Yes, you can typically do this with a relatively simple op-amp circuit.
Ideal - that's what I need to know - thanks.
Thanks to everyone for your input on this but it does seem to have taken on a life of it's own. Really, all I was trying to ask in my original question was: Can you configure a SINGLE supply opamp as an inverter that only ever has a positive output or does it HAVE to have a dual supply.
To answer Richard's questions:
1) Are you talking about a binary (on-off) -7 vs. 0 volts? Or are you talking about an analog voltage that ranges between -7V and 0V?
The latter. (To be fair I did say in my OP that the idea is to feed the Arduino ADC)
2) Where is this negative signal coming FROM?
The control interface of a plasma cutter.
3) Do you need to map -7V in to 0V out and 0V in to +5V out? Or do you need to map =7 V to +5V and 0 V to 0 V?
I have in mind the latter BUT I'm not sure it really matters. Remember this is going into the Arduino ADC so any amount of soft skullduggery can be done "later". As long as I have an ADC friendly representation of the 0 to -7 I'll be a happy bunny.
4) Do you have a bipolar source of power for the opamp? You will need at least -8V power and +8V power.
Now we're getting to the crux. This is really what I was asking in my original question. I've modelled this successfully in ltspice with a SINGLE supply, but you've confirmed that in the real world I need a split supply - thankyou.
To answer: I don't have a ready split supply so I'll have to arrange one.
5) Maybe you can borrow the biploar power from the (unidentified) source of the signal?
Afraid not - going "in the box" is out of bounds for warranty and H&S reasons. But I've an idea where else I may be able to get it from.