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The first of post#5 is basically the same as the one of post#1.
The other is better suited to your needs - at least I think so.
Give more informations:
* power supply
* DAC output voltage range
* desired speed / settling time / upper frequency limit
* load resistance range
* desired accuracy, precision
* other requirements
You will find a lot of 4..20mA circuit discussions in this forum.
To ease the design I recommend a VCC referenced load
To improve accuracy I recommend to use a mosfet instead the bjt.
1. You need a differential amplifier topology to make the output current independent from supply voltage variations.
2. A bipolar transistor introduces a considerable error by its base current. The suggested circuit uses a darlington transistor to reduce the error below 1e-4.
3. As shown, the output current is proportional to input control voltage. To introduce 4 mA offset, an additional reference voltage and respective circuit modification is required.
For 4..20mA outputs I recommend to add some headroom. I often go to 0..24mA.
The adjustment of 4.00mA and 20.00mA can be easily made by software. Without drift with time and temperature.
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