INA128: PSRR Equation

Part Number: INA128

Tool/software:

for op amps, I keep finding PSRR has two different formulas. I'm having a hard time understanding why the equation is sometimes inverted.

PSRR = 10 log (ΔV_supply²)/ ΔV_io²)

and

PSRR=-20 log (ΔV_io)/ ΔV_supply)

0 Gerasimos Madalvanos 5 months ago

Hello mp,

Could you please provide where you are finding the first equation for PSRR? The formula I am most familiar with is the bottom formula of the two.

Best,
Gerasimos

0 mp 5 months ago in reply to Gerasimos Madalvanos

+1 Gerasimos Madalvanos 5 months ago in reply to mp

Hey mp,

Thank you for providing the context of the equation!

Looking at the equation from the linked article,

PSRR = 10 log (ΔV_supply²A_v²)/ ΔV_out²)

First, this has mismatched parenthesis, so I will be assuming they meant PSRR = 10 log ((ΔV_supply²A_v²)/ ΔV_out²)

This inner term can be simplified to ((ΔV_supply* A_v)/ ΔV_out)²

This power can be brought out of the log using the power rule of logarithms, and the formula is now PSRR = 20log((ΔV_supply* A_v)/ ΔV_out)

ΔV_in*A_v = ΔVout, substituting this in to the above equation gives

PSRR = 20log((ΔV_supply* A_v)/ (ΔV_in* A_v)), the A_v term now cancels and you are left with the familiar form:

PSRR = 20log(ΔV_supply/ΔV_in)

Long story short, this is just a different way to write the same formula in terms of different values.

Best,
Gerasimos

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