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OPA4991: OPA4991IPWR Performance problem

yingjiao LI
Expert 1135 points
Part Number: OPA4991

Hi

24V passes through the 4mA constant current tube and resistor to the in-phase output end of the op amp. 4mA current flows to +15v of the op amp. The current cannot return to +15V. Could you please help confirm the cause

Thank you

  • 0
    TI__Mastermind 19587 points

    Hello, 

    Could you please describe the issue you are seeing differently? I do not understand what expected behavior is. 

    From my understanding, in order to maintain the 4mA current from the 24V, this would require 8.15V drop in the "constant current tube;" therefore the voltage before R15 should be 15.85V, please confirm. 

    With the 100Ω and constant 4mA there is 400mV drop across the resistor, making the voltage at pin 3 of the op amp (IN+) to be 15.45V. 
    This is within the absolute maximum ratings (yellow) of the op amp but outside the recommended operating conditions (blue). 

    It is also outside the input common mode voltage range: 

    If my assumption about the above is true, then the op amp is a non-linear region. 

    I have some questions: 

    1. Are there decoupling/bypass capacitors on the ±15V power supply pins of the op amp? 
    2. Can I see how channel B is configured? 

    All the best,
    Caro