Designer’s Guide Community

adesign

Community Member

Offline

Posts: 73

CMFB sensing circuit
Dec 10^th, 2008, 10:02pm

Hi,

I've come across an opamp for an integrator of a sigma-delta ADC. The figure given
in this mail shows the CMFB sensing circuit for this opamp.

In the attached figure, OUTP and OUTM are the differntial output nodes of the
opamp. phi1 is the integrating phase and phi2 is the sampling phase. phi1 and
phi2 are two non-overlapping clocks operating at around oversampling ratio of
128. CMFB is the output common-mode voltage sensed by this circuit.

My queries are:
1) How is this CMFB sensing circuit working?
2) For doing ac analysis of the opamp, what load should be connected at the
opamp outputs? There is no tranac(ac simulation at some transient point) option
in my simulator.

Please comment.

Regards,
ADesign

Back to top

cmfb.jpeg

IP Logged

ricky New Member Offline Posts: 4	Re: CMFB sensing circuit Reply #1 - Dec 25^th, 2008, 5:56pm Hi,ADesign i face the same question as you when using a sc CMFB for an integrator of a sdadc...do you get a solution?
Back to top	IP Logged

loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: CMFB sensing circuit Reply #2 - Dec 25^th, 2008, 6:29pm You need to show the sequence of the clock signals for this to be meaningful. It looks like you are doing capacitive voltage averaging, but without the clocking sequence it is tough to say.
Back to top	Jerry Twomey www.effectiveelectrons.com Read My Electronic Design Column Here Contract IC-PCB-System Design - Analog, Mixed Signal, RF & Medical IP Logged

adesign Community Member Offline Posts: 73	Re: CMFB sensing circuit Reply #3 - Dec 28^th, 2008, 11:57pm The clocking sequence is attached herewith.
Back to top	cs_001.jpeg IP Logged

HdrChopper

Community Fellow

Offline

Posts: 493

Re: CMFB sensing circuit
Reply #4 - Dec 29^th, 2008, 4:08am

Hi ADesign,

The circuit you are using is averaging the differential outputs of your amplifier, so that such voltage is afterwards amplified and fed back by means of your CMFB circuit.
Just think of each switched capacitor as if it was a resistor: in average it behaves as such. When you replace it by a resistor you are left with a resistive network and the capacitors connected in parallel provide a zero for compensating some other pole in the CMFB loop.

Hope this helps
Tosei

Back to top

Keep it simple

IP Logged

aaron_do

Senior Fellow

Offline

Posts: 1398

Re: CMFB sensing circuit
Reply #5 - Dec 29^th, 2008, 7:29am

Hi,

I'm not sure exactly how it works...are you sure you drew it correctly? I'm not too sure about switched cap circuits anyway, so maybe its right...

Anyway i just wanted to add that its probably feeding back the average of 2VC - (OUTP+OUTM) ... i.e. the error signal...I'd be interested in how VC comes into the picture though.

cheers,
Aaron

Back to top

there is no energy in matter other than that received from the environment - Nikola Tesla

IP Logged

adesign Community Member Offline Posts: 73	Re: CMFB sensing circuit Reply #6 - Dec 29^th, 2008, 8:16pm Hello Aaron, Thanks for your reply. VC is the desired output common mode voltage generated from some reference bias. -ADesign
Back to top	IP Logged

adesign Community Member Offline Posts: 73	Re: CMFB sensing circuit Reply #7 - Dec 29^th, 2008, 8:20pm Thanks Tosei. What is the purpose of R here? Is it current limiting resistor when the output swing is maximum? This condition may arise when the output swing is maximum.
Back to top	IP Logged

aaron_do Senior Fellow Offline Posts: 1398	Re: CMFB sensing circuit Reply #8 - Dec 30^th, 2008, 1:14am Hi, I understand that VC is the desired common-mode voltage, but i'm a bit puzzled how it actually fits in. When you write VC above the wire, is that supposed to be a voltage source , VC, accross the cap? Also a bit unsure why you need R when the switched cap is supposed to emulate a resistor... cheers, Aaron
Back to top	there is no energy in matter other than that received from the environment - Nikola Tesla IP Logged

adesign Community Member Offline Posts: 73	Re: CMFB sensing circuit Reply #9 - Dec 30^th, 2008, 1:27am Hi, Yes, VC is the voltage source. I think R is to limit the current when OUTP and OUTM are at +/-maxima. Please correct me, if I'm wrong. Thanks, ADesign
Back to top	IP Logged

HdrChopper

Community Fellow

Offline

Posts: 493

Re: CMFB sensing circuit
Reply #10 - Dec 30^th, 2008, 2:42pm

Hi ADesign,

I think the series R could be there to just provide a pole in your CMFB loop. Such pole might be there for stability reasons and/or for providing antialiasing means for your CMFB circuit. I do not think the second option makes too much sense though, since usually noise aliasing in a CMFB circuit is not relevant.

Regards
Tosei

Back to top

Keep it simple

IP Logged

XY-oriented

Junior Member

Offline

Posts: 23

Re: CMFB sensing circuit
Reply #11 - Jan 5^th, 2009, 2:08am

hi guys...
I don�t thing the schematic is correct because VC don't play any role � but if �the left PH2 switches are connect to GND instead of VC such that C1 is charged with a voltage of -VC then when PH1 is �active �averaging occur in each half branch (assuming C1=C2)
((VOUTP-VCMFB)-VC)/2 + ((VOUTM-VCMFB)-VC)/2
and VCMFB=(VOUTP+VOUTM)/2 -VC as expected

is my analysis right ? What do you think ?

Back to top

« Last Edit: Jan 5^th, 2009, 6:14am by XY-oriented »

IP Logged

aaron_do Senior Fellow Offline Posts: 1398	Re: CMFB sensing circuit Reply #12 - Jan 5^th, 2009, 3:21am Quote: but if the left PH2 switches are connect to GND instead of VC such that C1 is charged with a voltage of -VC that's what i was thinking too...
Back to top	there is no energy in matter other than that received from the environment - Nikola Tesla IP Logged

HdrChopper

Community Fellow

Offline

Posts: 493

Re: CMFB sensing circuit
Reply #13 - Jan 6^th, 2009, 6:39pm

Hi XY-oriented,

I think your analysis is correct, except that instead of GND you should connect the left PHI2 swtich to the desired voltage to bias any current source inside the opamp topology.

Such voltage difference (current source bias - VC) is the one performing the comparison in the SC CMFB circuit.
Ultimately what the circui shhould give at CMFB node is

VCMFB=(VOUTP+VOUTM)/2 -(VC-current source bias)

I think the posted circuit just shows the sensing part of the CMFB but it is not complete since the comparsion is missing.

Regards
Tosei

Back to top

Keep it simple

IP Logged

XY-oriented Junior Member Offline Posts: 23	Re: CMFB sensing circuit Reply #14 - Jan 6^th, 2009, 11:46pm Thanks Tosei, you are right ... I forgot the bias ;) Redouane
Back to top	IP Logged