• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 47 subscribers
  • Views 938 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

OPA521: Unity gain?

Taketo Sato
Genius 10075 points
Part Number: OPA521
Other Parts Discussed in Thread: OPA564

Hello team,

Can OPA521 work with unity gain if phase compensation is properly done externally and if GAIN_SET pin and -IN pin are shorted? If it is feasible, what is common mode voltage range?

Best regards,

  • 0
    TI__Guru* 93105 points
    Sato-san,

    The OPA521 is a de-compensated power op amp design optimized for closed-loop applications where the the gain is -7 V/V, or higher. The de-compensation maximizes the amplifier's bandwidth which is ideal for PLC applications. The design does not lend itself to being used for gains less than -7 V/V.

    The unity-gain stable OPA564 is the closest product to the OPA521. Its peak current capability is a bit less. However, if it meets the application requirements it is the obvious choice.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    TI__Genius 10075 points

    Hello Thomas-san,

    Thank you for the comments. OPA564 current capability is actually ok but its package is too big.
    I look for 1A output capability, +/-5V or more supply, same or smaller pcakage than HTSSOP-20 and unity gain solution.

    If OPA521 can be used with unity-gain with some external compensation, and if it has wider common mode input range, it meets the requirement so I wanted to confirm it.

    Best regards,

  • 0 in reply to Taketo Sato
    TI__Guru* 93105 points

    Sato-san,

    I need to check with the OPA521 designer regarding your questions. What are the characteristics of the input signal that would be applied to the non-inverting input (minimum negative and maximum positive voltage levels)? What are the characteristics of the output load i.e. resistive, capacitve, inductive, or complex?

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Taketo Sato
    TI__Guru* 93105 points

    Hello Sato-san,

    I have given further consideration to your question regarding the possibility of operating the OPA521 in unity gain. I spoke with the design engineer and he has concerns about attempting to this because that was not a consideration when the product was being designed. It is optimized for the fixed closed-loop gain of -7 V/V, and the datasheet specifications were developed around that particular gain.

    Nonetheless, we have been testing the OPA521 simulation model using different compensation methods. We have a compensation proposal that shows some promise. Shown in the TINA diagram below the OPA521 is set to a noise gain of 10 V/V by Rs and Cs. Doing so resulted in a closed-loop gain of 1 V/V at DC and low frequencies, but with considerable gain peaking at the higher frequencies. Adding the feedback capacitor Cf eliminated most of the peaking and brought the gain very close to +1 V/V across the entire bandwidth. You can see the results from simulations seen below.

    This compensation scheme hasn't been tested on the bench. Since TI isn't specifying the OPA521 for unity gain operation in the datasheet, that isn't likely something that we would undertake. TI wouldn't be able to assure that the OPA521 is able to maintain stability under all conditions.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Genius 10075 points

    Hello Thomas-san,

    Thank you for the comments.

    Best regards,