Using XTR family to sink current coming in on V+ on multiple channels

Hello Winston,

This sounds like an interesting application, thanks for using TI products. 

If you're familiar with 4-20 mA loop systems, the XTR117 is a "2-wire" transmitter.  This means the intended operation of the device is to be remotely located from the loop supply and return with all of the sensor/signal conditioning powered between the local VREG and IRET pins.  The input signal will be scaled such that the desired output range of 4-20mA (or other) is achieved.

With any 2-wire current loop, the current around the "loop" is constant.  Therefore the current delivered into the V+ pin of the XTR117 will be equal to the current out of the IOUT pin.  Therefore, placing the sense resistor between the loop supply and V+ of the XTR117 will produce accurate results.  It is uncommon to operate this way because the sensing circuitry is required to have a common-mode voltage that includes the loop-supply which is typically more difficult than sensing on the loop-return side where the common-mode voltage is GND.   

Although your idea of sensing the current on the loop-supply side will work,  it will be a problem if you connect the IRET pins of any of the XTR devices together.  Since the XTR devices are meant to be remotely located, the intention is that the input circuitry is fully powered between the VREG and IRET pins of the XTR and will float in relation to the +loop supply and loop-return.  This is required because as the output current changes, so does the voltage on the IO pin.  Therefore, if you tie the IRET pins to each other or loop-return then the operation of the circuit will fail to produce the desired output range and device damage may occur.  If you look at the block diagram of the device this becomes apparent because if you connect IRET and IO signals both to GND, then you've effectively shorted out the 25 Ohm sense resistor (R2) inside the XTR that is used to regulate the output current of the BJT.  As expected this will result in unintended circuit operation.

As shown in the datasheet if the desired operation is to use a DAC then the two common options are to isolate the analog output using an isolation amplifier or use a locally powered DAC and isolate the digital communication going to it. 

So I'll conclude by asking if you really want a 2-wire transmitter for this application, or if it could work with a 3-wire transmitter such as the XTR111 which is meant to be operated with an input signal that is referenced to the main GND of the IC.   

Hi collin

Can i use XTR115 as 3wire sinking? ??
or as Isolated? ?? 

Collin,

Thanks for the reply, I am not sure why I did not see this earlier.  I am putting together some test equipment, and need to put my "transmitter" in various parts of the loop.  n the most generic case, there is the the V+ of the loop, the sensing resistor, then my "transmitter", then a current limiting 100 ohm resistor, then the return line back to GND of the V+. individually isolated channels seems the best way to go. I get how to isolate the digital signals to the DAC, but can you describe the "isolation amplifer" to isolate the AO signal a bit more?