impedance of current sources

[senmeis]

Hi,

I’ve read the following statement about a 4 to 20 mA transmitter:

Current transmitters are less sensitive to noise than voltage transmitters due to their inherent low impedance.

As far as I know the ideal current source shall have an infinite high output impedance. Did I misunderstand this statement?

 

[danadakk]

Usually the loop terminating R quite low -

And of course the wire run Z enters the design considerations.


Regards, Dana.

 

[FvM]

As far as I know the ideal current source shall have an infinite high output impedance.

Yes, exactly. Not only the claim of the statement, also the conclusion is wrong. Where did you get it?

We should clarify which kind of "noise" we are talking about. A current loop specifically cancels the effect of common mode voltage differences between transmitter and receiver ground.

 

[senmeis]

Please refer to page 23 of https://training.ti.com/sites/defau...rrent-loop-transmitters-presentation-quiz.pdf

 

[danadakk]

Please refer to page 23 of https://training.ti.com/sites/defau...rrent-loop-transmitters-presentation-quiz.pdf

I dont see a page numbered 23 ?

 

[senmeis]

See the attachment.

 

[albbg]

Is exctly the contrary. The current loop is less sensitive to environmental noise due to the high impedance of the current generator (ideally infinite). Usually the noise is induced by an electrical field that causes a noise voltage across the wires. Since the high impedance of the generator a very small current can flows in the mesh (is a simple series circuit) that means only a small noise current can be superimposed to the transmitted current. This is even more effective if the receiver resistance (that has to be placed closed to the receiver) is low.