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OPA171 output

Darren Yang
Expert 2000 points
Other Parts Discussed in Thread: OPA171

we using OPA171 for our design，I have question as follow，if the PIN3 Voltage is equal to the PIN4 Voltage，however I want to konw the output is high or low？the Schematic and waveform as follows；but in the Datasheet I can not find the Description about it，can you help give data to explain output is high or low？tks！

  • 0
    TI__Guru 62765 points

    Hello Darren,

    Thank you for sharing your schematic and results when using the OPA171.  Since you do not have a feedback path from the output of the amplifier back to the input you are using the op amp as a comparator. When used as a comparator the output will go to positive supply rail if the IN+ pin is greater than the IN- pin and will go to the negative rail if the IN- pin is greater than the IN+ pin.  When the voltages applied to the IN+ and IN- pins are equal then the output polarity will be defined based on the polarity of the input offset voltage.

    We have two things to point out about your circuit.  First, the OPA171 is not a rail-to-rail input product and the input common-mode range extends from (V- - 0.1V) to (V+ - 2).  Therefore with a +5V single-supply the input common-mode range is -0.1V to 3V.  Your circuit configures the device with the IN+ pin at +5V which is outside of the usable common-mode range of this device.

    Second, the OPA171 features back-to-back diodes between the inputs.  These diodes will conduct current when the inputs are pulled more than ~0.7V apart which may impact your application.