# PSRR calculation for low voltage bandgap ref

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#### mike_bihan

##### Full Member level 3
psrr calculation

I have an interesting finding in calculating the PSRR of this simple bandgap reference.

I would like to define PSRR=Vgs_p1/Vpower

If we have an ideal opam and these is no gain from Vcc to poam's output, we will have the PSRR as 1/loopgain. When the loopgain is low, the PSRR is bad.

However, if the opam is not ideal. Its output probably offset some PSRR and increase the supply rejection ratio.

psrr db

Actually the PSRR you get here is very small. If you take the decibel value, it is negative. So, the more the gain of the opamp, more should be the PSRR because in your loop gain,which is Aβ, increase in A will decrease the PSRR which is negative becomes more negative.In fact, the PSRR is increasing here.

I hope that I am clear.............

bandgap psrr

Actually, the way you would define PSRR for this is

PSRR = deltaVref/deltaSupply.

As far as P1 is concerned, You actually want the gate of P1 to move along with
the source.,which is the supply. So you can actually put a big cap from the gate
of P1 to the supply if you need a good AC PSRR.

If you are interested in PSRR in a DC sense, then you need to increase the lengths of all the top PMOS transistors.,(better cascode them if you have enough headroom)

psrr formula

I have question about the same issue how to calculate ir using simulator like cadence?

thanks

• sami05

### sami05

Points: 2
site:edaboard.com bandgap psrr

AC simulation

low voltage bandgap

do ac analysis
the cap is add at gate to ground
let the gate not vary with power

• sami05

### sami05

Points: 2
calculating psrr of voltage reference

Hey Guys .... could anyone tell me the exact formula to calculate PSRR of the voltage reference in dB.

• sami05

### sami05

Points: 2
psrr decibel

arsh_josan said:
Hey Guys .... could anyone tell me the exact formula to calculate PSRR of the voltage reference in dB.

PSRR[dB] = 20*log(deltaVref/deltaSupply)

Ex.: deltaVref=1mV, deltaSupply=1V
PSRR[dB] = 20 * (-3)dB = -60dB

Cheers, erikl

psrr dc in db

do dc analysis
sweep the vdd
then use the formula which was mentioned by erikl

• Hardon

Points: 2