Galvanic isolation interfaces now dominate the market, they work on the principle1 as Zener barriers with the addition of isolation between input and output circuits.

There are essentially 3 general configurations of Galvanic isolator2
loop powered isolator loop powered where the isolator is powered from the signal itself, the most common being for 4-20mA loops and digital output signals.
Output voltage can be limited as the loop current has to power the interface
2 port isolator 2 Port isolated where a separate power supply (usually 24vdc) connection is required but this is not galvanically isolated from the signal lines.
Consideration must be given to the electrical configuration of any circuit the non-hazardous area signal terminals are connected
3 port isolator 3 Port isolation is probably the most common with signals and power supply isolated offers a greater flexibility and reduces the possibility of hook up problems

Due to their complexity they must be designed specifically for the application circuit or signal type, Analogue, Digital, input, signal type etc.
These are conventionally shown as 'positive' voltage but, unlike Zener barriers, they are isolated, so the concept of positive and negative barriers becomes irrelevant.


Notes


  1. The basic output circuit of isolation interfaces tends to be like that used in Zener barriers but, particularly for custom applications, output circuits can be complex and use other techniques and components for power limitation 

  2. Shown here with a 'representative' Zener diode and resistor output which is not necessarily the actual circuit used which will vary between isolator types.  

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