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OPA2182: unexpected output during power up

Aki Li
Genius 9205 points
Part Number: OPA2182
Other Parts Discussed in Thread: LM2904, OPA2192

Customer used OPA2182 for current sensing, and found during the power up, the OPA2182 output showed the unexpected voltage which was equal to VCC. And LM2904 did not show such phenomenon.

Could you please help check the possible reasons and any other parts to suggest to avoid this issue? We need a competitive part to compete against SGM8252 and SGM8255, with the Vos target less than 0.1mV, and lower cost.

  • 0
    Guru 140200 points

    Hi Aki,

    it's normal that an OPAmp is out of control during power-up until it reaches the specified minimum supply voltage. See figure 7-21. And even if the supply voltage rises instantaneously (which is not recommended for other reasons) the OPAmp needs some dozens or hundreds of microseconds to properly bias after power-up. How long exactly also heavily depends on the actual circuit. Keep in mind that eventually feedback or load capacitances have to be charged up which may take some time.

    One remedy is to actively mute the output signal until the output has fully stabilized. The control signal of muting can usually be derived from the supply voltage, eventually in combination with a simple RC-delay. Sometimes even a simple p-JFET can do. If the output signal of OPAmp goes to an µC, on the other hand, then also detect the supply voltage of OPAmp with a free input and do the glitch suppression in software.

    Another remedy is to add a simple low pass filter (eventually in combination with a suited diode clamp) with a time constant long enough to suppress or at least minder the voltage glitch.

    Kai

  • 0
    TI__Mastermind 23520 points

    Hello Aki, 

    I agree with Kai's comments. After power up does the device operate as expected once everything is powered fully on and settled?

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    TI__Genius 9205 points

    Chris,

    Agree that no issues after power up. However, since the output of OPA2182 was provided externally for their system. It is not acceptable for their product with such unexpected output during power up. It might hard to filter out with RC, since the max voltage might follow the supply voltage closed to 5V.

    Why LM2904 did not show such issue? And any other amplifiers with similar spec as SGM to show low output during power up?  

  • 0 in reply to Aki Li
    TI__Mastermind 23520 points

    Hi Aki, 

    The OPA2182 is a chopper architecture that is much different from the LM2904 bipolar architecture. In the OPA2182 there are oscillators and Mos transistors that need to power up and regulate. During the startup the output will not be predictable and this is the reason it is going to an unexpected value. Bipolar devices are faster in nature and this may be the reason the customer is not observing the same behavior as the OPA2182. The startup behavior is also dependent upon ramp rate. The faster the dV/dt the higher the overshoots prior to settling down once regulated. A bipolar device may be better in this regard however op amps will have an unpredictable output startup behavior. The OPA2192 is more controlled on it's output during power up however will still be unpredictable. 

    Let me know if you have further questions. 

    Best Regards, 

    Chris Featherstone