mA signal to voltage input accuracy

naegely · Apr 30, 2009

I have a very simple sytem w/ ML1000 analog processor (2 current analog inputs and 2 voltage analog inputs). The system has a pressure transmitter and temperature transmitter (both 4-20 mA) that are currently connected to the voltage inputs of the ML, with resistors across the terminals. This works OK, but I have the 2 current inputs available, and can easily move the instruments to them and modify the program. My question is, do you sacrifice any accuracy going to the voltage inputs through resistors vs. connecting to the current inputs?

Thanks for any info.

Rob

MartiniRossy · Apr 30, 2009

Yes you do. I had almost the same situation. An OEM peice of equipment came with an 1100 and a 4-20ma pressure transducer wired up to the voltage input with a resistor. I since switched it to a current input and found that it was far enough off for me to never do anything that way.

naegely · Apr 30, 2009

Thanks for the reply, I'm going to go ahead and move them over.

Lancie1 · Apr 30, 2009

Rob,

The voltage in a circuit is different at different points, due to the resistance of the wire. Also, if your current-to-voltage conversion resistor is not an exact value (precision with low tolerance range) then you will have a built-in error. On the other hand, the current in a series circuit is the same at all points, so that the miliampere value sent by the transmitter is the same value read by the PLC.

Vic · Apr 30, 2009

I Disagree

MartiniRossy said

Yes you do.

I disagree. One way is just as accurate as the other. The 4-20 mA inputs have resistors across them and the voltage across the resistor is sensed by the input circuitry.

What may change is the accuracy of the resistor. You need to use a precision resistor or accurately measure the resistance of the resistor and use the actual resistance in the scaling calculation.

rta53 · Apr 30, 2009

I know you didn't ask about this but if you are that concerned about accuracy I wouldn't use the ML1000. I've had problems with the embedded analog inputs. ML1200 would be better.

Lancie1 · Apr 30, 2009

The 4-20 mA inputs have resistors across them and the voltage across the resistor is sensed by the input circuitry.

True, but the point is that the voltage across that resistor is probably NOT exactly the same as the voltage generated by the voltage signal transmitter. The signal transmitter has no way of sensing how much voltage drop is happening in the signal wire from transmitter to PLC. That is the voltage reduction due to the resistance of the wire, which is never 0, and may be large for long runs of wire.

Now, for a current signal, the current transmitter can measure the current and adjust as needed to match the current to the measured variable. This will be the same current as seen by the PLC input, because there is no "current drop" in the wiring.

Ken Moore · Apr 30, 2009

If memory serves the ML1100 built in analog inputs are voltage only, 0-10 volts and are only 10-bit. So if you add a 500 ohm resistor, and use a 4-20mA instrument, then the range will be 2-10 volts, so you loose some resolution. If you buy an analog input expansion module, they have much higher resolution, can't remember if they are 12 or 16 bit cards.

Long story short, you get much better resolution with the add on analog inputs.

rta53 · Apr 30, 2009

He's actually using a ML1000 and not the ML1100.

George Graziano · Apr 30, 2009

I thought the 1000 was digital only. Since he said he has 2 voltage inputs, I believe it is an 1100. I used my first 1100 just recently. I used the 2 voltage inputs with the resistors for 2 levels that didn't need tight tolerence. If given the choice though, I would use a mA input.

George

Roy Matson · Apr 30, 2009

Vic is correct, provided you use a precision resistor across the module input there is no difference between Current or Voltage input. The A-D measures Voltage not current.
Roy

rta53 · Apr 30, 2009

George Graziano said:
I thought the 1000 was digital only. Since he said he has 2 voltage inputs, I believe it is an 1100. I used my first 1100 just recently. I used the 2 voltage inputs with the resistors for 2 levels that didn't need tight tolerence. If given the choice though, I would use a mA input.
George

There are 3 models of the ML1000 that have analog. They each have 4 analog inputs and 1 analog output.

mellis · Apr 30, 2009

According to the manual, the ML1000 analog inputs have the same specs for the voltage and current inputs. Both are 16 bit. Voltage range is 0-10.5v, current is 0-21ma. So, it's pretty clear that a 500 ohm resistor on the voltage input will make it the same as the current input. You want a precision resistor just like the current inputs use.

Voltage signal vs. current signal from the transmitter is a whole different discussion. If you are going any distance at all, current is superior. If the transmitter is in the same panel and the wires are short, it's no big deal to use voltage.

Lancie1 · Apr 30, 2009

Voltage signal vs. current signal from the transmitter is a whole different discussion.

Mellis, Yes, and I thought that Rob was proposing to use voltage transmitters instead of the present current transmitters. I was wrong. I see now that he was just debating whether to use the same current signals on a voltage or current input.

theDave2 · May 1, 2009

You do not need to use a precision resistor.

What you need is knowing exactly what the resistance is.

mA signal to voltage input accuracy

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