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OPA991: Design Help

Ray Vincent Aranzaso
Mastermind 18180 points
Part Number: OPA991

Hi,

Good Day. I have a customer who is working with OPA991. Please see below his query for your reference. Thank you very much.

Q1.  OPA991. We have an application where we'd like to use an OPA991, powered with 5VDC and GND in an unity gain configuration (G=1). The output is connected to a comparator thru a 10K resistor. Input voltage will be 0 to 20V, max. input current limited to 90 uA. Vin is much higher than the positive rail. I'd like confirmation that the output will be clamped to the 5V rail when the input voltage exceeds that rail, without phase reversal at max. 20VDC.

The assumption is that the output will follow the input for Vin 0-5V and be 5V for 5V<Vin<20V.

Q2. How soon will the output of OPA991 come out of saturation from the 5V rail when the input voltage drops to zero? The data sheet freq. response indicates 1us to 0.1% but this is a different situation with only 5VDC supply and heavy overdrive on input vs. the supply.

Best Regards,

Ray Vincent

  • 0
    Guru 140200 points

    Hi Ray,

    why not using a 4:1 voltage divider at the input of OPA991? And if the comparator circuit is earlier powered than the OPA991 even a diode clamp in front of the voltage divider would help to not only prevent damage but also decrease the risk of lock-up or latch-up. This would also prevent saturation delays.

    This thread might interest you:

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1097514/tlv9002-can-we-connect-ct-to-tlv9002

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 27590 points

    Hi Ray,

    Ray Vincent Aranzaso said:

    Q1.  OPA991. We have an application where we'd like to use an OPA991, powered with 5VDC and GND in an unity gain configuration (G=1). The output is connected to a comparator thru a 10K resistor. Input voltage will be 0 to 20V, max. input current limited to 90 uA. Vin is much higher than the positive rail. I'd like confirmation that the output will be clamped to the 5V rail when the input voltage exceeds that rail, without phase reversal at max. 20VDC.

    The assumption is that the output will follow the input for Vin 0-5V and be 5V for 5V<Vin<20V.

    Kai is correct. Having an input voltage far higher than the supply voltage is dangerous to the part, and violates the datasheet. I recommend implementing the 4:1 voltage divider at the input to avoid damaging the op-amp. 

    Ray Vincent Aranzaso said:
    Q2. How soon will the output of OPA991 come out of saturation from the 5V rail when the input voltage drops to zero? The data sheet freq. response indicates 1us to 0.1% but this is a different situation with only 5VDC supply and heavy overdrive on input vs. the sup

    I think the spec you want to look at is the overload recovery time, which is 400ns for the OPA991 under the specifications specified in the datasheet on page 12. However, the overload recovery time will be different depending on the output load the op-amp sees. 

    Best Regards,

    Robert Clifton