Problem with using XTR117

Hello Anna,

First, the transistor is connected backwards in the schematic.  Please reverse the collector and emitter connections and see if things behave better. 

Second, are you sure you want a 2-wire transmitter?  It sounds like this is a 3-wire system which could explain your issues.  Since the schematic is pretty bare-bones I can't tell where you connected the instruments to their respective GND potentials.  Is the 0-3V input to the XTR isolated from the 0-12V supply?  If it is not, then this design will not function.  You will find many references in the forum of us correcting improperly used 2-wire transmitters and replacing them with 3-wire devices.  

Please consider a 3-wire locally powered 4-20mA transmitter such as the XTR111 or XTR110 and see if it works better in your system.

Hello Collin,

Thanks for the fast response.

I drew the transistor symbol incorrectly, and the arrow should be pointing from the base to the emitter and not from the emitter into the base.

I made sure that the negative terminal of the 0-3V is not connected to the negative terminal of the 12V supply.

Can you clarify the difference between a 2-wire vs 3-wire?  We had assumed that we want a 2-wire system because we want to make our transmitter versatile such that it could be connected to a receiver that already had loop power or if the receiver does not have loop power, we could connect our on-board supply.

Thanks for your help,

Anna

Hi Anna,

As mentioned before there are already many posts dedicated to the subject of 2-wire vs. 3-wire operation, please give them a read.  In short, a 2-wire transmitter has two-terminals, V+ and IOUT, and is typically remotely located (up to distances > 5miles), powered by the loop, and must consume less than 4-mA to operate correctly.   All 2-wire transmitters sink current.  Look at the XTR105, XTR106, XTR112, XTR114, and XTR115- XTR117.  

A 3-wire transmitter is typically local and is powered from a local supply and GND and creates an output between IOUT and GND used to interface with remote instruments.  All 3-wire transmitters (except bi-directional ones) are current sources.  Look at the XTR110, XTR111, and XTR300.

If you want your design to be either a 2-wire or 3-wire transmitter it may not be possible with our dedicated 4-20mA ICs.  Would you please draw a full block diagram or share a full schematic?   This will be the easiest way for me to prescribe what type of system is likely best for you.   

Hi Collin,

After examining my schematic more, I think that I probably have to use a 3-wire device, since the DAC ground is a common ground for the whole board and from what I understand, the Iret (pin 3 of XTR117) should not be connected to a common ground to other devices.  Is this correct?  Please take a look at the schematic of my transmitter for reference:

4544.DOC050613.pdf

This transmitter needs to be able to interface to various types of 4-20mA current receivers.  The current receiver is a black box to me.  I only know whether it contains loop supply or not.  I was hoping that I could design the current transmitter such that I can manually add a shunt to an on-board 12 V supply if necessary.

If I were to use the XTR111, would it still be able to interface to receivers that already have loop power?

Thanks,

Anna

Hi Anna,

Where is the MCU/processor that controls the DAC?  

You are correct that if you're going to drive the input of the XTR with a DAC then the DAC needs to be powered between VREG and IRET.  In this configuration IRET cannot be connected to any other potential in the system (loop-supply, GND) or the device will not function correctly.

I've worked through several 4-20 mA systems and I haven't seen one like yours that may or may not be powered locally, something seems strange about the way you've connected things and I really wanted to see a full schematic showing what you're using to communicate to the DAC with and how your powering everything.  Is this system supposed to be a "Remote 4-20mA Transmitter"?  If so, the industry standard is that a loop-supply will be provided, that's how a 2-wire system works.  

How much current do the devices that control the DAC consume?  At a minimum this would be the controller and DAC or digital isolator and DAC.

If you use an XTR111 which would be a locally powered 3-wire transmitter then it can not be configured to operate as a remote 2-wire transmitter.  

I am using the AnalogOut pin on mbed with Cortex-M3 to control the input to the current transmitter.  There is an embedded system running Linux that is connected via internet to our server.

http://mbed.org/handbook/mbed-NXP-LPC1768

I will have external power to the board with a power block that supplies 12 V, up to 1.5A.  The AnalogOut pin can only source 4 mA of current.

So the industry standard is for the loop supply to be with the transmitter?  It seems that I should go with a 3-wire transmitter then.  This module that I'm working on is meant to be a portable unit that can be connected to either a 4-20 mA transmitter or a 4-20 mA receiver out in the field, for the purpose of monitoring power use.  It can then sense the 4-20 mA signal or send out a 4-20 mA control signal.

I am not familiar with 4-20 mA systems so thank you for your patience to work with me towards a working solution.  I also appreciate learning from your expertise with 4-20 mA systems.  I am certainly open to better suggestions for both the transmitter and receiver.  I had intended to use a TI-INA170 current monitor followed by a voltage buffer as the current receiver but that doesn't seem to be working either.  Do you have a suggestion for a better circuit in the context of my application?

Thanks, Collin.  I'll try these out.

Cheers,

Anna