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OPA2237: using OPA2237 as a comparator (without feedback resistor) whose non-inverting input becomes abruptly negative

Nitish Agrawal
Expert 1595 points
Part Number: OPA2237
Other Parts Discussed in Thread: LM393

Hello,

We are planning to use OPA2237 as a comparator where the input signal is not slow moving, but changes abruptly at the non-inverting input to a negative value.  I am posting this question in the Amplifier forum as I was advised to do so by the Comparator engineers.  The package we are using is 8-pin SOIC where only one opamp is being used.

OPA2237 is intended to replace LM393 in our circuit that is already in production.  The LM393 was getting damaged, due to the negative input voltage that it saw at its non-inverting input.

We decided to use OPA2237 as we needed a device whose abs. max input voltage rating was around -0.8V (where OPA2237 guarantees -0.7V).  Also, we needed a power supply value > = 28V.  OPA2237 was the only one to satisfy the requirement.

Attached is the application circuit.  We wanted to make sure it won't get damaged due to the negative input voltage.  Is there a clamping diode inside it that will clamp the non-inverting input to a safe level?

We also wanted to make sure that opamp won't oscillate when used as a comparator.  If there are any suggestions you have, they would be greatly appreciated.

Also, if there is anything else we need to be careful of, when using it as a comparator, please advise.  Would greatly appreciate your response.

Kind regards,

Nitish AgrawalOPA2237_as_comparator_TIE2E.pdf

  • 0
    Guru 140200 points

    Hi Nitish,

    why not changing the circuit in a way that no longer a negative voltage can reach the input?

    Kai

  • 0 in reply to kai klaas69
    Expert 1595 points

    Hi Kai,

    The opamp output doesn't need to make sense when the input is negative....our only concern is that it must not get damaged.

    At present, we don't have the liberty to change the circuit, as we are already in production.  Given this information, do you think it will get damaged?

    Also, if there is anything we can do to prevent potential oscillations, please let us know.

    Kind regards,

    Nitish

  • 0 in reply to Nitish Agrawal
    TI__Guru 67971 points

    Nitish,

    Because of the large input resistors, which limit the input current to 100uA, the circuit should not get damaged - see below.

    As far as your question of potential oscillation goes, please elaborate what exactly you mean since the output in comparator application outside the transition (slewing) time is slammed against one of its rails - see above.

  • 0 in reply to Marek Lis
    Expert 1595 points

    Hello Marek,

    Thank you for your response with simulations confirming the device won't get damaged.

    Regarding my question about potential oscillation, we wanted to ensure that when not transitioning from one level to the other, the output will be stable at either of the rails, since we are using this opamp without an external feedback resistor.

    For instance, in the first simulation, when VG1 = 12V, Vinn = 9.68V, Vinp = 9.61V (70mV difference between the inputs), the output will be stable at 11.33V in practice (as opposed to being unstable).

    Kind regards,

    Nitish

  • 0 in reply to Nitish Agrawal
    TI__Guru 67971 points

    When driven in an open-loop configuration, all op amps will be pushed against one of its rails.  The only time I may think of a potential problem is when the input voltage violates the specified input common-mode voltage range and the op amps suffers from the phase-inversion issue.  This will not occur here since OPA2237 Vcm input voltage, (V-)-0.2V to (V+)-1.5V, is never violated.