Cable Length for Analog Sensors

A question came in recently concerning the maximum recommended cable length for analog sensors.  Even as digital interfaces gain popularity, sensors with analog interfaces (0-10V, 4-20 mA, etc.) still represent the overwhelming majority of continuous position sensors used in industrial applications.

The question about maximum cable length for analog sensors comes up pretty frequently.  Generally speaking, the issue is that electrical conductors, even good ones, have some resistance to the flow of current (signals).  If the resistance of the conductor (the cable) gets high enough, the sensor’s signal can be degraded to the point where accuracy suffers, or even to the point where it becomes unusable.  Unfortunately, there is no hard and fast answer to the question.  Variables such as wire gauge, whether or not the cable is shielded, where and how the cable is routed, what other types of devices are nearby, and other factors come into play, and need to be considered.  A discussion about all of these variables could fill a book, but we can make some general recommendations:

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E = IR: It’s Not Just a Good Idea, it’s the Law

I recently had a conversation with a customer that resulted in one of those forehead-slapping “duh” moments for me, and I thought it might be worth passing along. Here’s the story:

The customer had an application that required an analog linear feedback sensor that provided an output of 1 volt to 5 volts over the linear stroke range. Now, a 1-5V output is not very common, and the particular sensor he was interested in was only available with either a 0-10V or a 4-20 mA output. What to do? Perhaps the answer should have been obvious to me, but it was the customer who provided the solution this time: “couldn’t I use a 4-20 mA output and 250 ohm resistor to get my 1-5V output?” Why, yes….yes you could (smack…..duh!). And I know it will work, because we have the law on our side. Ohm’s Law, that is: E = IR, or voltage equals current x resistance.

Let’s check it:

4 (mA) x 250 (ohms) = 1 (volt)

20 (mA) x 250 (ohms) = 5 (volts)

So there you have it. Take a very common 4-20 mA output and drop it across a 250 ohm resistor and, lo and behold, you have your less common 1-5V signal. And, if you do this conversion right at the input to the controller, you get the added benefit of increased noise immunity of the 4-20 mA signal.

And, yes, I’m sure I knew of this little trick at one time. Maybe the part of my brain where this information was stored got overwritten by the names of the contestants on The Amazing Race or by the rollout plans for my million dollar consumer product idea: Dehydrated Water (just add water). But let’s keep that just between us, ok?

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