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Rail to Rail output for OPA2330

Jonas Ketterle
Prodigy 40 points
Other Parts Discussed in Thread: OPA2330

I would like to use the OPA2330 op-amp in a low side current sensing circuit I am designing. Having 0V output when there is no current across the sense resistor is particularly important to us. In the diagram below, the opamp is running on 3.3V, and 12V_GND is the current return path to our battery.

The spec says the output will be within 30mV typ, 100mV max of the rails. In my testing so far, I've observed 0V-3.28V output on a 3.30V supply (fully within spec). However, when I have no current on the input, I'm not observing that the output voltage is ever anything near the 30mV value in the datasheet (rather, it is 0V). This is just as we'd desire, but I need to ensure that in manufacturing all the op-amps do in fact behave similarly.

The opamp output feeds directly into an ADC unit with 10MOhm impedance, so I'm not pulling much current. Under what conditions should I expect to see a nonzero voltage on the op amp output when the input is 0V? Thanks for your help.

  • 0
    Guru 21975 points

    Jonas,

    Our op amps are generally specified with a load connected to mid-supply. This forces the op amp to sink current when attempting to swing as close as possible to ground. This is a robust test for sourcing and sinking current but may not be the way your circuit operates. If the load current approaches zero when the op amp output approaches zero, it will reliably swing within a few millivolts of ground.

    You did not attach a circuit diagram. If you want further validation of your circuit please attach. Don't forget that the feedback network may require some output current from the op amp so feedback circuit configuration and load both can matter.

    Regards, Bruce

  • 0 in reply to Bruce Trump
    Prodigy 40 points

    Hi Bruce,

    Thanks for your answer. My circuit diagram is attached - seems like it didn't upload correctly the first time.

    Best, Jonas

     

  • 0 in reply to Jonas Ketterle
    Guru 21975 points

    Hi Jonas,

    You have not shown any additional load to the op amp. This is where you could encounter issues. If the load requires sinking any substantial current it will degrade the output swing.

    It does not appear that the feedback network requires any sinking of output current. I assume that R65 is connected to the same ground as the V- terminal of the op amp. This means that when the output of the op amp attempts to swing close to ground, it does not need to sink any current. Any positive-going output voltage requires current to be sourced by the op amp, flowing through R64 and R65. This is what limits your swing to the positive rail.

    R28 is not required to achieve your desired transfer function. You've probably included it to isolate the op amp from possible noisy pulses at the sense point--a good practice.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 40 points

    Thanks Bruce, that fully answers my question and increases my understanding of designing with op-amps. Looking forward to working with TI in the future.

  • 0 in reply to Jonas Ketterle
    Guru 21975 points

    Jonas and all,

    One last comment for clarification. I don't want to leave an impression that all op amps will swing very close to the rail if they are not sinking any output current. This is a characteristic of most CMOS op amps.

    Op amps fabricated on a bipolar process (BJTs--Bipolar Junction Transistors) are unlikely to achieve output swings a few millivolts from the rail even when they are not sinking output current. They will generally show improvement over their output swing performance when they are not sourcing or sinking current but not as good as a CMOS op amp. The best that a rail-to-rail-out BJT op amp is likely to do is approximately 40mV to the rails without load current.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 40 points

    That makes a lot of sense. Thanks for the clarification.

    Best, Jonas