Books aren't specific enough. But if you show us your layout, I will comment on it!


Make a central star ground, where the signal ground of mcu voltage and 12V are connected to each other, best near the voltage regulators. Connect this ground point with very shortest connection (!!!!!) to the same radio frequency plane to which you connect the cable shields and input output filtering. Often the radio frequency plane is one wall of the metallic enclosure, the side where all the cables enter or leave the enclousure.

From this point route the signal ground via wide copper traces, big wires or even a solid ground plane (best!) to the 4-20mA section and to the relais section. Now, feed the 12V supply also to these two sections, without forming a loop and with close distance to signal ground (to prevent here a loop either). Now, filter the supply voltage at both sections via a RC- or RLC-filters to the local signal ground, directly at the remote loads producing the switching noise. The effect of this filtering is, that by this the path on the signal ground to the star ground point is free from ground return currents, which otherwise would erode the signal ground and make one section feel the action of the other and vice versa. Also, by the filtering the ripple on the 12V is eliminated, so that the 4-20mA section will no longer "see" the relais switchings.

By consequent filtering of supply voltage at all places where interference might be injected into it due to load changes (relais switchings, etc.) and by star point grounding (best use a solid ground plane, which is a big star ground point) you can totally isolate the sections from each other. You can everything hook to the same signal ground and the same supply voltage without having the individual sections "cross couple".

If I understand you correctly, then there's absolutely no reason to even isolate the relais coils from your circuit.


Ok, because the sensor sees earth you want the circutiry connected to it isolate from earth. So, use a saftey class II power supply for the circuitry, which has no connection to earth (protection earth). If this isn't possible and you must use a saftey class I supply, or if the circuitry comes in contact to earth elsewhere (connection cable to PC, e.g.), use some galvanic isolation by the help of optocouplers or else between the sensor and the circuitry or between the circuitry and "the rest of the world" to prevent low frequency ground loops.


Here situation looks different, because 4-20mA signalling usually has no problem with ground loops, just because the signal is a current and not a voltage. A proper 4-20mA emitter can eliminate the effect of ground potentials developping in such a ground loop. That's just why 4-20mA current signalling is used at all, instead of voltage signalling.
Nevertheless, if you desire to isolate it by the help of optos, do it.

But this all does not explain to me, why you want to isolate the relais coils in your circuit. There is no way for developping a ground loop at all.

Kai Klaas