# How to measure CMRR for differential amplifier?

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#### surianova

h iall,

For differential amplifier, CMRR should be measure single ended or differentially?
(i shorted both input with ac=1). When i measure differetially, i get 300dB, when i measure single ended, get 90dB.

#### richloo

##### Full Member level 1
what do you mean by cmrr of an amplifier?

where:
Acm=common mode gain

With both input shorted together, it means that u are measuring the Acm.

#### electronics_kumar

calculate cmrr

dear surianova can we conclude for mentioning ideal opamp we refering Acm

J

#### jiangwp

##### Guest
cmrr half circuit full circuit

With aspect to the definition of CMRR, the differentiall output has to be used, i think, when the differential input ports short together.

#### alok_ky

##### Full Member level 1
define cmrr in a differential amplifier

it is measures differentially,,,refer sedra/ smith for details.

#### nandu_r

##### Member level 1
cmrr of single ended output amplifier

the measurement depend opon how you tap the output
if your amplifier has single ended output, then you have to measure w.r.t single end output

#### surianova

define cmrr in differential amplifier

electronics_kumar said:
dear surianova can we conclude for mentioning ideal opamp we refering Acm

ya, i mean Acm. All the value i get is in negative dB.

#### wanily1983

##### Member level 5
to calculate cmrr

in many classic referrence book, there are two definition of CMRR.
i think we often care the later one. but i am not sure what the usage of the first one. hope for more.

the first definition is used in sigle-end output and the second difenition is used in fully differential opamp. in fact, they are the same just definiting the reject degree.

#### prabhu_psg

##### Junior Member level 1
what do you mean by cmrr of an amplifier

hI WANILY

I feel the later definition hold only when u have a mismatch in the diffrential pair.I am very confused abt the first definition.How to calculate it using spice .

#### Humungus

##### Full Member level 6
ideal cmrr

It depends on which parameter are you interested. In a fully-diff amplifier there are several gains:
Ad_d : change in the differential output due to a change in the differential input
Ad_cm: change in the differential output due to a change in the common mode input
Acm_d: change in the common mode output dua to a change in the differential input
Acm_cm: change in the common mode output dua to a change in the common mode input.

#### gajen

##### Junior Member level 1
cmrr=a/acm

You should measure output differentially. And measurement will be change in differential output to change in common mode input.

#### _AndrewS

##### Newbie level 1
cmrr differential output

By the way: do you have any idea how measure CMRR in Cadence Composer schematic? Which simulation I can use?

#### wanily1983

##### Member level 5
cmrr ideal opamp

hi,AndrewS
you can find what you want in chapter 6 of allen's book "cmos analog circuit design".
hope it helps u.

#### chu

##### Junior Member level 3
Re: CMRR

hi all , i'm new in analog design and i just read abt Acm in Gray/Meyer,
Acm=voc/vic and voc=(vo1+vo2)/2 ... so does it mean that we need to measure single-ended output on each side of outputs and calculate voc to obtain Acm ??

#### jinxingsun

##### Member level 5
CMRR

chaper 4 of Razavi's book discusses this point in detail

#### leonlin

##### Newbie level 4
Re: CMRR

measures differentially

Re: CMRR

#### huojinsi

##### Full Member level 5
Re: CMRR

I am confused by up posts.For differential amplifier, i think CMRR should be measured single ended. Because normally CMRR is defined at single-ended out mode.

#### electronics_kumar

Re: CMRR

for ideal opamp,CMRR will be 80~90dB

##### Junior Member level 2
Re: CMRR

Hi, could someone explain generally when we use a half circuit.

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