What is the purpose of the switch on the RTS - HART_OUT line (on datasheet
diagram figure 28 and 29)
Is it needed?
The switch is needed in that position in some cases:
If the analog input current changes from 20mA to 4mA, it generates change of
-4V across the 250ohm resistor. That change is coupled through the 2.2uF
capacitor. The right hand side of that capacitor is biased to about 0.75V – by
the 75kohm/22kohm resistors in this picture, but also by resistors of total
~75kohm impedance inside the AD5700. If there was no external switch, then the
change of -4V coupled by the would be pushing the AD5700 output below ground
voltage – and it would be clamped by the AD5700 internal protection diode. From
the HART AC signal point of view, the circuit under these conditions looks like
2.2uF connected to ground, and if there was any HART receive signal coming in,
it would be effectively shorted to ground. Other words, the HART receive would
not be possible while the analog input current is changing its value.
This problem gets even worse if the shunt resistor is higher value, or if the
circuit is used as a "secondary" device, where the shunt resistor is outside
the circuit and could be sitting on top of relatively high – and changing –
common mode voltage.
In these cases the switch isolates the AD5700 output from the rest of the
circuit in HART receive mode, and makes the circuit work.