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Mahesh New Member Offline Posts: 5 ahmedabad	Optimum Bias current value for MDAC in PipelineADC May 12^th, 2009, 11:35pm Hi all, I am working on 1V 12 bit Pipeline ADC @ 80 MSPS design.I am badly stuck in finding out the optimum value for the bias current(tail current source) for the OTA used in the 2-bit/stage MDAC(Multiplying DAC).Kindly help me out.
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kidman Community Member Offline Posts: 42	Re: Optimum Bias current value for MDAC in PipelineADC Reply #1 - Sep 13^th, 2009, 1:29am It all depends on the specs of the OTA. You can use the gm/id method http://bwrc.eecs.berkeley.edu/classes/icdesign/ee240_s09/
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Optimum Bias current value for MDAC in PipelineADC Reply #2 - Sep 13^th, 2009, 4:29am hi, i didn't understand what do you mean by optimal..generally for getting required bandwidth bias current will be fixed. Thanks, rajasekhar
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RobG

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Bozeman, MT

Re: Optimum Bias current value for MDAC in PipelineADC
Reply #3 - Sep 13^th, 2009, 12:10pm

raja.cedt wrote on Sep 13^th, 2009, 4:29am:

hi,
� �i didn't understand what do you mean by optimal..generally for getting required bandwidth bias current will be fixed.

Thanks,
rajasekhar

This has been my experience... current is set by the load capacitance, and load capacitance is set by the desired noise.

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Optimum Bias current value for MDAC in PipelineADC Reply #4 - Sep 14^th, 2009, 1:23am hi, if you fix gm for noise reduction how you will get desired bandwidth? Thanks, rajasekhar.
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RobG

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Bozeman, MT

Re: Optimum Bias current value for MDAC in PipelineADC
Reply #5 - Sep 15^th, 2009, 2:50pm

raja.cedt wrote on Sep 14^th, 2009, 1:23am:

hi,
� �if you fix gm for noise reduction how you will get desired bandwidth?

I can't tell if this was in response to my comment so let me expand. In an MDAC there are 2 major sources of noise: 1) Thermal noise from the opamp and 2) thermal noise from the sampling switch (i.e. kT/C noise). To decrease opamp thermal noise you need to increase the current. To decrease the kT/C noise you increase the capacitance. But by doing this you ALSO have to increase the opamp current so that it can drive the additional load capacitance.

So regardless of the noise source you have to increase the opamp current.

With MDACs it has been my experience that kT/C noise dominates. This means the design flow is: 1) Pick sample cap size based on kT/C noise. 2) pick opamp current based on need to drive the sample cap.

So, to optimize you should choose the caps sizes, which determines the current. Not vice-versa.

FWIW Chiu/Gray's 2004 JSSC work touches on optimizing MDAC scaling on down the pipeline.

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