Difference between Offset Control & Common Mode Feedback

[wylee]

difference dc offset cancellation cmfb

I am in a confusion about this 2 functions: common mode feedback (CMFB) and offset control/cancellation.

:?: What is their difference?

:idea:
As for my knowledge, CMFB is use when you want to maintain the output common mode of the circuit at a specific voltage. Offset control/cancellation is to control the DC offset of the output waveform. But they seems the same to me.

:arrow:
Assuming I have a differential-in differential-out opamp. I want to lock the output DC point to be at 1V regardless the small DC point shift (1V +-30mV) at input (due to process variation & temperature), which circuit technique should I use to implement this? CMFB or offset control?

 

[selvaraja]

Re: Difference between Offset Control & Common Mode Feed

hi

Offset due to the threshold voltage mismatch of the device which is trade with the channel capacitance .For example , a thershold mismatch of 0.01mV translates to roughly 100fF of channel capacitance for each transistor in a 0.18 um technology.If you where to connect many stages of diff. amp parallelly , the input capacitance becomes large, this can degrade the speed and consume huge amount power in the preceding stage. So to overcome this you have to a offset cancellation techniques.

regards,

selva

 

[maxwellequ]

Re: Difference between Offset Control & Common Mode Feed

The common-mode feedback is a negative feedback network that must be used in all amplifiers with differential output. This is because the feedback network external to the amplifier only sets the differential output voltage. For example if the gain of the circuit is 2, then the differential output voltage will be 2x the differential input voltage:

vop-von=2.(vip-vin)

However the external feedback network is not determining the common mode of the output voltages. Imagine that vip-vin=0.5 V -> Then vop-von=1V. But you can have this with vop=1.5 and von=0.5, or with vop=4 and von=3 (in this example the differential output voltage is the same but the output common mode voltage changes).

You must, therefore, use a negative feedback mechanism that sets the common mode voltage -> this is what the CMFB does.
In differential systems the "information" in in the difference of voltages and not in their common-mode. Therefore it is usually not important to have a VERY well controlled common mode value.


The offset cancelation is an entirelly different thing. The mismatches in the components will cause an offset voltage which will affect the differential output voltage. I the previous example you would have:

vop-von=2.(vip-vin)+VOS

Note that the offset voltage affects the differential voltages, i.e., your "information". The offset cancellation techniques suppress (or decrease) the offset voltage.

 

[wylee]

Re: Difference between Offset Control & Common Mode Feed

maxwellequ,

Thanks for clarifying my confusion,

I have another question, if somehow I need to have a chains of indentical opamps connected cascadingly (such as limiting amplifiers), and I want to make sure the common mode of the eye diagram is really locked at the center, is it vital to have both CMFB and Offset cancellation implemented in the system?

From your explaination, I have a fairly good guess that in my scenario here, having CMFB will help centering the eye diagram at the desire CM while Offset cancellation will ensure 50% duty cycle of the waveform. Am I right?

 

[maxwellequ]

Re: Difference between Offset Control & Common Mode Feed

Dear wylee,


The usage of CMFB is mandatory in when fully differential amplifers are used. As I explained they are necessary to set the common-mode of the output voltages in a certain value which, usually, is does not have to be defined very strickly (as long as it guarantees that the input and output stages od the amplifiers are biased correctly it is ok).

Regarding the offset voltage, you must see what is its effect on your system.


I'm not an expert on that type of systems, but what you say seams to make sense.. If you post more details (like a schematic), maybe I can help you further.

Regards