Confused about CMFB in one of Schreier's Amplifier

Hi: all
Now I am reading one of Schreier's paper: "A 375-mW Quadrature Bandpass Delta Sigma ADC With 8.5-MHz BW and 90-dB DR at 44 MHz". during reading, a question lingers. what is confusing me is the amplifier he used, more accurately, the CMFB. The amp is showed below
My question is the first stage CMFB(indicated by the red rectangular) seems got the feedback in the wrong direction, that is, IMO it is a CM feedforward..... I know maybe something dumb in my mind. so forgive me if I mess up...

Also could anybody point out what is the purpose of adding a series RC network(marked be blue rectangle) across the output of the first-stage amplifier. Is this amplifier specific? or just a functional part of the whole system(a Continuous Bandpass Sigmal-Delta AD)


Any help would be much appreciated.

Julian

A question to think about.
1) Yes it seems to me too from the circuit that the cmfb loop has a positive feedback.
2) CM feedforward: This is a common mode feedforward. I think the capacitor averager is used for compensating the cmfb loop by accomplishing this feedforward technique.
3) The series RC circuit introduces a zero to the differential circuit. I don't know why it is used. The series RC circuit will have an effect on the common mode loop response (if at all the common mode loop was a negative feedback one).
4) Is this amp specific? : Hmmm, I think this amplifier might be used within an integrator and not stand alone. I inferred that from the high differential DC gain it has.

Has this circuit not been explained atleast briefly in the paper?
PS: I might be wrong in my above inferences as I do not have access to the above paper. All the suggestions are based on my inferences of the circuit.

Hi amriths04
sorry about the CM feedforward stuff. what I really want to mean is Positive feedback.
when you see the capacitive CMFB path in 1st stage, that FB is negative. but the resistive CMFB path is positive based on what I have learned these years. maybe it is just a typo or what...
As to the original paper, it just give a brief comment on the circuit, as I quote it below

Figure shows a schematic of the amplifier used in the first resonator. The signal-path of the amplifier consists of a cascade of two differential amplifier stages in parallel with a feedforward stage. Common-mode feedback is accomplished by sensing the common-mode of each stage and developing a control signal for the associated tail current. In order to avoid resistive loading of the first stage, its common-mode is sensed with source followers driving a pair of resistors. The common-mode of the second stage can be sensed directly with resistors since its output is resistively loaded anyway. .....

Click to expand...

no explanation is given with regards to series RC.

Best Regards
Julian

The series RC is to compensate the amplifier because it is a two stage amplifier when looking from the input of IF to Op. Then there is another amplifier which is just a single stage amplifier the feedforward part, no compensate needed only one stage! The RC is used to not load the single stage amplifier, If normal miller compensation was used you would load the single stage amplifier while also compensating the two stage, not good if you want high GBW. Also it does look like a mistake on the CMFB 1, it is positive feedback, its a small mistake and could easily be made...If you are redesigning this just change it to negative

Jgk