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OPA354: ( Non ) Inverting attenuator is stable?

Ras Sharif50
Genius 3455 points
Part Number: OPA354
Other Parts Discussed in Thread: OPA836,

Hi All,

I see in the OPA836 datasheet page 32, the attenuation property.

Does have OPA354 attenuation property for stable working? Please consider inverting and non-inverting circuit.

Must to care special design procedure to stabilize the attenuator?

Regards, Ras

  • 0
    TI__Genius 15555 points

    Hello Ras,

    I could not find the OPA386 part that you were referencing. The OPA354 can be used as an attenuator since it is unity-gain stable, and the noise gain of the part will always be 1+(RF/RG) whether the part is being used as inverting or non-inverting configuration. I've attached a TINA circuit below that you can use to check the phase margin.

    Ras.TSC

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Guru 48395 points

    Hey Hasan, 

    The is a typical test circuit that misses an important issue. Your sim shows 60deg phase margin, but if you do this correctly (where the device output impedance see the feedback path and partcularly the input C added externally here, it goes to 42deg. Still stable, but quite different. 

    Here is the file, 

    Inverting attenuator LG sim OPA354.TSC

  • 0 in reply to Michael Steffes
    TI__Genius 15555 points

    Hello Micheal,

    Thanks for the input, you are right that my sim file doesn't include the effects of the output impedance in the response. You seemed to have used 4pF for the input capacitance when it is only 2pF, however. Updating your file to reflect the 2pF cap, phase margin seems to be around 54 degrees. 

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Genius 3455 points

    Hello Hasan and Michael,

    Thank you for the reply.

    The OPA836 is correct. I edited the first post.

    Please consider the section in the OPA386 datasheet I cut and paste here:

    "9.1.4 Attenuators
    The noninverting circuit of Figure 53 has minimum gain of 1. To implement attenuation, a resistor divider can bepla ced in series with the positive input, and the amplifier set for gain of 1 by shorting VOUT to VIN– and removing RG. Because the operational amplifier input is high impedance, the resistor divider sets the attenuation. The inverting circuit of Figure 54 can be used as an attenuator by making RG larger than RF. The attenuation is the resistor ratio. For example, a 10:1 attenuator can be implemented with RF = 1 kΩ and RG = 10 kΩ."

    My question:

    Considering both of inverting and non-inverting attenuators proposed in the OPA836 datasheet ( also i the sloa163.pdf ) do the circuits work properly using OPA354 in any configuration? Or some stable condition should to be cared?

    Regards, Ras

  • 0 in reply to Hasan Babiker31
    Guru 48395 points

    For loop gain purposes you add the Ccm+Cdm

  • 0 in reply to Ras Sharif50
    Guru 48395 points

    Any VFA can be used in any inverting configuration - if you test and adjust for low phase margin should it occur - the details of your implementation matter a lot - higher R's in that kind of circuit can produce another loop pole - nothing to do with op amp stability itself, just LG analysis - this comes up so often, I recently published this article covering this. 

     Planet Analog broke the links to expand the figures to readable size, here is the pdf that you can expand to read if desired, 

    Designing Inverting Attenuators Aug2_2019.pdf