Hi Kaos,
My apologies, I'm still unsure what you are asking. I'll try to explain the circuit so that it might answer the question.
Please let me know if your query is answered/unanswered.
The CN0267 represents a field instrument.
The ADC's that you see in figure 1 are used to measure a sensor (Example: Pressure). Without HART , the signal from the sensor is calculated on the microcontroller and the ADuCM controls the AD5421 to output a current (4mA to 20mA current loop). The higher the current, the higher the pressure. This is returned to the master controller
With HART, extra information can be added to the 4mA to 20mA loop (Sensor ID, Sensor Type, Faults). The ADuCM communicates with the AD5700 via serial, and the AD5700 sends the data on the 4mA to 20mA loop (via HART_OUT to the AD5421).
When the master communicates data back, it can do so via HART. The AD5700 on the field instrument reads the current loop:
If filter is external, ADC_IP is the pin. If Filter is internal, HART_IN is the pin. This data is read via serial to the microcontroller.
Hopefully this helps, please let me know if anything is unclear.
Kind Regards,
David