How to measure CMRR for differential amplifier?

[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]

what do you mean by cmrr of an amplifier?

CMRR=Adm/Acm
where:
Adm=differential mode gain
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

 

[jiangwp]

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]

define cmrr in a differential amplifier

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

 

[nandu_r]

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

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]

to calculate cmrr

in many classic referrence book, there are two definition of CMRR.
one is Adm/Acm, the other is Adm/Acm-dm.
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]

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]

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]

cmrr=a/acm

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

 

[_AndrewS]

cmrr differential output

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

 

[wanily1983]

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]

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 ??

Please reply, thanks.

 

[jinxingsun]

CMRR

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

 

[leonlin]

Re: CMRR

measures differentially

 

[beckchm]

Re: CMRR

depend on your choosing configuration

 

[huojinsi]

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

 

[alizadeh_m]

Re: CMRR

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