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I've got an MCU with a 3.3V DAC onboard and I'd like to convert the 0-3.3V signal to a 4-20mA output. Any ideas on how to do this?

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  • \$\begingroup\$ How About using a mosfet transistor?, keep in mind they are not linear, so you will have to look at the data-sheet of your mosfet. You can also use op-amps I Googled voltage to current converter using op amp and there are lots of solutions. \$\endgroup\$ – Falk Mar 5 '14 at 23:51
  • \$\begingroup\$ ti.com/lit/ds/symlink/xtr300.pdf \$\endgroup\$ – Ecnerwal Mar 6 '14 at 0:03
  • \$\begingroup\$ is this to interface with other 4-20mA circuitry? Such a std is usually terminated by a 50->750R resistor thus 1V --> 15V at the termination end THUS higher at the transmission end. \$\endgroup\$ – JonRB Mar 6 '14 at 2:04
  • \$\begingroup\$ I'm well aware of the hundreds of parts from TI, Analog Devices, and others. What I'm not clear on is if any part that takes a 3V3 analog input as full-scale input, nor am I clear on if this is the best approach/should I do something else/is there a trick. \$\endgroup\$ – Pugz Mar 6 '14 at 6:46
  • \$\begingroup\$ I Googled this too... There is no published data on what the stability/full-scale deviation of the signal is when put through an op-amp circuit. Any idea if this type of info or an application note for this style of converter is published? \$\endgroup\$ – Pugz Mar 6 '14 at 6:50
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You can use one of the ICs called "voltage to current" converters and follow one of the appnotes or datasheet examples to achieve your goal. You get a lot with them, but of course you pay in terms of cost and single-source.

A way is to use a +V power source to provide whatever voltage compliance you want (with an output impedance of some kind, usually sized larger for perhaps a watt or more) and a BJT to sink current to ground. In the emitter leg of the BJT, there is a current measuring resistor that develops a ground-referenced voltage that is used to feed back to an opamp that is driving the BJT. But you need to worry about oscillation and perhaps calibration with offset and gain adjustments. Often, that is handled by using a dual op-amp package like the cheap and decent TL062A, with one of the opamps accepting the voltage input and providing offset and gain adjustments and the other opamp doing the current monitoring and driving the BJT sink.

You haven't said a word about the necessary voltage compliance for the 4-20mA output. Nor the specific MCU and the characteristics of the DAC (like its precision, for example, let alone output characteristics.) Nor what kind of accuracy you need and whether or not you care about offset errors and/or gain errors, differential or integral linearity, etc.

Also, what have you googled and rejected?

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  • \$\begingroup\$ DAC is 12-bit on the MCU. Loop V needs to be between 9 and 36V. I do need this to be VERY linear. The output must have as little variance in linearity as possibly. As I stated in another comment, I don't know "what the stability/full-scale deviation of the signal is when put through an op-amp circuit" and can't really find any info on it, though if it's like a many circuits made from discrete components I fear it might be to bad for what I need. I've looked at devices such as the TI DAC8760 which uses SPI in and can output 4-20 and 0-10V and take a 3V3 reference... just so expensive ($10/each) \$\endgroup\$ – Pugz Mar 6 '14 at 7:22
  • \$\begingroup\$ I guess what I'm saying is - I don't care if I use the onboard DAC if it's too much trouble. I'm fine with a discrete DAC that can do 4-20 if it works better. \$\endgroup\$ – Pugz Mar 6 '14 at 7:25
  • \$\begingroup\$ I think you need to pay the price. I don't know what "very linear" means, but if you want top quality and can't do the design then you need to buy one of those expensive devices with specs you can read about and support staff who can help make sure you use it right. I suppose that is where some of the money goes. \$\endgroup\$ – jonk Mar 6 '14 at 7:51
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You could use an XTR115 by TI. Or you could skip the D-A step and get a digital 4-20 driver like the AD421 by Analog Devices. There are newer ones but those are 2 I've used in the past.

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  • \$\begingroup\$ I called TI about this part today, and the others in the XTR family... I'm still not clear on whether they take 3.3V as full scale input/how the reference works. He did say these parts are legacy... most of the XTRs were released around 2000. \$\endgroup\$ – Pugz Mar 6 '14 at 6:52
  • \$\begingroup\$ From what I can tell, the XTRs require a 5V analog input voltage reference and can't do 3.3. \$\endgroup\$ – Pugz Mar 6 '14 at 7:15

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