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tldr: Can I transmit an analog signal through a noisy environment without it being corrupted, or should I use a digital signal instead?

My requirement is to transmit a signal from a MA3 absolute magnetic encoder 75 feet in the sky to a DAQ module on the ground. This model comes with an analog or digital interface. I would prefer to use the analog, because it is easier to interface with a DAQ module.

With the analog interface, the sensor has 10 bit resolution, 2.6 kSample/s, 0-5V output. To keep linearity of the sensor data around the 0 and 5V boundaries the output load impedance should be greater than 4.7kohms and less than 100pF.

The digital interface uses PWM. It has 10 bits of resolution and has a frequency of approximately 1kHz. The RC oscillator for PWM is only within 10% of its rated pulse period, but this can be accounted for by calculating the ratio of tOn to tOff for each duty cycle. In order to interface this with a DAQ module I would have to use counting logic. I am not a fan of this approach because the counting approach seems error-prone, and it will require learning software for the DAQ. However, I will use it if the analog route is impossible.

For the shape of the signal, I expect it to constantly vibrate slightly from the sensor position changing slightly, but change overall very slowly since the shaft it is monitoring rotates at less than 1 RPM.

There are three complications to this.

  • the signal has to pass through a slip ring
  • the signal has to travel from a height of 75 feet to the ground - I have heard that long wires act as antennae for EMI and the height of the cable in the sky may affect this
  • the wire carrying the signal will be no more than a foot away from wires carrying 3 phases of up to 40V of wild AC

My first question is: based on all of these sources of interference, does the analog signal stand any chance of maintaining integrity when it reaches the DAQ module? I lack the experience with signals to be able to predict whether this is feasible or not.

If you think this is possible, do you have any recommendations for making it work? The only approach I know of is shielding the wire, and keeping it as far away from the power lines as possible.

Thanks for your help

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    \$\begingroup\$ "transmit a 10 bit resolution, 2.6 kSample/s, 0-5V analogue signal" An analog signal doesn't have a bit resolution or sample rate, so I assume you are digitizing it on the receiving end? And you need the noise floor to be below whatever your LSB is? What bandwidth do you need? \$\endgroup\$ – John D Jan 30 at 20:46
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    \$\begingroup\$ Why can't you digitize the signal at the source? Or perhaps encode it with a V/F converter? \$\endgroup\$ – crj11 Jan 30 at 20:56
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    \$\begingroup\$ Please add that information to the question itself. You will not be able to satisfy that 100pF load restriction through a 75ft long cable unless you use a driven shield. And that text suggests an output impedance on the order of 400Ω, not too problematic. As you already have a local power supply, you would be better off doing the conversion near the sensor anyway. \$\endgroup\$ – Edgar Brown Jan 30 at 21:03
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    \$\begingroup\$ "I would like to have a bandwidth of +/-0.00244V, but I need at worst +/- 0.0278V." - that's resolution, not bandwidth. What is the maximum frequency of the waveform, and what shape is expected? What is the device being sensed? \$\endgroup\$ – Bruce Abbott Jan 30 at 21:13
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    \$\begingroup\$ @user211492 That would be hard to low pass while still getting 10 bits of resolution and fast response. However, since it is slow, you could use counting logic to digitize the PWM. Counting the high and low time at 2 MHz would give you 10+ bits of resolution. You could probably do it in firmware with the right microcontroller. \$\endgroup\$ – crj11 Jan 30 at 21:25
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Can I transmit an analog signal through a noisy environment without it being corrupted, or should I use a digital signal instead?

Yes, you can transmit analog signals long distance without them being corrupted, but proper EMI technique needs to be followed. The usual route is to gain the signal up at the sensor and then use shielding on the cable from the sensor to prevent interference from electric fields but not magnetic fields. 10bits of resolution on a 5V signal equates to 5mV, which isn't terribly difficult in most cases but in your environment with AC running close to the cable keeping the signal clean might be tricky.

The other problem with a cable would be currents (especially non constant/AC currents) on the shield of the cable which must be kept to a minimum or the current through mutual inductance between the shield and the inner conductor(s) can create voltage noise on the inner conductors. There can be problems with running a long cable, such as creating a ground loop. Since you also need to go through a slip ring this could also be a potential source of noise since it would difficult to maintain the shield. The slip ring could be a big source of noise for an analog signal.

With a digital signal (and differential signaling such as RS485) these problems can be avoided and in my opinion it is easier to isolate a digital signal than an analog one. I would say go the digital route considering your environment.

If you want to go the analog route, if you had a power supply, you could run an experiment and set it to a known voltage, like 5V and then measure the noise on the other end of the cable with a DMM, if the noise is below 5mV, then it would probably be feasible to go the analog route.

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  • \$\begingroup\$ Thanks for the response. I have decided that the difficulty of counting logic PWM on the DAQ module is worth the less corruptible digital signal. Do you know of any resources to study digital signal integrity? \$\endgroup\$ – user211492 Feb 1 at 20:30
  • \$\begingroup\$ You could maybe get one of these, I haven't checked the voltage levels and I don't know if this one will work, but there are devices that measure duty cycle laurels.com/transmitter-duty-cycle.php The sampling time or bandwidth needs to be more than the thing your measuring. Look at ways to "measure duty cycle" on your favorite search engine. \$\endgroup\$ – laptop2d Feb 1 at 21:14
  • \$\begingroup\$ You could also do a simmilar thing with a raspberry pi raspberrypi.stackexchange.com/questions/63409/… \$\endgroup\$ – laptop2d Feb 1 at 21:15

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