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OPA140: Explain the content of the technical documentation

Part Number: OPA140

Hi team,

One of our customers saw the following document in the OPA140 datasheet

FEEDBACK PLOTS DEFINE OP AMP AC PERFORMANCEhttps://www.ti.com.cn/cn/lit/an/sboa015/sboa015.pdf

During the study of the SBOA015 document, this customer has the yellow portion of the screenshot below that he does not understand what it means to say. I hope someone can give some explanation. Any comments and explanations would be much appreciated.

Best Regards,

Amy Luo

  • Well Amy, this is not exactly true. The idea here is the overall gain equation has  a resistive element and a LG element - if the NG is rolling off with the Aol curve than that Loop Gain term is "constant" and (if the Gainbandwidth product were identical part to part) you could adjust the R' to account for the LG error - however, that adjustment would have to be for each individual device as the Gainbanwidth product varies part to part some

  • Thank you ,after your answer ,i konw paraIlel a resistive can  improve accuracy ,but  can not batch use .even so , I want to know "a span of 100,000:1",refer to what?

  • Hi Amy,

    Thanks Michael for the reply!

    Here is the math to explain the paragraph. 

    The Acl has two parts. 1/β the closed loop gain when loop-gain is infinitive large. (1 - 1/T) is a part of closed-loop gain error term, where the error will be the minimum if Aol*β is large. 

    For OPA111, Aol ≅ 125dB or 10^(125dB/20) = 1.78e+6. If Acl = 1/β=10, then Aol*β =1.78e+5 or 105dB. 100,000:1 refers to the loop-gain, T, where T = Aol - Acl in dB. If the integrator circuit is able to maintain T> 80dB, then it will increase the dynamic range, and keep the error at <0.01% level, where 1/T = 1/80dB = 0.01%.

    If T=100dB as in OPA111 (properly choose op amp for the integrator with high accuracy integrator application), then the closed-gain error will be at a level 1/T = 1/100dB = 0.001% from the ideal gain equation. 

    So the highlighted paragraph is saying that the application when selecting integrator op amp, designer should pick higher Aol gan op amp and configure high closed-loop gain (lower β) in order to increase the dynamic range and maintain high accuracy for the application.

    If you have additional questions, please let us know. 

    Best,

    Raymond 

  • Hi Amy,

    I am going to close this inquiry. If you have additional questions, please let us know. 

    Best,

    Raymond