Isolated thermistor measurement

Question

I'm considering re-implementing my thermistor measurement circuit to be isolated, since it's destined for an industrial environment. The thermistor is a simple passive sensor and is not grounded on the far end.

U1 is a 5V/5V regulator rated for 3kV isolation. The line out to the thermistor is connected to protection shottkys, a TVS zener, and caps that set the time constant at (I think) ~1s.

U3 is a "linearized" optoisolator rated to 3.75kV isolation.

Back-of-the-envelope calculations for the following nominal values:

• the input and output of U2 at 2.5V,
• the LED current at 8.7mA,
• Irm1 = Irm2 = 90uA,
• line time constant = 853ms

^{simulate this circuit – Schematic created using CircuitLab}

Questions:

• Have I made any mistakes in how the circuit is connected or the passives are chosen?
• Is it even necessary for this isolation to be done, given that the far end does not have its own ground?
• Are there any rules I should follow with regard to PCB layout (especially shuffling around the diodes and caps, etc.)

Answer 1

Thermistors are relatively high level devices and are not generally grounded so it's not usually necessary to isolate them. You can add a low pass filter and use shielded wire to the thermistor if you expect significant EMI.

You should check the reverse leakage of those Schottky diodes- if they are the type I think I see-- at -4V and 75 degrees C they will typically leak 200uA!

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Edit:

^{simulate this circuit – Schematic created using CircuitLab}