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Original question:

OPA2180: OPA2180 used as a Comparator

OPA2180-Q1: Output Voltage Constraint of Op Amp Follower

Peiheng Liu
Prodigy 620 points
Part Number: OPA2180-Q1
Other Parts Discussed in Thread: OPA2197-Q1, OPA2192-Q1

Hi,

I am testing OPA2180-Q1 in the simplest follower configuration without any external current limit resistor in the input path or the feedback path. Vout deviates from the input voltage when +IN pin > 4.1V, measured Vout = 4.1V at +IN = 5V.  OPA2180-Q1 was powered single-ended, positive supply V+ = 5V and V- tied to GND in my case. IN+ pin is fed by a resistor divider.

I am going through e2e and found the previous discussion about back-to-back protection diode. It looks like the diode from V+ to V- would be forward biased when the voltage on pin +IN is close to Vs, drawing excessive current from the divider. Would it be the cause of my issue? 

Thanks for your help!

  • 0
    TI__Guru* 93105 points
    Hello Peiheng,

    The OPA2180-Q1 has a common-mode input voltage range VCM, specified from V– to (V+) – 1.5 V. If you are using a single +5 v supply, and drive the non-inverting input with a voltage greater than about +3.5 V, the input stage is not necessarily operating in its linear operating range and the electrical behavior becomes degraded and won't produce expected output.

    The OPA2192-Q1 and OPA2197-Q1 are our low-offset, precision amplifiers using our e-trim™ technology. They are specified with an input VCM range that extends 100 mV beyond each supply rail. You should be able to use a +5 V input without the input leaving the linear input range. However, do have a look at the datasheet to make sure the output can swing as close to the supply rail as you are planning.

    You can find the OPA2192-Q1 datasheet here:

    www.ti.com/.../opa192-q1.pdf

    Regards, Thomas
    Precision Amplifiers Applications Engineering