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INA180: OPA991 + INA180 current loop

Georges Maccario
Prodigy 40 points
Part Number: INA180
Other Parts Discussed in Thread: OPA991, TINA-TI

Tool/software:

Hello,

I am working on a current source (100/200mA).

To minimise power losses in the shunt, I've chosen to use an INA180 in the feedback loop.

The system does not require a high bandwidth (10Khz or less).

Doing an AC sweep there is a amplitude ''bump'' between 200Hz and 5Mhz (about).  I can reduce the amplitude or the shape of the frequency response but I still get this ''bump''. 

The problem that is reflected in the overall noise current transmitted to the load (laser diode), this shows in the laser light FFT, as well as the boost converter (but that is taken care of).  I could get read of the INA180 and use a bigger shunt, but I do not want to do that for now.

I suspect it is the INA180A2 reduced BW in the feedback loop and the driver ''integrator'' + Mosfet configuration messing-up the response ?

The question is what to do to reduce the bump...

1. Type II controller (RC// with 47nF) ? how to calculate the values ?

2. Finding a similar INA180A2 op-amp with higher BW, or making one with a faster op-amp

3. Fine tuning the integrator capacitor ? how to calculate.

To be fair, there are a lots of App. Notes on that, but none of any practical REAL use, it is all put together with little finesse and probably never used in the real world.

Thank you for your support.

 OPA991+INA180A2_I_Source.pdf

  • 0
    TI__Expert 5060 points

    Hi Georges,

    This could be a case of impedance mismatch or resonance, could you share your Tina-TI simulation file please?

    Best,
    Mallika Senthil

  • 0 in reply to Mallika Senthil
    Prodigy 40 points

    Hello Mallika,

    Thank you for your help.

    Please find attached the TINA sim. file

    Regards

    GeorgesV-I_OPA2991_03.TSC

  • 0 in reply to Georges Maccario
    TI__Expert 5060 points

    Hi Georges,

    Please check your decoupling capacitors to ground near your 5V supply. The capacitor should be between your supply and ground, currently they're on the other side of the supply.

    Best,
    Mallika Senthil

  • 0 in reply to Mallika Senthil
    Prodigy 40 points

    Hello,

    Good try, but that will not do anything to the loop frequency response.

    As well, in non transient mode, that wont make any difference. I doubt in this mode the simulation will take that into account as you will have to model as well the supply source and track impedances...

    This has nothing to do with my initial query, which is about the response of the control loop and the current flowing through Z2/R6...

    Please, check with an other of your colleague, maybe more expert in loop control and compensation.

    Regards

  • 0 in reply to Mallika Senthil
    Prodigy 40 points

     Hello,

    I tough this is correct: 1. supply (via, track, plane) >> 2. decoupling capacitor(s) >> 3. IC pins.

    During transient and HF, the capacitors and low impedance tracking will cater for transient power and lowering noise reinjected into the circuit ?

  • 0 in reply to Georges Maccario
    TI__Expert 5060 points

    Hi Georges,


    Please find below expert's response:

    To reduce the "bump" you see in your transient response, you can go the Type II compensator route. The following application report will tell you how to calculate the values. 
    https://www.ti.com/lit/an/slva662/slva662.pdf

    In your schematic, 

    Move your Rf (10k) to be in the op-amp's negative feedback loop and make your R4(1k) larger, (e.g. 10k because) the INA180 can't sink currents larger than a few mA without loosing output swing headroom (see the claw curve below).

    Following the suggestions above your schematic and simulation result will look like the below. 

    The bump in AC sweep now comes after 14kHz, and given that in your application 10kHz is sufficient, this should work. You can fine tune this graph by adjusting the feedback loop capacitor and resistor values following the App note linked above.

    Best,
    Mallika Senthil