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OPA4171: Comparator

Brian Wang0928
Genius 14840 points

Part Number: OPA4171
Other Parts Discussed in Thread: OPA172, OPA170, OPA171, OPA2990, TLV9062, LMV2011, OPA333, TLV171

My customer wanna to know if OPA can be used as Comparator? And Can Comparator be used as OPA? why? or Why not?

Thanks. 

Regards,

Brian

  • 0
    TI__Genius 15410 points
    Hi Brian ,

    The OPA171 is not recommended to be used as a comparator. The OPA171 (and also the OPA170 and OPA172) has internal back to back diodes, when a differential voltage is applied to the device, the internal diodes will conduct causing current to flow through the diodes. Since the diodes are only designed to protect the device and not to be repeatedly turned on or remain on the device is not recommended to be used as a comparator.

    I recommend the customer use the OPA2990 which does not have internal back to back diodes.

    Thank you,

    Tim Claycomb
  • 0
    Guru 140200 points
    Hi Brian,

    even if an OPAmp allows a high differential input voltage, it might nevertheless run into trouble and could show unexpected behaviour like huge overload recovery time or increased supply current. So, be very careful when using an OPAmp as comparator.

    An OPAmp is designed to have both inputs sitting at almost the same potential all the time and to have the output operating in the linear output voltage range. A comparator is designed for just the opposite situation.

    Kai
  • 0 in reply to kai klaas69
    TI__Genius 15410 points

    Hi Kai,

    Actually the opposite of that is true. A larger differential input voltage will decrease overload recovery time and increase slew rate. Please see Figure 43 in the TLV9062 datasheet.

    There is nothing wrong with using an op amp as a comparator as long as it does not have internal back to back diodes and it meets the timing requirements of your system. The transition time will be dependent on the overload recovery time of the op amp plus the time to slew from high to low (or low to high).

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    Guru 140200 points

    Hi Tim,

    yes, the TLV9062 shows a very nice behaviour. Unfortunately, many other OPAmps do not. Extreme recovery times can be seen in the datasheets of OPA333 and LMV2011, e.g..

    And regarding the increase of supply current, please see this thread:

    e2e.ti.com/.../799954

    Kai

  • 0 in reply to kai klaas69
    TI__Genius 15410 points

    Hi Kai,

    My response regarding Figure 43 in the TLV9062 datasheet was to point out that a higher differential voltage does not increase overload recovery time or decrease slew rate. A larger differential voltage decreases overload recovery time and increases slew rate.

    Every op amp will have an overload recovery time whether it is used as a comparator or in normal closed loop operation.

    Hi Brian,

    There is nothing wrong with using an op amp as a comparator as long as it does not have internal back to back diodes and it meets the timing requirements of your system. The transition time will be dependent on the overload recovery time of the op amp plus the time to slew from high to low (or low to high).

    And in your case I recommend using the OPA2990 not the TLV171 because the TLV171 has internal back to back diodes.

    Thank you,

    Tim Claycomb