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OPA209: OPA209 have around 0.5V offset at no load

Part Number: OPA209
Other Parts Discussed in Thread: OPA365

Hi team,

My costomer use OPA209 for output current sensing and Rsense is 0.25 ohm, please refer to below circuit.

But the OPA209 have around 0.5V offset at no load, after change OPA365 this issue solved.(verify on 20 samples)

I would like to know the root cause or which OPA parameter cause this issue.


  • Hi Ryan,

    the OPA209 is no input output rail-to-rail OPAmp. The input voltage must stay 1.5V away from the supply rails. See the "common mode voltage range" specification in the datasheet of OPA209. So, you violate the common mode input voltage range of OPA209. Also, the output of OPA209 cannot full go to the supply rails.

    The OPA365, on the other hand, is an input output rail-to-rail OPAmp and can better do what you want. But even this OPAmp cannot fully reach the supply rails at its output. See the "voltage output swing from rail" specification in the datasheet. Is this a problem for you?

  • Does it violate the common mode input voltage range of OPA209 cause around 0.5V offset at no load?
  • Hi Ryan,

    there are two mistakes in the circuit with the OPA209: 1. Violation of common mode input voltage range (see page 6 of datasheet) and 2. The output of OPA209 cannot fully go down to 0V (see page 7 of datasheet).

  • Hi Ryan,

    And in addition to to the OPA209 Vcm and output voltage concerns that Kai mentions the OPA209 minimum operating supply voltage is +4.5 V. The schematic you have provided indicates 3V3_SEC which surely means a +3.3 V supply in being used.

    If the input Vcm range is violated the op amp is forced out of its linear operating mode and operating region. When that occurs the op amp output can move to an undefined voltage level. An op amp with few exceptions is expected to be operated in its linear input and output ranges, and within its specified power supply range.

    The OPA365 is a specified as a rail-to-rail input/output voltage RRI/O, low-voltage single-supply op amp. It is much more suited for this kind of application. The OPA209 is high-performance, high-voltage op amp designed for a very different set of operating conditions.

    Regards, Thomas
    Precision Amplifiers Applications Engineering