Is there any way to connect an externally powered instrument to an active analog input. I ask this questions just to try and clarify isolation methods.

Also, is there such a thing as an active isolated analog input? Or by the their nature of isolation does that force them to be passive?

Thank you for the help!

 

I am only familiar with a field device's output (not an analog input) being classified as active or passive, where active outputs are powered from field device's power but passive outputs are powered from a power supply external to the field device, as in a 2 wire, loop powered output that uses an external power supply. Active outputs come from 3 or 4 wire instruments.

Some analog input I/O modules have a terminal that is or can be connected to a DC power source for powering passive field devices which is there for convenience in field wiring, but I don't recall those analog inputs being referred to a active or passive. But the world is full of manufacturers . .

How does a power supply source affect isolation? If the field instrument uses a power supply whose ground reference is at a different potential than the analog input's ground potential, then current will flow between the grounds. This current adds or subtracts to the loop current that's supposed to be a measurement creating an error, if not driving the signal right off scale, or frying it.

Loop isolators are frequently needed to prevent ground loops between active output field devices and the analog input, because of the difference in ground potential between the two. This situation is very common with 3 and 4 wire active output instruments. I figure you're lucky if you don't need a loop isolator.

The ground loop issue diminishes when a single power supply, typically at the AI end is used to provide power over the loop to a 2 wire loop powered field instrument(s). The real benefit is seen when there are multiple field instruments supplied from the same power supply. Although the instruments always have an earth ground terminal, the field device electronics are isolated from the earth ground. So a ground loop is limited to the power supply and the AI ground reference.