Voltage drop on mV signal

[PhaseShift]

How does distance and signal wire size effect a mV signal?

With a 4-20mA signal the current is constant around the loop and the total resistance of the loop needs to be restricted to a certain value in order for the voltage supply to be able to push the maximum 20mA current throught the loop at any given time. Adding length or resistance to the loop will not effect the constant current signal as long as total resistance in loop is kept below maximum value.

What happens with a mV signal or 0-10V signal when resistance is changed? Does the signal drop voltage as it travels across the wires or resistance? For a 4-20mA signal you dont have to worry about drop acrross cable becasue mA signal stays constant and reciever is looking only at current signal? But what about a mV signal where reciever is looking a voltage signal, which may drop across signal cable?

 

[Smart $]

Is there a specific issue that prompts your query?

Of course, there is going to be voltage drop. However, a lot of mV signals are encoded, so the information transmitted isn't dependent on voltage, but rather the integrity of the signal.

 

[ELA]

You are really talking about a voltage divider issue.
In the current loop example the typical input impedance would be 250 ohms.
With a 0-5,10 volt signal the typical input impedance might be 100K -1 Meg (don't hold me to an exact number as it has been a while, but in that neighborhood).

So you can see the resistance of the wire in the voltage input scenario is not that significant.

One very big difference and concern is noise susceptibility. 0-5,10 volt signals run over a long distance can become more easily upset by noise sources.

 

[gar]

100529-1426 EST

PhaseShift:

Real simple. If the receiving end has infinite input impedance and there is no leakage along the cable, then the voltage at the receiving end is the same as the sending end assuming shunt capacitance of the cable is insignificant.

Change the circuit so that the source impedance at the sending end is 100 ohms, the wiring resistance is 900 ohms, and the load resistance at the receiver is 9,000 ohms, then the voltage at the receiver is 90% of the source voltage.

If the wiring resistance changes to 350 ohms, then the receiver voltage is 9000/(9000 + 374 + 100) = 0.9499683 or 95% of the source voltage.

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