The PSRR and CMRR definitions

[chang830]

I found the psrr in some books is defined as the ratio between the gain from the input to the output and the gain from the supply to the output. In other books and some product datasheets,it is defined it as the ratio of the change of output and the change of the power supply.

Same questions applied to the CMRR definition. I wonder what's the descranpancy between these two defintions? Or in what conditions, I should use whaich one of them?

Thanks

 

[tam81]

definition psrr

PSRR is a concept to measure the rejection of supply voltage to the output which is apply to the whole frequency range
CMRR is a concept to measure the rejection of input signal to the output which is usually a dc concept, I think

 

[melc]

define power supply rejection ratio

Beside the point of math and definitions:

PSRR is telling you how much ripple may have the power supply and how much from this ripple will be (in)visible at the OA output.

CMRR is telling you how much from a rectangular signal applied simultaneous to both in- and in+ (in your favourite AO configuration) will be visible at the OA output. Should be 0 but is not.

 

[chang830]

definition of psrr

Beside the point of math and definitions:

PSRR is telling you how much ripple may have the power supply and how much from this ripple will be (in)visible at the OA output.

CMRR is telling you how much from a rectangular signal applied simultaneous to both in- and in+ (in your favourite AO configuration) will be visible at the OA output. Should be 0 but is not.

hello melc and tam81,
Thanks for the reply. I do understand what is the PSRR and CMRR and the descranpancy between them.

What I want to know, the PSRR have two diferent definitions in applications. I wonder which one should I select in differnt applications?

Regards

 

[sigui777]

define power supply rejection?

By definition Power Supply Rejection Ratio is the ability of an amplifier to maintain its output voltage as its power-supply voltage is varied. I understand that psrr is the "the ratio of the change of output and the change of the power supply" like you said. However in voltage regulators (LDOs) It will be define as the ratio of the output to the input since the input in this case is the raw power supply you are trying to regulate.