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Problem INA155

Daniele Cortellazzi
Prodigy 110 points
Other Parts Discussed in Thread: INA155, TIPD128

Hello, 

i have a problem with the INA155, I have the circuit posted in the schematic. In the bottom part there is the reference use to create the Vref (400mV) on GND1 and at the input pin of the opamp i have a dc voltage to amplify. I have a problem with the circuit because there is an oscillation of this generated by the opamp, i have tried to put a 0ohm instead C5 but i can measure on oscillation on the pin GND1. Why the system enter in oscillation? 

Vdd (3.3V) is stable and if i disconnect the ference voltage from the ina155 it not present oscillation but it is stable

Thanks

  • 0
    TI__Mastermind 25795 points
    Hello Daniele,

    Here are a few observations and recommendations (in no particular order):

    1) I recommend using a 'quiet' analog ground for analog circuitry. It appears as though both the INA and op amp are connected to "DGND", which I assume is a noisy digital ground.
    2) It looks like "GND1" is not 0V...you're using the ground schematic symbol to represent a reference of 400mV. Assuming you're using the LM4132-2.048, the actual reference voltage is closer to 409.6mV.
    3) The supply bypass capacitors should be connected between pins 7 and ground, or "DGND" as you have currently in your schematic. The same holds true for the input filtering. I'm also surprised that you reference the output to "GND1" instead of "DGND".
    4) Since there is an oscillation when you short the inputs and then it goes away when you disconnect GND1, this makes me believe that you have a stability issue with the op amp. I recommend that you add an isolation resistor for the reference amplifier. I highly recommend reading TIPD128. It fully describes the appropriate circuit for this application along with measured results for various capacitive loads(please refer to figures C.7 to C.10).
  • 0 in reply to Pete%20Semig
    Prodigy 110 points
    Pete Semig said:
    Hello Daniele,

    Here are a few observations and recommendations (in no particular order):

    1) I recommend using a 'quiet' analog ground for analog circuitry. It appears as though both the INA and op amp are connected to "DGND", which I assume is a noisy digital ground.

    ok, we will implement in the next pcb


    2) It looks like "GND1" is not 0V...you're using the ground schematic symbol to represent a reference of 400mV. Assuming you're using the LM4132-2.048, the actual reference voltage is closer to 409.6mV.

    yes, gnd1 is a virtual ground at 410mV respect GND


    3) The supply bypass capacitors should be connected between pins 7 and ground, or "DGND" as you have currently in your schematic. The same holds true for the input filtering. I'm also surprised that you reference the output to "GND1" instead of "DGND".

    ok, we will implement in the next pcb


    4) Since there is an oscillation when you short the inputs and then it goes away when you disconnect GND1, this makes me believe that you have a stability issue with the op amp. I recommend that you add an isolation resistor for the reference amplifier. I highly recommend reading TIPD128. It fully describes the appropriate circuit for this application along with measured results for various capacitive loads(please refer to figures C.7 to C.10).

    yes there are stability problem with the ina155. Tomorrow i will do more test to find the problem



    Thanks