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I am working on a DC load circuit and have put together a simple configuration based on templates available online. As you can see in 3rd image attached, I am noticing a small ringing in the load circuit measured at R1 which is a current sense resistor. I am assuming its being generated from the OPAMP not being able to stablise quick enough and so I tried different random solutions like adding 10 pF capacitor between MOSFET base and ground however I am unable to completely get rid of the ringing.

Can anyone suggest a methodological approach to stablise the opamp and remove this ringing?

Thanks,

  1. Schematics Schematics

  2. Full output Vsingal vs I(R2)

Full graph

  1. Ringing zoomed Ringing

Edit: Final schematics with added RC filter on 2nd stage opamp to filter out ringing. Adding to original post in case anyone's looking for similar solution:

Final Schematics

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    \$\begingroup\$ R5 should be much lower ... (100 Ohm or 0 ?). \$\endgroup\$
    – Antonio51
    Commented Sep 17, 2023 at 7:23

2 Answers 2

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Here is an additional tweak that will help ensure stability.

enter image description here

The output in my simulation above is more-or-less accurate if V3 is current-limited. In reality U2 will be destroyed if V3 is too low in source impedance.

You should limit the input voltage to U2 to >= 0V. If it goes negative by more than a few hundred mV and the current is limited you'll see the phase reversal in my simulation, and if it's not limited you'll probably damage the chip.

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  • \$\begingroup\$ Thanks for your response. It seems the 1nF capacitor and changing R3 to 680 ohms did the trick. I am now getting full current resolution without any ringing. Regarding the V3, I am only using sine wave to test the characterstics of variable DC input. In real world it will be 0 - 5VDC and thus my input reference will never go in negative. \$\endgroup\$
    – Prashant
    Commented Sep 17, 2023 at 13:23
  • \$\begingroup\$ Also, may I ask the theory behind the capacitor and resistors (R2, R3 and C1) and how's it helping eliminate the ringing ? My understanding is that the three elements create a RC filter which elimitated the harmonics (ringing). Is my assumption correct ? \$\endgroup\$
    – Prashant
    Commented Sep 17, 2023 at 13:27
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    \$\begingroup\$ @Prashant Capacitive loading (such as the gate in the MOSFET with only 1Ω in series effectively) can destabilize most op-amps. The series resistor makes it much worse (the op-amp itself has poorly documented ~100Ω output resistance). By adding significant phase-shift to the feedback you're (at a minimum) decreasing phase margin. The theory behind in-loop compensation such as I used is covered in this AD paper, but note that it's somewhat guesswork since the output resistance is not well known. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Sep 17, 2023 at 14:06
  • \$\begingroup\$ To test this (including in simulation), you should use square waves which are much more likely to excite oscillation or ringing than sine waves. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Sep 17, 2023 at 14:07
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    \$\begingroup\$ I feel like I'm learning more from just being on this forum then my entire 4 yrs of engineering. Thanks a lot for clear , crisp and real-world explaination for this problem. \$\endgroup\$
    – Prashant
    Commented Sep 17, 2023 at 14:32
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Here is what you should have as outputs ... when changing your R5 to 100 Ohm.

enter image description here

Here is the "response" with a square wave at 10 kHz.

enter image description here

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  • \$\begingroup\$ Thanks for the suggestion. Replacing R5 to 100 ohms did help a bit. \$\endgroup\$
    – Prashant
    Commented Sep 17, 2023 at 13:24

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