Common mode feedback circuit

Hello

Can any one tell me about the common mode feedback circuit (CMFB) which is required in a differential amplifier. Does this CMFB circuit only detects the DC voltage because the input of differential pair is Vin=Vcm+Vd. where Vcm is the common mode dc voltage and Vd is the diffferetial ac signal voltage. So if this CMFB is for detecting the common mode output of differential pair it means its going to detect the DC voltage at the output only?

Please can anyone tell me why for only dc voltage CMFB is used not for ac?

Thanx

I have difficulties to relate your question to the usual topology of a CMFB circuit. Can you clarify it referring to the terms in the below diagram (copied from Gray/Hurst/Lewis/Meyer, Design and Analysis..)?

Yes I want to ask from figure vocm is the dc output voltage of differential amplifier. why only for dc voltage a CMFB circuit is required? why not for ac output? what should be VCM?

The circuit doesn't distinguish between AC or DC. It works for any signal between DC and amplifier bandwidth limit.

Vcm is usually biased to mid supply. Respectively the CMFB amplifier enforces an output common mode voltage centered between the supply rails, allowing maximal output swing for the differential signal.

It means the CMFB circuit is basically for DC voltage. The Common Mode sensing circuit will sense the DC output voltage of the Op-amp.? Am I right?

- - - Updated - - -

Can you please refer some solved examples of CMFB. because I am still confused about Ac voltage and Dc voltage of op-amp which will go to the CMFB circuit.

The CMFB circuit also cancels (or at least reduces) AC common mode output voltage. It's not true to say that it only senses DC voltage.

You can operate a true differential amplifier as unbalanced-to-balanced converter, its performance depends on the gain and bandwidth of the CMFB circuit.

Thanx for your reply. what voltage (VCM) do I have to apply at the input of the CM sensing circuit. as shown in the diagram you attached? Is it a DC voltage. If yes then how much DC voltage do I have to apply?

CMFB circuit explanation

Hello

Can anyone tell me what does the common mode feedback cicuit sense. Does it sense only the DC voltage because the common mode voltage is the DC voltage. and is the vref also a DC voltage.

Thanx

Is it a DC voltage. If yes then how much DC voltage do I have to apply?

Click to expand...

Please review post#4.

Vcm is usually biased to mid supply.

Click to expand...

Hi,

If yes then how much DC voltage do I have to apply?

Click to expand...

Usually you should know.
Why do you use differential signaling?
And where do the signals go to? And what is it´s input specifiactions?

In my case the differential Opamp drives an ADC. ADC with differential input. And this ADC produces V_CM. In my case 0.9V.

--> There is no general rule what voltage to use for V_CM.

Klaus

how you considered VcM 0.9V? There is a vref point in a common mode circuit what should it be?

Hi,

As said: I didn´t consider. It is produced by my ADC, because the ADC needs a common mode voltage of 0.9V.
--> In my case: common mode voltage is specified by my ADC.

In your case:
* As asked before: Where do the signals go to?
* Who or what specifies V_CM?

We will always have difficulties to understand your problem as long as you don´t answer our questions.

Klaus

Common mode feedback is used in an ECG heart pulse circuit to cancel DC from other muscle movements and AC from mains hum picked up by the patient.

Thanx for your reply. I am making an OTA and for Fully differential OTA we have to use CMFB circuit to stabilize the output. But in CMFB there is a Vref point and that vref should be equal to the common mode output level of the differential amplifier . The output of Differential amplifier goes to the CMFB where it sense the output take its average and then compare the average of the result with vref. If there is an error that CMFB return that error to the biasing circuit of the fully differeential amplifier. Now I want to know What Vref do I have to set.

- - - Updated - - -

I have applied a 100mV DC common mode input voltage at the input of a differential pair with no Ac signal. My fully differential amplifier output common mode DC voltage is 456mV at one end and 450mV DC voltage on the other end of the differential amplifier. Now what Vref should I place at the Vref of the CMFB circuit?

I have read somewhere it should be at the mid supply voltage. I have used +/- 1V supply means do I have to place the vref at 1V or 0.5V.? what should be the region of operation of the transistors in CMFB circuit. Does the Gain of the common mode feedback not show any impact on the fully differential amplifier?

can you explain in detail

Hi,

Asking the same questions again, but not answering our questions won´t bring a result.

Klaus

The OTA designed is for ECG detection system it will got to an ADC . I couldnt undetstand what would be the Vref for CMFB circuit. I took differential out put referring to different research articles.

Hi,

it will got to an ADC

Click to expand...

How many different ADCs do you think do exist?

--> Tell us what ADC exactely.

Klaus

single 7 bit ADC.

Hi,

single 7 bit ADC.

Click to expand...

Ah yes. The one with the black plastic case and a couple of connections...
Are you joking?

My last attempt:
ADC: Brand, type. Exact type. And complete circuit...

We are asking for your application details for more than a month now.
Is it really that hard to give detailed informations?

Klaus

I am designing OTA for ECG acquisition system that OTA I will be using this OTA for designing of filter. Here is the attached figure

- - - Updated - - -

I am not designing ADC. I am designing OTA and for that fully differential OTA, CMFB circuit is required which I dont know how to implement. What should be Vref