The response of mA is adequate for the vast majority of process sensing needs. The original published standard for mA , ISA SP50, was published in , almost 50 years ago, generating widespread acceptance and creating an installed base of millions of points. Electricians find it easy to wire and troubleshoot mA. Its working principles are easily understood DC electronics. When used with a nominal 24 VDC loop power supply, no one who touches the wire terminals gets shocked or even feels a tingle.
Disadvantages: The current can induce a magnetic field and cross talk to parallel lines. Like this: Like Loading Follow Following. Instrumentation Application Join 67 other followers.
Sign me up. Already have a WordPress. Log in now. Loading Comments Sensors range from familiar examples like photodiodes and photomultipliers that produce current when exposed to electromagnetic radiation, to submersible pressure transducers that are used to measure fluid levels.
For example, before the transistor, mA current loops were common. Live zero is measured as zero input even though the actual signal is 4mA, not 0mA. A dead zero would be 0mA, at which point you have an open circuit. Of course, if you aren't already aware, one of the main advantages of current loop over voltage driven analogue signals is that a current driven loop allows greater distances to be achieved - typically lengths up to m are possible. Any signal sent over a long distance produces a voltage drop which is in proportion to the length of the cable cable resistance , however when a mA signal is used to transmit the analog signal as opposed to a VDC signal , the voltage drop is irrelevant, since the same current has to pass through the circuit loop it has no where else to go!
You may already be aware that having a non-zero current 4mA for representing the "zero value" of the analog signal allows the controller to detect a broken wire 0mA as well as allowing loop-powered transmitter design. Having the 20mA correspond to the 'maximum value' of the analog signal is practical for industrial process instruments, since having larger currents would result in larger voltage drop for the same size cabling, thereby limiting the cable length.
It comes down to maximizing the SNR or signal to noise ratio. This constant current analog bus is relative low impedance and thus immune to stray leakage noise. The longer the line and higher sensor resistance, higher the voltage you need. Low current near zero meant detecting the threshold near the supply rails which was not easily done long ago as it is now. Thats also a consideration.
The current source has high impedance source which results in most of the voltage across the driver and small amount on the receiver. It could be any current levels you wanted unless you wanted to be compatible with standard receivers with a reference levels. You could use mA but would lose some margin from offset.
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