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Design >> Analog Design >> CMFB for a voltage difference
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Message started by fran2k5 on Jun 27^th, 2005, 6:46am

Title: CMFB for a voltage difference
Post by fran2k5 on Jun 27^th, 2005, 6:46am

Hello everybody,

I would like to ask you all if you know any technique in which a feedback circuit defines the voltage difference between two nodes.

Normal CMFB circuits sense two nodes and feed back a voltage. I would like to feed back a voltage difference. In other words I want to define the voltage difference between two nodes. Something like a floating battery used in class AB output stages but in which I can control somehow the voltage across it.

Could anybody suggest any good reference?

Thanks,

frank

Title: Re: CMFB for a voltage difference
Post by ywguo on Jun 27^th, 2005, 7:56pm

Hello, Frank,

Do you want to design a differential voltage reference? I am not clear what is your application.

Please turn to Larry Singer's "A 3-V 340-mW 14-b 75Msample/s CMOS ADC with 85-dB SFDR at Nyquist Input", JSSC 2001.

Best regards,
Yawei

Title: Re: CMFB for a voltage difference
Post by fran2k5 on Jun 28^th, 2005, 1:52am

Yawei,

thank you very much for your answer.

The reference you gave me considers a classical CMFB though. I am looking for a way of feeding back a difference of voltages and not just one voltage.

Any other suggestion is welcome.

Regards,

frank

Title: Re: CMFB for a voltage difference
Post by Vivek Sharma on Jun 28^th, 2005, 7:09am

Hi,

It is not clear what you want to do? If you just want to compute the exact difference between 2 voltages and feed it back, then you just need a simple difference amplifier easily realized with 1 opamp and 4 resistors.

Are you trying to transform a VDD-referred signal to a VSS-referred one by any chance?

Regards
Vivek

Title: Re: CMFB for a voltage difference
Post by fran2k5 on Jun 29^th, 2005, 2:33am

Hello,

sorry for not being clear. I'll try again.

I am wandering if it is possible to feed back two voltages to the bias circuitry instead of one.
Normal CMFB circuits feed back one voltage. In other words I want to define the voltage of two nodes in the bias circuit or their difference.

Thank you very much for your patience.

Regards,

frank

Title: Re: CMFB for a voltage difference
Post by vivkr on Jun 29^th, 2005, 7:13am

Hi Frank,

You can generate such a floating voltage in several ways. One way is to use a DDA (Differential Difference Amplifier). This has 2 sets of differential inputs. For some information on this, you can refer to the JSSC paper by Sackinger and Guggenbuhl.

"A Versatile Building Block: The CMOS Differential Difference Amplifier", JSSC, April 1987.

You must keep in mind though that DDAs suffer from various performance limitations and might not be practical for your task.

Another option is using a fully differential difference amplifier. Such a block is widely used in analog design. You will need 8 resistors and a fully differential opamp though, not to mention a CMFB for this opamp.

If you are planning to use this floating voltage reference as a CMFB for another opamp, then you will quickly discover that stabilizing the circuit is nontrivial.

Iif you want to track 2 mutually varying voltages to generate a floating supply, you may have to do something like this. However, if the 2 voltages are question but only with a fixed difference, a level shifter may be all you need.

Hope this helps.

Regards
Vivek

Title: Re: CMFB for a voltage difference
Post by fran2k5 on Jun 30^th, 2005, 3:35am

Vivek,

thank you very much for your answer.
I will read the paper you suggested me. It seems really close to what I am trying to achieve.

Best regards,

frank