This is a difficult one. Clearly, earth bonding of equipment is a useful safety feature (it's a mandatory requirement in safety standards). Unfortunately it also creates problems in other ways, so simply connecting 0V to ground may not be the best way to do it.
USB was designed for peripherals like mice and keyboards, it wasn't designed as a general purpose comms connection, although that is what it is now used for. What to do with 0V and shield on the USB connector does not appear to be specified in the USB standards, so some people leave the shield unconnected, a lot of people tie it to their 0V, as the Due designers did. Other people connect USB shield to 0V via some protection components, which seems to be best practice.
You can get USB isolators, which are magnetically coupled. I have found these may not always work reliably. Really we would want to use a comms connection with designed-in isolation like ethernet.
We have heard of many USB problems people have when things go wrong on these forums, e.g. with systems hanging, hot USB cables, components burning on the Arduino etc. When designing a shield though, most of these issues are out of our hands, since we can't dictate the design of the Arduino or how people wire up their systems. We can do whatever we can to alleviate the problem, but ultimately I think all we can do is to recommend some best practice.
Personally I use isolated PSUs and don't connect anything to ground, so I wouldn't see these type of problems.
I did wonder if we should try to improve the analog performance, e.g. by using a proper voltage reference, but the analog signals pick up so much noise it didn't seem worth it. Since we are only measuring temperature, which is relatively slow to change, just adding a lot of filtering reduces that problem. For some reason 4k7 is used in the voltage divider, but that is not the best value as it leads to a high slope in the temperature range of interest. It would be much better to have values suited to extruder temperatures, and a different value for heatbed.
USB was designed for peripherals like mice and keyboards, it wasn't designed as a general purpose comms connection, although that is what it is now used for. What to do with 0V and shield on the USB connector does not appear to be specified in the USB standards, so some people leave the shield unconnected, a lot of people tie it to their 0V, as the Due designers did. Other people connect USB shield to 0V via some protection components, which seems to be best practice.
You can get USB isolators, which are magnetically coupled. I have found these may not always work reliably. Really we would want to use a comms connection with designed-in isolation like ethernet.
We have heard of many USB problems people have when things go wrong on these forums, e.g. with systems hanging, hot USB cables, components burning on the Arduino etc. When designing a shield though, most of these issues are out of our hands, since we can't dictate the design of the Arduino or how people wire up their systems. We can do whatever we can to alleviate the problem, but ultimately I think all we can do is to recommend some best practice.
Personally I use isolated PSUs and don't connect anything to ground, so I wouldn't see these type of problems.
I did wonder if we should try to improve the analog performance, e.g. by using a proper voltage reference, but the analog signals pick up so much noise it didn't seem worth it. Since we are only measuring temperature, which is relatively slow to change, just adding a lot of filtering reduces that problem. For some reason 4k7 is used in the voltage divider, but that is not the best value as it leads to a high slope in the temperature range of interest. It would be much better to have values suited to extruder temperatures, and a different value for heatbed.