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Design >> Analog Design >> Fully Differential Folded Cascode OTA With SC-CMFB design HELP
https://designers-guide.org/forum/YaBB.pl?num=1249070011

Message started by cdbular on Jul 31^st, 2009, 12:53pm

Title: Fully Differential Folded Cascode OTA With SC-CMFB design HELP
Post by cdbular on Jul 31^st, 2009, 12:53pm

Hello,
I have designed a fully differential folded cascode OTA with SC-CMFB.
I have tested the circuit using a SC-CMFB network using Verilog-A macromodel switches. And the OTA seems to work very well.

When I replace the switches with transistors the clock injection is so bad that the CMFB voltage (which should be around 900mV) never is mantained at a fixed value and varies in a much wider range . The common mode open loop gain seems to be as large as the differential gain (~70dB) as well is the Unit gain Bandwidth (>125MHz).

I attached the images of the schematic and the simulations.

When using the MOS switches the CMFB voltage varies in a much wider range. As you can see in the simulation screenshots.

Schematic Screenshot

Transient simulation using Ideal Switches
Left: Diferential output, Rigth: VCMFB, Differential input: 300uV

Transient simulation using MOS Switches
Left: Diferential output, Rigth: VCMFB, Differential input: 300uV

Title: Re: Fully Differential Folded Cascode OTA With SC-CMFB design HELP
Post by wave on Jul 31^st, 2009, 6:07pm

I don't see the clock waveforms -- where is your Non-Overlapping clk generator?

Title: Re: Fully Differential Folded Cascode OTA With SC-CMFB design HELP
Post by raja.cedt on Jul 31^st, 2009, 8:03pm

hi,
have you checked normal operation properly? because i don't think Charge Injection will give this much problem.... some times inadequate bandwidth in CMFB also troublesome..

Thanks,
Rajasekhar.

Title: Re: Fully Differential Folded Cascode OTA With SC-CMFB design HELP
Post by cdbular on Aug 1^st, 2009, 12:10pm

Hello, thanks for replying
Wave, I am generating non-overlaping clock phases with Vpulse sources in cadence.

Rajasekhar, What do you mean with normal operation? How should I check this?
How should I check if the CMFB bancwidth is correct?

Thanks
Carlos Bula

Title: Re: Fully Differential Folded Cascode OTA With SC-CMFB design HELP
Post by thechopper on Aug 2^nd, 2009, 11:14am

cdbular wrote on Aug 1^st, 2009, 12:10pm:

Hello, thanks for replying
How should I check if the CMFB bancwidth is correct?

Hi Carlos,

One way to check the CMFB BW is to use Periodic STB analysis (Pstb). This tool allows you to analyize the stability of any loop that has two different periodic operating points, like the case of the SC CMFB you are showing.

Another simpler approach is just to apply a step to the input of your CMFB and see how it looks and what the settling time is. From there you can check what the BW is for such circuit.

Hope this helps.
Tosei

Title: Re: Fully Differential Folded Cascode OTA With SC-CMFB design HELP
Post by Berti on Aug 2^nd, 2009, 11:54pm

Quote:

...never is mantained at a fixed value and varies in a much wider range .

The common-mode seems to settle, I don't see a big problem. But it looks like that you have inadequate CMFB bandwidth as Rajasekhar pointed out.

Cheers

Title: Re: Fully Differential Folded Cascode OTA With SC-CMFB design HELP
Post by cdbular on Aug 3^rd, 2009, 7:29am

In case that I have inadequate CMF bandwitdh, How should I correct this?