... transmitting a sensor’s output as a voltage over long distances has several drawbacks. Unless very high input-impedance devices are used, transmitting voltages over long distances produces correspondingly lower voltages at the receiving end due to wiring and interconnect resistances. However, high-impedance instruments can be sensitive to noise pickup since the lengthy signal-carrying wires often run in close proximity to other electrically noisy system wiring. Shielded wires can be used to minimize noise pickup, but their high cost may be prohibitive when long distances are involved.
Sending a current over long distances produces voltage losses proportional to the wiring’s length. However, these voltage losses — also known as “loop drops”—do not reduce the 4-20mA current as long as the transmitter and loop supply can compensate for these drops. The magnitude of the current in the loop is not affected by voltage drops in the system wiring since all of the current (i.e., electrons) originating at the negative (-) terminal of the loop power supply has to return back to its positive (+) terminal—fortunately, electrons cannot easily jump out of wires!
Emm... I'm sorry to say that I don't really understand this point... Would you pls elaborate a bit further? Thank you very much in advance...IanP said:...however, noise interference in current loop, because of its nature, is almost negligaible....
I don't understand the sentence highlighted in RED color above. Would u pls advise how a low impedance across the input of an A/D will reduce the amount of noise? Thanks.VVV said:Noise and long distance between the sensor and the measurement A/D are the reasons that led to the development of the current loop.
Noise in industrial environments is always present and always strong.
To reduce the amount of noise, it is desirable to have a low impedance across the input of your A/D.
But a low input impedance will introduce large errors in the measurement if sensor outputs a voltage and the distance between the sensor and the A/D is long, which is often the case in idustrial environment. The problem occurs because the connecting wires have resistance, which coupled to a low impedance input creates losses and hence errors. Plus, various sensors can be at various distances, so it is impossible to compensate for these errors based on the known distance.
The soulution is of course a current loop. The input resistance is small, to keep the noise low. Then, by regulating the current, the voltage drop across the connecting wires does not matter. In the end you measure the voltage across the low input resistance (typical values for this resistance are 100 ohms to 1kohm; 250 ohm is very popular, since it produces a 1-5V signal across it, easy to measure). V=R*I. Since R is constant, V depends only on the current.
The 4mA lower limit is useful in detecting faulty wires: if the wires are broken, the current is zero and so is the voltage, so you know something is wrong.
If the wires are shorted, the current through the resistor will be almost zero, the voltage will be almost zero so again the fault is detected.
It beats me that no one has mentioned that many 4-20mA transmitters are "loop powered". The 4mA is used to power the electronics and the transducer.VVV said:The 4mA lower limit is useful in detecting faulty wires: if the wires are broken, the current is zero and so is the voltage, so you know something is wrong.
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