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LM358B: LM358 as an amplifier has a large amplification error

Gabriel Wang1
Genius 4735 points
Part Number: LM358B
Other Parts Discussed in Thread: LM358, LM7705

Hi,sir .

FA FORM huntkey 的副本.xlsxLM358B  is used as an amplifier to detect the current of 250A, but the actual deviation of +/ -5A is too big. A total of 200K is used, and only 150 pieces have this problem.

  • 0
    Guru 140200 points

    Hi Gabriel,

    it's hard to say anything useful without knowing the component values :-)

    I would remove CA27, CA28 and CA31. They are decreasing the phase margin and are destabilizing the OPAmp. As consequence the OPAmp can behave strange or can even oscillate.

    Can you give some more details on the circuit?

    Kai

  • 0 in reply to kai klaas69
    Genius 4735 points

    Dear Kai

    Thanks for your reply.

    Could you please tell me how to test whether LM358B fails or not? What parameters do I need to test?

  • 0 in reply to Gabriel Wang1
    Guru 140200 points

    Hi Gabriel,

    is the circuit having problems with very low measuring currents? This could mean that there is an issue with the input offset voltage of LM358 or with a limitation of linear output voltage range of LM358 because the output goes into saturation.

    You could disconnect the shunt and input a test voltage which simulates the measuring voltage (voltage drop of measuring current across the shunt).

    Eventually, a remedy is to add the LM7705 to allow the output of LM358 to fully swing down to 0V.

    Kai

  • 0 in reply to kai klaas69
    TI__Genius 15410 points

    Hi Kai,

    Thank you for the help. 

    Hi Gabriel,

    What output voltage change would correspond to a +/-5A current? For example, a change in 100mV on the output represents a change in current of +/-5A.

    What is the gain of the circuit?

    What is the shunt resistor value?

    It would be best to provide a schematic with the component values labeled as Kai has mentioned.

    Thank you,

    Tim Claycomb