Hi,

Many thanks for the additional information of the PGA309+XTR111 module schematic and connections to the PGA309EVM-USB.

I went ahead and reviewed in detail the MULTI-CAL Master board Schematics that includes the loop supply multiplexer circuits and current measurement multiplexer circuits. The MULTI-CAL master board is not able to support 3-wire current transmitters.  The hardware would need to be modified to add a separate multiplexed Iout current measurement path and multiplexed loop power supply path (GND).  Both the software and hardware would need significant changes. On the MULTI-CAL system, the current loop transmitter measurement is performed on the positive loop supply side, which does not work for 3-wire current transmitters.

Below is a more detailed explanation.

PGA309EVM-USB Current Measurement (can be set up for either 2-wire or 3-wire current transmitters, voltage output 2-wire and 3-wire)

On the PGA309EVM-USB, the transmitter current measurement is sensed on the Iout (XTR-loop-) connection of the board
The USB-DAQ senses the current across the burn resistor on the negative connection of the loop supply (XTR-Loop-).
Alternatively, since this is a single-site solution, the user can easily connect an external current meter on the (XTR-Loop-) connection and sense the current loop output on the negative supply connection.  This approach works well for both 2-wire or 3-wire current transmitters.
When using the XTR111 (3-wire current transmitter) case, as long there are separate cables connecting the loop supply GND and IOUT, this approach works well as you have set up on your system…   

Below is the USB-DAQ current sense measurement circuit (from Figure 16, p13 of the USB-DAQ Platform User Guide) on PGA309EVM_USB using USB–DAQ board; sensing the measurement on XTR_Loop- negative side (or IOUT):

MULTI-CAL-SYSTEM Current Measurement (supports only 2-wire current transmitters, voltage output 2-wire and 3-wire)

However, one key difference, on the MULTI-CAL system, the loop current is sensed on on the positive Vpos loop connection, using an external current meter. The external current meter is multiplexed to each test card site using the Vpos (XTR-loop+) supply connection (on the positive loop supply side).

When using a 2-wire current transmitter, the total 2-wire XTR transmitter signal output current (Iout), includes the 2-wire current transmitter consumption, and this approach works well for 2-wire current transmitters such as the XTR115/6/7.

Unfortunately, the MULTI-CAL hardware set up will not work for the XTR111 (3-wire current transmitter), where the +V loop supply current is not the same as the XTR111 current transmitter output (lout).  In other words, on the XTR111 case, the current output measured will be the desired transmitter current signal (4-20mA) plus an additional error: the 3-wire transmitter/sensor circuit current consumption producing an error…

See below Figure 7, page 7 section of Multi-Cal-Master User's Guide; where the current signal is sensed on the Vpos (+) multiplexed loop supply connection.  This measurement is only accurate for 2-wire current transmitters

Please let me know if you have additional questions, 

Thank you and Kind Regards,

Luis