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Problems in building a front end with INA333

Sergiu Man
Prodigy 30 points
Other Parts Discussed in Thread: INA333, OPA333

Hello

I have some problems in simulating a front end amplifier. No operating point found error appears for both of the posted circuits. I cannot understand the problem.

The final circuit shoud be an EMG front end amplifier.

 I would rely appreciate any help.

  • 0
    Guru 21975 points

    Sergiu,

    It appears that your output DC control loop, U2, is attempting to drive the output of U1 to 0V, its rail voltage. Perhaps you intended to connect the non-inverting input of U2 to J2. This would bias the output of U1 into its linear operating range.

    The second circuit will have a similar problem with U4. I believe R3 should be connected to J2.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 30 points

    Bruce,

    Thanks  for the quick answer.  I made the modifications but, with the supply of 2x1.5 (first figure) it gives me the same error. It works with 2x3 (seccond figure) witch is actually out of the INA's supply range (max 5.5).

    Could you give me a hint?

  • 0 in reply to Sergiu Man
    TI__Expert 7185 points

    Sergiu,

    The INA333 is a very complex model and sometimes requires a bit of care to make sure that it converges, especially when you add another complex model such as the OPA333 in the feedback path.  I cannot really explain why your simulation converges with a different supply voltage; however, here are some things to try that can usually help with convergence:

    (1)  In the analysis-->set analysis parameters tab make sure that the shunt conductance is set > 0 (i.e. 1p)

    (2)  Bias the common mode at the inputs to the mid supply value (i.e. J1/2), not 100mV.

    *Another thing that sometimes will help with convergence is to place large load resistors on the output of each device in the schematic (i.e. INA333, U2, U4). 

    I hope this helps.

    Matt

     

  • 0 in reply to Matthew Hann
    Prodigy 30 points

    Thanks Matt.

    I will keep in mind your advices. One more thing. If I physical complete the circuit, the one with 2x1.5 batteries, should I expect problems (I am quite new in electronics)?