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Currently I use a LT3083 as a constant current source more or less according to the datasheet as given in the first image. Instead of a single set resistor however is use a fixed resistor in series with a variable resistor connected via a cable to the case of my device which is generally not recommended as I'm told.

Constant current source

In the future I would like to set the current with a DAC if possible like this:

DAC controlled regulator

Unfortunately, the datasheet does not offer any more details to this configuration and it only works as a voltage regulator. How would I have to modify the second schematic to create a DAC controlled current source with the LT3082? Is this even possible?

Thanks!

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  • \$\begingroup\$ "variable resistor connected via a cable to the case of my device which is generally not recommended" - Who told you that and do you have details of the cable length/thickness/connections to the PCB? I personally don't see why this setup wouldn't work, you would need to take into account the cable resistance to accurately control your current output. It is possible that the parasitic inductance of the cable plays a small "evil" role in this but I don't see a reason not to try it. \$\endgroup\$ – eeintech Dec 12 '19 at 15:26
  • \$\begingroup\$ @Cisco25 From the datasheet: "Since the SET pin is a high impedance node, unwanted signals may couple into the SET pin and cause erratic behaviour". The PCB layout even shows how to put a guard ring around the pin. \$\endgroup\$ – Finbarr Dec 12 '19 at 15:54
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Once again, the 4-resistor difference amp is your friend. The circuit they show you has the output of the LT1991 referenced to ground, but it can also be referenced to Vout, like this:

schematic

simulate this circuit – Schematic created using CircuitLab

OA2 acts to keep the voltage difference between OUT and REF equal to the voltage difference between IN+ and IN-. This means that you can now control the voltage across R1 — and therefore the output current — by means of a ground-referenced voltage Vset coming from your DAC.

This circuit shows a gain of 1, which means that Vset must be limited to 0 - 1V, but you could actually use a gain of less than 1, which would allow you to use a wider range of DAC voltages. Just keep R3 = R5 and R4 = R6.

You can get amplifiers with matched resistors built in, such as the LT6363 family. In particular, you might want to use the LT6363-0.5.

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  • \$\begingroup\$ This seems to works (up to a certain output current) with a very bad non-PCB-based setup. Surprisingly, currents and voltages are as calculated within a 10%-error margin. I can not say of course, how well this will perfom on a well designed PCB but I expect this will be exactly what I wanted. Thank you for explaining what OA2 exactly does - this helps me a lot as I'm not well trained in using OPs. \$\endgroup\$ – Peder Dec 13 '19 at 10:57
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The configuration with the LT1991 would work for adjustiblity but would only provide open loop voltage control as there is no feedback. If you monitored the output current with an ADC and a sense resistor, the loop could be closed and feedback provided digitally with a digital PID controller. Usually the datasheets don't go into digital control, but you might be able to find an app note somewhere or talk to a rep.

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  • \$\begingroup\$ Your idea is not bad and as a matter of fact, I already measure the resulting current with an INA226. Dave Tweed's answer however suits me more, but thank you anyway! \$\endgroup\$ – Peder Dec 13 '19 at 11:01

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