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OPA209: OPA209

Part Number: OPA209
Other Parts Discussed in Thread: OPA171,

Hi support team 

When the input voltage (pin3, 4) is greater than VCC (pin 5) , the output pin (PIN1) will be locked at high level, causing OCP to be detected when start up.

There is no such problem when switch to OPA171. I would like to ask where the difference between these two OPs even those two rail to rail output only, 

Thanks a lot 

  • Hi Red,

    It is never a good practice to have a voltage applied to the input, especially one exceeding the supply level of the operational amplifier, applied at start up. Doing so can create unintentional current paths in which a parasitic latch structure can form and the Op amp may not function normally. Different Op amps will respond in different ways to these conditions depending on their designs and processes. 

    Likely, the different behavior that you are observing is related to the very different common-mode voltage (Vcm) ranges of the OPA209 and the OPA171. Anytime you drive the input of an Op amp beyond its specified Vcm range there are the risks of highly degraded performance, a complete shutdown of operational amplifier function, or the latch condition already mentioned.

    The OPA171 has a specified Vcm, Common-mode voltage range of (V–) – 0.1 V to (V+) – 2 V. But the first page of the datasheet also indicates that "Input Range Operates To Positive Supply." The OPA209 Vcm range is specified  as (V–) + 1.5 to (V+) – 1.5 V, and there is no note in the datasheet indicating that it will continue to be functional once the Vcm is exceeded. The OPA171 appears to be more tolerant of the particular input over-voltage condition than the OPA209. TI makes no assurances about performance or operation under such conditions.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Hello Red,

    We haven't heard from you in a while, we hope your issue is resolved. I am going to close this thread now. Feel free to reply to to re-open if you have any questions moving forward.