$ If you have to ask these particular questions you are in danger from this mains-connected circuit. \$\endgroup\$
    – Spehro Pefhany
    Jun 15 at 18:41

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That optical isolator tells me that the designer intended the left half of this circuit to operate completely isolated from the right. That's a good design choice, since the left side is connected to mains.

I can't read the IC designators for the opamps, but I assume those two opamps are from different physical packages. All opamps in the same package share the same power supply, they are all physically connected to the same supply rails inside the package. If those two units in the schematic come from the same package, that entirely defeats isolation.

I am fairly certain that the left opamp is housed in one IC, and the right opamp is in another completely different IC, and that each IC is powered from its own independent, isolated supply.

The left opamp is probably powered from the supply that provides VCC2 and GNDA, (the one that you also show us), since VCC2 is used on the input side of the opto-isolator.

The right opamp is probably powered from a completely separate and isolated source of +12V, as suggested by its use on the output side of the optical isolator, a supply which we don't see here.

The left opamp is a differential amplifier, and responds to the difference in potential between mains live and neutral, there's no need to connect mains neutral or live anywhere other than shown.

GND for the +12V side of the circuit should not be connected to GNDA from the VCC2 supply side. The whole idea of the opto-isolator is to eliminate the need for such connections.

VCC2 is derived from a bridge-rectified AC source. The use of a diode bridge means that connecting either side of that source (neutral or live) to GNDA or any other point in the DC side of the circuit will make fire.

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  • \$\begingroup\$ Sorry if I didn't make my question clearer. Op-amps on both sides are indeed separate packages and use isolated power supplies. My question was.. some context before that: I have two of those AC sensing circuits, and the left op-amp of both the circuits use the same VCC/2 and GNDA. VCC/2 and GNDA are also derived from one of these AC inputs (inverter). My question was if I use the same VCC/2 and GNDA for both left-side op-amps would that connect the neutral lines of the two AC supplies inside the op-amp. \$\endgroup\$
    – nottherealfaraday
    Jun 16 at 1:39
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    \$\begingroup\$ @nottherealfaraday Forget right side. Left side, you have one supply producing GNDA/VCC2, and two opamps+periphery powered from that same supply. Right so far? If so, there's only one INV AC source in use, and your question must be about isolation between the MAINS AC source and the INV AC source. Is that right? \$\endgroup\$
    – Simon Fitch
    Jun 16 at 2:56
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Should I connect the neutral line of the AC supply in the op-amp circuit to GNDA?

It depends on if you want the design isolated or not. The LM3240 on the left side of the LOC110 should be isolated from the LM3240 on the right.

So if the LM3240 (on the left) is using VCC/2 (which it should be the same as the left side of the LOC110) then it should use the same ground (GNDA).

The VCC and ground for both LM3240's should be marked on the schematic (which it isn't). An opamp requires a positive and negative power rail to work. In this case your positive rail is VCC/2 and your negative rail is GNDA. The negative rail provides a return path of current back to the power supply.

You need two separate LM3240's, otherwise the isolation won't work Connecting the GNDA to both LM3240's will connect the grounds and make the isolator pointless if isolation was required by your design.

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  • \$\begingroup\$ Apparently, my question wasn't clear. I've made edits. Also check my comment on @Simon Fitch's answer. Both op-amps are indeed optically isolated and use separate power supplies. And yes, I've connected the rails. It's an LM324 package, so the supply rails are in a different part of the schematic. \$\endgroup\$
    – nottherealfaraday
    Jun 16 at 1:45

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