4-20ma analog input - direct vs convert to voltage via resistor?

clay.float

Member
Join Date
Apr 2017
Location
Austin, TX
Posts
4
Greetings all,

In a project I'm working on, we have an 4-20ma temperature transmitter linked to an S7-1200's analog input (0-10V reading 2-10V, 12-bit) via a 500Ω dropping resistor. There are some issues occurring in the field where the temperature is fluctuating from the reference calibrating probe more than we would expect.

Would we improve reliability, accuracy, precision, or general frame of mind by moving the temperate transmitter to an analog input like the one on the SM-1234? (0-20ma reading 4-20ma, 13-bit (12+sign) )?

Specs for the S7-1200 input give an accuracy of +/-3%-3.5% while the SM-1234 has an accuracy of +/-0.1%-0.2%. If our reading on a 0-100°C probe is 3% off, that's over 6°F; if it's 0.2% off, that's just over 0.4°F.

Could that the be the source of our inconsistencies? Otherwise, are there any specific advantages / disadvantages to using a native 0-20ma input vs 0-10V via resistor?

Many thanks in advance! :)
 
In general, any time that you convert a signal you are introducing error. First off, in your case, field installed dropping resistors are very seldom the exact resistance that they are labeled with. 2nd. as you mentioned, resistors change in value with temperature change. Last, after the conversion to 0-10 volts, you have the voltage drop of the wiring between the resistor and the analog input. So, based on this alone I would go with the 4-20ma option. Add the improved accuracy of the SM-1234 that you mentioned, you should be in great shape.

All of that being said, you can improve your existing situation by using a better class of resistor (tighter tolerance and lower temperature drift) and making sure that it's mounted as close as possible to the analog input.
 
Thanks for the reply Lynx777!

The resistor is installed across the input terminals on the PLC, so that's about as close as it can be! ;)

The near and future development of this project will allow us to move this input to an input on a SM-1234, so we'll do that regardless. Hopefully that will banish this specter of floating temperature readings once and for all...

Cheers 🍻
 
As Lynx777 said, you will gain accuracy by using the SM-1234 module. That said, all PLC analog inputs are eventually converted to voltage signals on the field side of the IO module. At that point, a voltage controlled oscillator provides optical coupling to the backplane side of the input module.
 

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