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saber

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Settling of Diff AMP
Oct 18^th, 2004, 9:53am

Hello,
I have designed a folded-cascode differenctial amplifier used in the pipelined ADC. In settling simulation, I found that differential output (=vop-von) was well settled, but single-ended outputs (vop, von) were quite bad in waveform. I tried to make a good settling for single-ended output, but it was not so easy. Since differential output will be used in level-decision(comparators), could I ignore the single-ended result?

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ywguo Community Fellow Offline Posts: 943 Shanghai, PRC	Re: Settling of Diff AMP Reply #1 - Oct 19^th, 2004, 6:39am Hi, Saber, Why do you require the single-ended outputs settling very well? Are those comparators differential or single ended? Thanks Yawei
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John Galt

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Re: Settling of Diff AMP
Reply #2 - Oct 19^th, 2004, 9:33pm

Hi!
Since your circuit is differential, I guess it doesn't make a difference whether the individual single-ended outputs settle properly or not as long as the differential output settles well and the output common mode voltage is within bounds. The next stage of the pipeline is sensitive only to the difference of the two outputs and their common mode, so as long as these two are accurate, you don't have a problem.

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John Galt

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sheldon

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Re: Settling of Diff AMP
Reply #3 - Dec 19^th, 2004, 6:36pm

Hi,

Have you checked your common-mode loop characteristics?
For example, is the slew rate and the bandwidth of the
common-mode loop faster than the differential mode? If
the differential mode is faster than the common-mode
loop. Then you could be have issues with the common-
mode levels not settling well even though the differential
mode characteristics look good. If this is the case, then
you may want to modify the design so that the
common-mode loop performance is sufficient for your application.

Sheldon

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