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OPA2354: Slew rate for gain=11 / rule of thumb for slew rate vs gain

Ue
Genius 16535 points
Part Number: OPA2354
Other Parts Discussed in Thread: TINA-TI

Hi all

My customer is evaluating the OPA2354 for a low side current sensing function that should be fast.

They have this question:

What is the expected slew rate for a gain=11 ?

In the datasheet we specific the slew rate only for a gain=1 (110V ... 150V/us).

And is there a "rule of thumb" as to how the slew rate would scale with the gain of the OpAmp ?

Best regards

Ueli

  • 0
    Guru 140200 points

    Hi Ueli,

    you may find this thread interesting

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/290261/slew-rate-vs-gain

    And why not running a simulation with the

    OPA2354 TINA-TI Reference Design

    to see the fine details of slewing?

    Kai

  • 0
    TI__Genius 15661 points

    Hello Ueli,

    The slew rate (SR) of an op-amp is not wholly dependent or related to the set gain.  An op-amp's ability to slew at 150V/us, for example, does not depend on the gain, but rather the voltage step size applied (the large signal voltage) and supply voltage.  This information is present in the Electrical Characteristics Section 7.7.

    Could you elaborate on your customer's design/application and why there may be an anticipated need for a certain slew rate at a certain gain (Av = 11 V/V)?

    If I can understand the need or curiosity of the customer, I can better assist in advising you (and your customer) on this matter.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Genius 16535 points

    Hello Alec

    Thank you for these comments, I will follow up by email.

    Hello Kai

    Thank you for the quick reply and for pointing to the forum post covering this topic.

    I would be reluctant to rely on a simulation here, be it TINA-TI / PSpice or other as I would first have to analyze the model to check if slew rate is modelled correctly.

    Normally frequency domain simulations are less critical but when you are looking at a time domain simulation (as would be the case here) you might find that models are not optimized for a detailed analysis.

    Best regards

    Ueli

  • +1 in reply to Ue
    Guru 48395 points

    hey Ueli, 

    The amplifier will try to follow the ideal small signal response shape until it hits a nonlinear limit on dV/dT. Running non-inverting, there is a possibility that the input stage will be the limit at low gains since you are putting say the full desired output step on the input stage at gain of 1. The effect goes away at higher gains or running inverting. 

    It is somewhat confusing, but I did try to explore  these issues in a 2 part article, part 1 is here, 

    https://www.edn.com/what-is-op-amp-slew-rate-in-a-slew-enhanced-world-part-1/

  • +1 in reply to Ue
    TI__Genius 15661 points

    Hello Ueli,

    I have read your email; please let us know any further assistance or explanation you require on the forum/email.  Michael & Kai have shared resources and I can work to provide additional information if you request it.  

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Genius 16535 points

    Hello Michael

    Thank you for the clarification and for pointing to the article.

    -------------------------

    Hello Alec

    Thank you the support, this fully answers the customers question.

    Best regards

    Ueli