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.

[Overspeed]

Hello

Yes thanks , that can work just need to design a PCB and purchase the components

Question : do you have the gerber ? or a PCB view / pic  I can redraw in my CAD soft and convert to Gerber ?

30 V can work as I think they use high voltage to reach high level of electrical field in the soil an reach high level deepness which is not my goal as with some meters deep that enough

Regards
OS

[jbb]

Sorry, that’s all I’ve got. It’s more of a sketch than a design. Doing a complete design for an instrument would be a lot of work.

Some notes if you want to continue the design:
- U1 and U3 need to be (somewhat) precision amplifiers. I just grabbed LTC2057HV for convenience; it may be a poor choice
- U2 and U4 need to be power amplifiers. I just used LTC6090 for convenience, it’s probably a poor choice. Of of the OPAxxx line of power op amps from Texas Instruments would probably be a better fit
- there’s no output protection in the sketch. This is required to protect the system from ElectroStatic Discharge if nothing else

Also, you’ll need a voltage measurement system. I was thinking an ADA4254 programmable instrumentation amplifier might hit the spot. If you want to use that, you’d probably want to change the circuit above to a +-24V split supply rather than a 48V single supply.

Edit: ElectroStatic Discharge = ESD