A question about sigma-delta ADC

[Alan_Nesta]

I've heard some one said the OPs in it should use 2-stage class AB better,
but I am doubt that if the GBW is enough for it?
And I am wondering if 1-stage folded cascode OP is enough for the design?
Thank you!

 

[A.Anand Srinivasan]

in 2stage they model the OP such that the second pole is far off from the dominant pole that we are introducing due to our design.... using single stage your gain would be very low but bandwidth higher.... it is just a trade off....

since you are talking about GBW why dont you try using a telescopic op.....

 

[Alan_Nesta]

when low power working, telescopic op ICMR is very low,
so I think folded is better.

 

[A.Anand Srinivasan]

ok i agree on that..... but what about the power dissipation, GBW, offset voltage etc etc..... actually what are you designing this opamp for.....

 

[Alan_Nesta]

Need I care offset voltage for fully-differential one-stage folded-cascode OP?
the system offset is zero for presim?right?
I only need care for layout?

 

[A.Anand Srinivasan]

actually offset can cause change in the system considerably.... especially in the case of high gain systems like opamp you are supposed to take care that it is minimised.....

 

[caosl]

I've heard some one said the OPs in it should use 2-stage class AB better,
but I am doubt that if the GBW is enough for it?
And I am wondering if 1-stage folded cascode OP is enough for the design?
Thank you!

The dc gain requirement in SDM design is not strict ,so the one stage telescopic or folded is enough in many cases, and larger bandwidth could be obtained, some power consumption is saved too. But in cascade architecture, the first stage requires more larger gain to surpress the leakage and distortion, in this case, you can use 2-stage, but many designs still use one stage opamp.

 

[amriths04]

you just need to do an accurate design for your first integrator for the others you can choose any topology you want as it's not that stringent so as to affect the system's performance.

it does not matter even if you choose a 3 stage , it's just the way you are going to compensate it.

anyway no one can decide the appropriate topology just by seeing only GBW. there are many other constraints right?

 

[Alan_Nesta]

ok
thanks to all of you very much
I will try use 2-stage op in first integrator and 1-stage op in others