I'm looking for general advice on how to know (up-front) when you need the benefits of a 4-20mA output. I'm working on electrical design and sensor choices for an updated commercial espresso machine design. I know from past experience that there are numerous noise sources in our machines that cause various problems:
So, I'm trying to decide how much I should prioritize 4-20mA output. Is it a deal-breaker? We're dealing with pretty short wire runs - possibly only a foot or two if we do what I want, and less than 10 feet long even in the worst-case scenario. So, the voltage drops over long wire runs aren't an issue. I'm just wondering about noise on a low-impedance vs high-impedance signal carrier. I know that inputs with really high impedance, like multimeters, oscilloscopes, op amps, etc. can be incredibly sensitive to any sort of induced noise, and that current loop inputs are able to shed most of that because they're low impedance. What I don't know is where the cut-off is.
The Sensata part (0.5 - 4.5V ratiometric output) recommends a 10kΩ pull down as the load (input impedance on the board we would design.) The Honeywell part recommends a load >2kΩ. Are these input impedances low enough to help with noise rejection, or would 4-20mA current loop be a much safer bet?
For what it's worth, response time needs to be somewhat quick as it's part of a closed-feedback loop, but I'm sure a few milliseconds of lag wouldn't hurt anything, so we can add some amount of filtering too, whether in the form of input capacitors, digital filters, or both.
- Arcing relays for 1/3HP pump motors have caused RS485 communication dropouts fairly often, sometimes fouled the data for a 4x20 LCD display, and even caused microcontroller reboots occasionally.
- High current wires for heating elements (over 10A) induce noise in nearby RTD sensor wires, skewing temperature readings (admittedly, a better wiring layout would help a lot with this.)
- Numerous solenoid valves all create smaller noise spikes throughout the machine, occasionally causing problems similar to the motor problems described above (local snubbers help, but aren't 100% reliable.)
So, I'm trying to decide how much I should prioritize 4-20mA output. Is it a deal-breaker? We're dealing with pretty short wire runs - possibly only a foot or two if we do what I want, and less than 10 feet long even in the worst-case scenario. So, the voltage drops over long wire runs aren't an issue. I'm just wondering about noise on a low-impedance vs high-impedance signal carrier. I know that inputs with really high impedance, like multimeters, oscilloscopes, op amps, etc. can be incredibly sensitive to any sort of induced noise, and that current loop inputs are able to shed most of that because they're low impedance. What I don't know is where the cut-off is.
The Sensata part (0.5 - 4.5V ratiometric output) recommends a 10kΩ pull down as the load (input impedance on the board we would design.) The Honeywell part recommends a load >2kΩ. Are these input impedances low enough to help with noise rejection, or would 4-20mA current loop be a much safer bet?
For what it's worth, response time needs to be somewhat quick as it's part of a closed-feedback loop, but I'm sure a few milliseconds of lag wouldn't hurt anything, so we can add some amount of filtering too, whether in the form of input capacitors, digital filters, or both.
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