Confused about PSRR formula

psrr formula

from gray's "analysis and design of analog integrated circuit" pg 422 and razavi's "design of analog cmos integrated circuit" pg 335, psrr is defined as differential gain divided by gain from supply to output. however, from allen's "CMOS Analog Circuit Design" pg 314, the psrr is defined the other way as gain from supply to output divded by differential gain. can anyone explain y this is so?

psrr analysis

Hi.
I have not read Allen's book. However by saying PSRR we mean Power Supply "Rejection" Ratio. So it is about REJETION, which means the more rejection ratio, the more PSRR and this means, the less gain from supply to output, the more rejection occurs. So on the basis of linguistics, PSRR is what Razavi and Gray have mentioned. However, it is somehow impossible that dr. Allen has made such a big mistake, so read that book again. Maybe you've got it wrong.

Regards,
EZT

how to analyse psrr

Both definitions give the identical absolute value of parameter in dB, but they have various sign. For example, PSRR equals +70 dB or -70 dB for different definitions.

psrr equation

my idea is that PSRR equal to output's variation (caused by power variation)

divided by power variation.

for example, if power variation is 1V, output variation is 1mV,

then we can say PSRR is 60db.

I don't know if my opinion is ok.

best regards

kiat said:

from gray's "analysis and design of analog integrated circuit" pg 422 and razavi's "design of analog cmos integrated circuit" pg 335, psrr is defined as differential gain divided by gain from supply to output. however, from allen's "CMOS Analog Circuit Design" pg 314, the psrr is defined the other way as gain from supply to output divded by differential gain. can anyone explain y this is so?

Click to expand...

analyze psrr tutorial

hi kiat,
allen book has given right formula.actually you misunderstood it.

PSRR=Av/Add

Vout={Add/(1+Av)}Vdd
≈(Add/Av) Vdd
=(1/PSRR) Vdd

hence 1/PSRR=Vout/Vdd

with this formula we get negative PSRR.by just changing the polarity we get the actual PSRR.
i hope you got it.

Added after 15 minutes:

one other way i can make u understand is in following way.


allen has used unity gain opamp to derive that equation.

as we know PSRR=[ΔVout/ΔVin] / [ΔVout/ΔVdd]

in unity gain opamp ΔVout/ΔVin = 1

hence PSRR = 1 /ΔVout/ΔVdd
i.e. PSRR = ΔVdd/ΔVout
i.e 1/PSRR = ΔVout/ΔVdd

so if simulate unity gain opamp by providing frquency analysis at Vdd.we get the change at ouput . if we draw the frequency responce of output ,we get the negative result in dB (because of 1/PSRR).we make it positive and we get the actual PSRR.

similar analysis can be made for Vss change.
hope you get it[/u][/url]

Added after 6 minutes:

if i have made you clear about the cofusion on PSRR ,just click on helped me button below.it will not cost you anything.it will just provide me some points from the forum if i helped you.