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I'm building a programmable current load. The big picture is : A microcontroler (stm32f303) drives (through its dac) some n-mos (tested with PSMN034 and IRL2203) that drawn current. The microcontroler also measure current and adjust the gate voltage until the current reach the target.

My problem is I have some strange behaviour while controling the mos' gate with the DAC.

For this test, I only output a fix value on the DAC. With nothing connected to DAC, the output is stable. When connecting the gate to the DAC, I measure voltage oscillations on the DAC output. I supposed that the DAC was not able to provide enough current, so I added a AOP but the result is the same. With a fix DAC output, the oscillations' frequency is slowing down while the current is raising. The speed is "exponential". Current is growing from 20mA up to 1A, when I stop the experience.

I expected everything to be stable : DAC voltage and current drawn in mos.

Do you know what it can be ?

Thanks.

Here's a schematic:

Power is connected to a +8V power supply. The microcontoller is powered by a 3v regulator connected to my computer's usb port. AOP is powered by USB. enter image description here

Here's a screenshot of the measures. The top curve is the DAC output. The bottom curve is the output of the AOP (tied to mos' gate).

enter image description here

and a video to see the frequency changing: Video

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    \$\begingroup\$ What is "Power" connected to? And where do those connections either side of R35 go? \$\endgroup\$ – Finbarr Nov 7 '17 at 14:03
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    \$\begingroup\$ Perhaps using a current mirror is a more viable solution. \$\endgroup\$ – Marko Buršič Nov 7 '17 at 14:08
  • \$\begingroup\$ @Finbarrr : Power is connected to +8V, microcontroller is powered by the usb cable (it's a nucleo eval board). Ground of the two power supplies are connected together Connections from R35 are going to an amplifier used for future current measurement. \$\endgroup\$ – Julien Nov 7 '17 at 14:15
  • \$\begingroup\$ @MarkoBuršič: I don't know this design. I will ask google. Thanks. \$\endgroup\$ – Julien Nov 7 '17 at 14:16
  • \$\begingroup\$ @Finbarr: More exactly the microcontroller is powered through a 3.3v regulator connected to USB. AOP is powered by USB. \$\endgroup\$ – Julien Nov 7 '17 at 14:30
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Ok, this behaviour out comes when a AOP drives a capacitive input like a mos' gate. Added some filter and no more oscillation.

BTW, I found dozens of project like this on the net and obviously, like Finbarr says, using adc/dac to drive the system definitely not the best way. A much better way (and an interesting way to learn AOP) is to let the AOP drive the mos with a loop back coming from low side current measurement. See here: enter link description here

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