There are some quite cheap tiny modules that produce an isolated 5 volts out from 5 volts in. You would need to have that at the power supply end of the cable to avoid getting a whole lot of ground line noise. But then you could have your opamp running on + and - 5 volts and anything near zero would be in the center of the range. That would totally solve the problem of not working next to the negative supply rail. And that supply could probably power a few other amps as well.
It is probably the simplest and cheapest fix, and since you have 8 conductors in that cat5 cable it ought to be fairly easy to add the additional power connection. But probably you will also need to add filter capacitors near the opamp to quiet whatever the long cable picks up. 47Mfd at 16 volts, or similar, from both V+ and V- to the common.Probably a 0.1mfd in parallel with those just to provide a low impeadance at higher frequencies.
What a cracking little amp the AD8628 is. Thanks for the information.
I just set one up as a voltage follower and down at IN+ connected to ground, even on the mV range of my DMM, it was no more than 0.1-mV at the output. At the supply V+ end, the output was as near as damn it exactly the same.
The only downside so far is SOT23-5 package, but tacky ChipQuik 291 flux and an adaptor board, fairy easy to solder.
I also tried the LM7705 charge pump negative rail generator and again, another cracking chip (VSSOP-8 aside). Specified -0.232-V out and measured output exactly the same. When combined with the TLE2141 as a voltage follower, with input connected to ground, the measured output was still only 1-mV. Well pleased with that.
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