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How to improve CMRR(common mode rejection ratio)?

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yellowtooth

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common mode rejection ratio

In OPA(operational Amplifier) circuit design, the input is differential, the output is single ended.

How to improve CMRR(common mode rejection ratio)?


Thank U!
 

how to improve cmrr

For improving the CMRR of the differential Amplifier using opamp, u need to match all the ressistors in the differential Amplifier to around 0.1%, becauuse 10% mismatch in resistors causes 1/5th of common mode voltage to be appeared as differential voltage, which will be amplified and appears as output noise, or go for instrumentation amplifier.
 

common mode rejection ratio cmrr

Also it is necessary to increase output resistance of current source for differential cascade.
 

improving cmrr

animeshjn is right in the case of whole case testing stage. All process mismatches will harm the CMRR, including resistor mismatch, transistor size mismatch, threshold voltage mismatch.

On the other hand, in the case of schematic simulation, process mismatches are not considered at this stage yet. CMRR is depending on your OpAmp design. For example, doing the Opamp by a differential pair with current tail, make sure the current tail (current mirror) has a very high output impedance. In the common mode, the current tail's high output impedance degenerates the common-mode gain, which results in a high CMRR.
 
how to increase cmrr

Can U tell me in detail by the figure?

Thank U.
 

improve cmrr

animeshjn said:
For improving the CMRR of the differential Amplifier using opamp, u need to match all the ressistors in the differential Amplifier to around 0.1%, becauuse 10% mismatch in resistors causes 1/5th of common mode voltage to be appeared as differential voltage, which will be amplified and appears as output noise, or go for instrumentation amplifier.
Click to expand...

Do U mean R1=R3 and R2=R4?

Added after 7 minutes:

gevy said:
Also it is necessary to increase output resistance of current source for differential cascade.
Click to expand...

How to increase ?

The output resistance indicate which one , R1,R2,R3,R4 or some other?

Added after 6 minutes:

jlee said:
animeshjn is right in the case of whole case testing stage. All process mismatches will harm the CMRR, including resistor mismatch, transistor size mismatch, threshold voltage mismatch.

On the other hand, in the case of schematic simulation, process mismatches are not considered at this stage yet. CMRR is depending on your OpAmp design. For example, doing the Opamp by a differential pair with current tail, make sure the current tail (current mirror) has a very high output impedance. In the common mode, the current tail's high output impedance degenerates the common-mode gain, which results in a high CMRR.
Click to expand...

What is "current tail"?

What the "output impedance" indicate? inside OPA ??

Added after 14 minutes:

Another figure.

A little different.

One conected to GND, another conected to VCC/2.

Any different result ?
 

cmrr improve

In addition to the answears, you can test your circuit and improves by Vin1 = Vin2 to a sinusoidal wave with great amplitud, ideally Vout = 0, so you can get a test if it get better or worst.
 

site:www.edaboard.com improve cmrr

Thank U.

But I want to know what to be changed to improve CMMR.
 

common mode rejection test

Hi yellowtooth,

CMMR will be more for three stage opamp than single opamp differential opamp.

Go through the following for more information.
**broken link removed**

In the attached figure V4= v2- [(v2 -v1)* R/R gain] should not become negative for opamp biased with the single supply for faithful amplification.
 

improving common mode rejection ratio

Thanks.
 

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