Electronic – Nmos trigerring in 4-20mA converter

I don't recommend you to use the design in the picture. OAs ( Operational Amplifier) are good for Voltage gain and High impedance output, but not to "play" with current, the current output is sensible with the components tolerance. Try to simulate your circuits with variation of 5% in your devices (resistors & VCC) and see the output. When you will buy the components they will have deviations about 1% to 5%, depend on the quality of the materials you will buy.

Transistors are the devices to play with current gains, but impedance could be the problem.

See also the tolerance for the Valve mid-position for current, it is 1.3%:

Voltage type [V DC] 5 (±0.1)
Current type [mA] 12 (±0.16)

Your are right the 12.267 mA are not correct ( max. acceptable would be 12.16 mA), take this into consideration because it means: the tolerance current output of your circuit must be max. 1.3%!!!.

It could means also that maybe temperature compensation of your output must be implemented depend on the range of temperature where the device will operate and ICs you will use.

I have seen the Page 6 Datasheet of the valve:

Setpoint value
Voltage type [V DC] 0 … 10
Current type [mA] 4 … 20

Why don't you try to control the valve with "Voltage type"? I think it is easier to transform your +-5V to a 0-10 Volt linear signal with mid-position on 5V. And you have a little more tolerance (2%) requirement for your output. It is easy, you can use an OA adder with a two inputs: one a +5V offset and the second your +-5V signal (you must check the tolerance).

It will work. You might want to use a somewhat lower value of resistor so you can detect overrange on the 4-20mA. The offset zero allows you to detect underrange (this is useful for sensor break detection and such like). Of course you lose a bit of resolution on the ADC- 20% on the bottom. You could also use a stage with some gain rather than a voltage follower which would allow an even lower resistor (say 49.9 ohms) to be used and would allow a better performance op-amp to be used (relaxing the requirements to input common mode range includes ground and R-R on the output only).

Take care as to the layout- use Kelvin connection to the sense resistor, especially if you want to have a high accuracy (< 0.1%) circuit and are using a lower value resistor.

The compliance of the transmitter will determine the minimum supply voltage, and your 3.3V subtracts from that, so a lower input voltage is desirable, however too low and you lose accuracy. If several devices are connected in series on the loop then 12v or even 24V may not be enough (the transmitter may require 10V, an indicator another 10V and your circuit 3.3V is getting close even at 24V).

Note that a short across the transmitter will damage the input resistor most likely and probably blow the *** out of the op-amp and likely other stuff too. This can be protected against, at some cost in input voltage and parts. If you put a series resistor on the op-amp inputs it may limit the damage to just the input resistor.