Most industrial interfaces use the 4-20mA interface as a discrete interface to represent 0s and 1s with those two levels. I need to have an interface to a 4-20mA input, that can vary at least 1.6mA increments from 4-20mA. 4mA represents a off state, and 20ma is a full on state, and I need to be able to vary throughout this range, with fairly course steps. I would rather have 0.8mA increments throughout that range to give me better resolution. Is the load looking into a 4-20 interface standard? Is it resistive at a fixed load? I'm not familar with this type of interface. I assume I need a voltage source, perhaps a DAC, and then a current driver of some sort. Any help will be appreciated.
Thank you for your interest in a 4-20mA transmitter. We have a variety of parts available which can output any current within the 4-20mA range, so achieving 0.8mA increments will be no problem as long as the input signal is configured properly. The load of a 4-20mA transmitter is commonly a 250-ohm resistance, so then by Ohm's Law a voltage drop of 1-5V is created which can easily be measured. However, some 4-20mA receivers such as our RCV420 have a lower input resistance and then create the desired output voltage with an internal transimpedance amplifier.
It sounds like you plan on using the transmitter to control a remotely-located piece of equipment, such as a valve. Is this the case?
Ian WilliamsLinear Applications EngineerHigh Performance Linear
You are correct, the load is rated at 250ohms (according to my customer) and yes, I am controlling a remote industrial control mechanism with the 4-20mA interface. I will take a look at the RCV420 device and if that is a family of devices, I will look at those as well. What I don't know about, is if my load device has a power source integrated (Type 2) or if I have to provide the voltage source (Type 3, Type 4) into a passive load. Also my customer described a 0-20mA control mechanism? Not sure how the load device will respond to 0mA or why I would want to use that.
Thanks for the quick response, I appreciate that.
I just looked at the data sheet. I need a transmitter, so I need a voltage to current converter. This part is a receiver (RCV420). Do you have a recommendation on a transmitter?
The RCV420 is indeed a receiver - you asked in your original post about loads and whether they were fixed/standard, so I included the RCV420 as an example of a load with a non-standard input resistance.
I'd recommend the XTR111 in your application. It is an easy-to-implement voltage-to-current converter that can be used for 0-20mA or 4-20mA output. 0-20mA might be preferred by the customer if a 0-5V range is optimal for the mechanism being controlled.
We also have an EVM (evaluation module) for the XTR111 available here: https://estore.ti.com/XTR111EVM-XTR111EVM-Evaluation-Module-P1244.aspx
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