Isolated pH-meter with the help of optocouplers

Other Hardware Science and Measurement
1

Here's a question.
We assembled a device for isolated pH meter with the help of optocouplers. And there is such a problem: there is a device PPH-1000, which has a voltage from -600mV to 300 mV, i.e. from 0 to 14 pH. It is necessary to connect the electrode to the PPH-1000 via the arduino with a PWM signal, but the problem is that the PWM is from 0 to 5V. How to make the range from -600mV to 300 mV

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It is very difficult to imagine why anyone would want to do that, and there is no simple solution. Please explain.

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Why did you design the system to require that? The PPH-1000 certainly does not require a PWM signal. Does the PPH-1000 work properly with it's own PH probes?

edit: Why not isolate the PPH-1000 with an appropriate power isolation transformer?

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