Understanding Common Mode Rejection Ratio (CMRR) in Op-Amps

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Discussion Overview

The discussion revolves around the Common Mode Rejection Ratio (CMRR) in operational amplifiers (op-amps), focusing on input signal types (AC vs. DC), interpretation of datasheet specifications, and the implications for circuit design. Participants explore both theoretical and practical aspects of using op-amps in various applications.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • Some participants suggest that op-amps can amplify both AC and DC signals, depending on the application.
  • There is a query about whether a constant output DC current can be achieved with an op-amp.
  • One participant seeks clarification on the input voltage specifications for the op-amp pins from a datasheet, indicating urgency for a project.
  • Another participant points out the importance of the "common mode voltage range" in the datasheet for understanding input voltage limits.
  • There is confusion regarding the relationship between circuit design and datasheet interpretation, with one participant expressing uncertainty about the relevance of the two topics.
  • A later reply emphasizes the ideal function of an op-amp to amplify only the difference between the inputs, highlighting the significance of a high CMRR for optimal performance.
  • Participants discuss specific voltage ranges for inputs to achieve the best performance from the op-amp.

Areas of Agreement / Disagreement

Participants generally agree that op-amps can handle both AC and DC inputs, but there is no consensus on the specifics of input voltage requirements and the interpretation of datasheet information. The discussion remains unresolved regarding the clarity of the datasheet specifications and their application in circuit design.

Contextual Notes

Some participants express uncertainty about the definitions and implications of terms used in the datasheet, particularly regarding voltage levels and common mode rejection. The discussion reflects a mix of theoretical understanding and practical application challenges.

Who May Find This Useful

This discussion may be useful for students and practitioners in electronics and electrical engineering who are working with op-amps and need to understand their specifications and applications in circuit design.

lkari
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So if we have a Opamp component, do we have to give the input as sine wave or as a dc supply? I think it has to be at a DC voltage. please correct me.
Thanks in Advance
Lakshmi
 
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Most opamps will amplify a sinusoidal or dc signal.
 


So I can get a constant OUTPUT DC current with this opamp rt?
 
Reading a datasheet

Hi I need to understand few things about the Datasheet. I want to know what the input voltages for the pins vIN+ and VIN-could be. Please find the enclosed datasheet for reference. I need it very urgently for a project.
Thank you in advance
Kari
 

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Look at the item marked "common mode voltage range" in the datasheet.
 


I reffered to it but, what do they mean by saying the following there? (V+) –3
 
(I merged the two related threads into one -- both were about the opamp datasheet)
 
lkari said:
So if we have a Opamp component, do we have to give the input as sine wave or as a dc supply? I think it has to be at a DC voltage. please correct me.
Thanks in Advance
Lakshmi

If you mean V+ or V-, those should be DC voltage supplies.

If you mean V+IN and V-IN, it could be either AC or DC ... depends on the application.

(Refering to figure on p. 1 of the datasheet:
https://www.physicsforums.com/attachment.php?attachmentid=17873&d=1236355902 )
 
i really don't understand why they are on the same subject. one is about the circuit the other is about reading the data sheet for the component.

<< post edited by berkeman to clean up txt speak >>
 
Last edited by a moderator:
  • #10


lkari said:
I reffered to it but, what do they mean by saying the following there? (V+) –3

ideally, you want an op-amp to amplify only the difference between Vin+ and Vin-, not to amplify the average of Vin+ and Vin-. the higher the common mode rejection ratio, the closer you are to this ideal. to get the best rated performance for this op-amp, keep the inputs between (V-) - 0.1, and (V+) -3V.
 

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