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Hello Rodrigo,

The reason that your circuit will not operate correctly is that the XTR115 is a two-wire current transmitter.  In order for the XTR115 to operate correctly, the device's input must be allowed to completely float with respect to the output stage.  Specifically in looking at the IRET pin, the voltage on the pin is determined based on the output current commanded and the value of the load resistor.  As the output current changes, the voltage developed across the load resistor changes which moves the voltage at the IRET pin respectively.  In your circuit you have drawn the IRET pin of both XTR115 devices connected to the same ground potential.  This potential is being held with respect to the other XTR115 regardless of the output current and load resistor.  The device's internal circuitry is then unable to correctly bias itself resulting in what appears to be wrong correlation between the input and output.

The reason that the circuit works when only one channel is used is that the IRET pin floats completely with respect to the power supply ground.  The other channel has no effect on the IRET pin so it is able to achieve the necessary voltage to bias the circuit correctly.  Having multiple channels is going to require that the input signal to each of the channels be isolated separately from the multi-channel DAC and each other in order to not have the IRET interaction between the two channels.

Hello Scott,

It is possible to develop an two wire looping using the XTR111 for this aplication??

Thanks,

Rodrigo.

Rodrigo,

The circuit as was originally shown will not work for a 2-wire system.  The inputs to each of the channels must be isolated in order for them to be allowed to float independently of each other.  We do not recommend using the XTR111 in a 2-wire architecture. 

An alternate configuration would be to use two single channel DACs and optically isolate them from the controller rather than trying to isolate the analog output signals which is more involved than the digital isolation.  Each DAC could then be referenced to the IRET of that channel so there would be no interaction between the two channels.

Hello,

I am trying to do the same as Rodrigo.  Was a solution found? and may I see a schematic please?

In the 2-wire system, you have to isolated the input to create a loop. If you want to use XTR111, it often use in 4-wire transmitter.

Scott,

          I want to do something similar to Rodrigo, instead of using a DAC i'm using a msp430 to generate two PWM's that will be connected to two XTR117. I was thinking to use two ISO,one for each XTR117  (to avoid interaction between Iret of each XTR117), is this possible? Is there an IC like the XTR117 but instead of have one 4-20 output has two output 4-20?

Best Regards

Gastón