AC current source ? any idea for soil resistivity purpose

[jbb]

I see I shouldn’t have used ‘ground’ there. What I meant was that if you short Vb to DC- or DC+ then the circuit as designed doesn’t provide current limiting on U4. This is a fault situation where eg a wire has come loose inside the instrument box.

In normal usage, even RLoad = 0 ohms should be OK.

Fortunately, you could just get an opamp with appropriate internal current and temperature protection for U4.

The real question is: would this style of excitation circuit, which is limited to about 30 V RMS, be sufficient for your purposes? Those professional units deliver a lot more than that… but are intended to survey across greater differences.