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Transmitter
- Thread starter Iqbal
- Start date
It depends.
If there is excessive common mode voltage between the grounds in the loop then ground loop current will occur that will either
- be low enough to not matter
- enough to offset the true signal
- so much that it saturates the analog input and drives off scale upscale or downscale depending on polarity
- too much and it burns up an analog input
Passive signals from loop powered, 2-wire transmitters are almost always isolated and the ground references are the DC power supply and the analog input.
Active output can be isolated but are frequently not isolated and reference the ground at the field device.
Single ended analog input cards (all inputs use a common (-) return) with multiple inputs have the greatest likelihood of 'seeing' the ground loop.
Connect it up using a fixed value output and see what happens to signal when connected to more than one analog input (the fixed value will tell you if there's an offset). There's as much a reasonable chance that it will work as that it will not work.
In all these years of hanging around web fora, I've only ever read one report of someone who encountered a ground loop, located a bad ground, fixed it and eliminated the source of the common mode. Otherwise the fix for ground loops that appear as offset/saturation is a signal isolator or a signal splitter with isolated outputs.
If there is excessive common mode voltage between the grounds in the loop then ground loop current will occur that will either
- be low enough to not matter
- enough to offset the true signal
- so much that it saturates the analog input and drives off scale upscale or downscale depending on polarity
- too much and it burns up an analog input
Passive signals from loop powered, 2-wire transmitters are almost always isolated and the ground references are the DC power supply and the analog input.
Active output can be isolated but are frequently not isolated and reference the ground at the field device.
Single ended analog input cards (all inputs use a common (-) return) with multiple inputs have the greatest likelihood of 'seeing' the ground loop.
Connect it up using a fixed value output and see what happens to signal when connected to more than one analog input (the fixed value will tell you if there's an offset). There's as much a reasonable chance that it will work as that it will not work.
In all these years of hanging around web fora, I've only ever read one report of someone who encountered a ground loop, located a bad ground, fixed it and eliminated the source of the common mode. Otherwise the fix for ground loops that appear as offset/saturation is a signal isolator or a signal splitter with isolated outputs.
Isn't there also a "limit" to the total amount of resistance some devices (4-20 mA, particularly) can see in their entire output loop? I typically see limits in transmitter loops of 500 ohms. So, many input channels use 250 ohm dropping resistors to develop 1-5 VDC on the input circuitry (which can usually only read voltage, not current) which means if two device inputs each use 250 ohm dropping resistors then the 4-20 mA signal can only be connected to two of those devices and no more.
Some device inputs use 100 ohm dropping resistors. I have successfully connected multiple devices in a loop--as long as the total resistance of the loop did not exceed 500 ohms (which seems to be the typical limit I have encountered).
Some device inputs use 100 ohm dropping resistors. I have successfully connected multiple devices in a loop--as long as the total resistance of the loop did not exceed 500 ohms (which seems to be the typical limit I have encountered).
