I've been reading about the EIA-485 standard and they talk about how even with twisted pair wire they need a 3rd connection for common because sometimes the devices need to be isolated. I was thinking about it and I've always taken isolation for granted, but never actually understood why it would be necessary. Why is it that a device would ever need to be electrically isolated from another?
Electronic – Electrical Isolation
isolation
Related Solutions
Specialty optoisolators that can handle 50KV working voltage are available off-the-shelf, if you don't mind doing a little more work to properly isolate the I2C signal:
Yeah, that's an optoisolator. That's what you need for 50 KV isolation.
Slightly lower isolation rating parts (10 KV) are also available for considerably less money:
Note that these are all discrete optoisolators. If you can switch to a SPI ADC, it would make the isolator considerably easier to design, though you can isolate I2C with discrete isolators:
Here is a whitepaper about I2C isolation topologies.
Depending on the I2C data-rates you want to achieve, it may be fairly involved.
Further follow-up:
Here is an excellent white-paper about safety concerns in power-supply designs.
You can connect the two grounds if they're truly floating wrt each other. Then the CM range is controlled and you're back to the original situation. Several volts of difference is okay (-7 to +12) so any reasonable connection over a short distance should meet this requirement. A high CM range transceiver would still require some kind of control of the common mode voltage.
Isolated DC-DC converters are very common, just do a parametric search at some distributor such as Digikey. I see 176 pages (25 per page) of different isolated DC-DC board-mount modules before filtering for required power and for input and output voltages.
Edit: One of my favorites is the TI (nee Burr-Brown) DCR01 series, which has an LDO (no problems out-of-spec voltages with light loads) and fits in an SOP that is 2.5mm high. It can supply 1W, so 200mA at 5V.
There are also RS-485 hybrid transceiver modules with the DC-DC converter built-in, but they're a bit expensive, and the DC-DC converter is probably a bit inefficient compared to one with a less constrained transformer.
You should connect the isolated ground to the ground on the remote system to control the common mode range.
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Best Answer
There are several reasons why you would want your signals to be electrically isolated between devices.
The main reason I usually think of first is different ground potentials. The simplest way to think about this is if you are connected 2 nodes of a network together that are using different ground references. I have one real life example that I have personally dealt with. We had 2 computer network switches that were connected with a CAT5 cable in 2 adjacent buildings. The ground potential was different enough between each switch that it caused the ports being used to stop working.
Another extension of my previous example is being able to protect a failure in one device to not damage the other. This could be from a current surge, or static, or many other things.
Another reason is to limit the noise in one system from working its way into another system.
And one final reason that I know of is to help prevent electrical shock, especially in marine environments. There are certain items on a boat that will come in contact with water that have a good chance of having a voltage on them. You don't want salt water to be shocking anyone.
There may be other reasons, but this is my extent of knowledge.