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The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> Question on input switch size ? https://designers-guide.org/forum/YaBB.pl?num=1251197564 Message started by nobody on Aug 25th, 2009, 3:52am |
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Title: Question on input switch size ? Post by nobody on Aug 25th, 2009, 3:52am I studied materials about bottom plate sampling by Prof. Song and found something interesting. Here is the first figure about bottom plate sampling. I am looking forward any materials about the figure in the 2nd post . I also found a figure looks pretty much like the one in the 2nd post. I tried to post the 1st and 2nd figures in one post but can not make it. Sorry for any inconvenience I made. |
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Title: Re: Question on input switch size ? Post by nobody on Aug 25th, 2009, 4:07am Figure and another figure looks pretty much the same. http://books.google.com/books?id=MpEEAMHuRjoC&pg=PA107&lpg=PA107&dq=%22size+ratio+between+the+input+and+sampling%22&source=bl&ots=AbGh7nXZ5x&sig=0f4qX0j4XOHreRrLvABu9t9RZnw&hl=zh-TW&ei=8sWTSuzDJMiUkAWWoeS9Cg&sa=X&oi=book_result&ct=result&resnum=1#v=onepage&q=%22size%20ratio%20between%20the%20input%20and%20sampling%22&f=false |
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Title: Re: Question on input switch size ? Post by ywguo on Aug 27th, 2009, 11:07pm I don't know what your problem is. |
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Title: Re: Question on input switch size ? Post by nobody on Aug 28th, 2009, 2:14am What i am looking for is some explanations about the selection of input and sampling size. I found that figure and want to know more details like why input switch with smaller Ron is better. |
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Title: Re: Question on input switch size ? Post by Berti on Aug 28th, 2009, 5:05am Ron needs to be small enough to satisfy the settling/linearity requirements. You can do hand calculations to get an idea. Regards |
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Title: Re: Question on input switch size ? Post by nobody on Aug 28th, 2009, 6:39am Thanks for your reply. I know one requirement on settling time but do not know the relationship between Ron and R, which means which resistance should be larger and how much larger is better. Like the example in the post, it is better to have larger Ron/R or Rtop/Rbottom for good THD. |
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Title: Re: Question on input switch size ? Post by RobG on Aug 28th, 2009, 11:02am Berti wrote on Aug 28th, 2009, 5:05am:
The settling would be determined by the total Ron, but I think he is looking for the optimal relative sizes of the two switches. I hadn't seriously analyzed it, but I would think you would want the top switch resistance to be as small as possible (i.e. make it a wide switch). This is because its resistance is signal dependent so you want to minimize its effect. In addition, charge injection effects are minimized because the bottom switch is open so there is little penalty to a large switch. On the other hand, the bottom switch contributes to the charge injection. Most is common mode and signal independent, but some isn't. You would want to minimize its effects by making the switch as small as possible... that is, you would want the bottom switch resistance to dominate the settling time. I'm just rattling this off the top of my head, but the above seems to make sense. It doesn't seem to be consistent with the plot. Ideas? rg |
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Title: Re: Question on input switch size ? Post by subgold on Aug 29th, 2009, 6:50am RobG wrote on Aug 28th, 2009, 11:02am:
is that because of the parasitic cap of the switch, for example, the source-bulk cap. so the clock feedthrough, or the limit on the settling together with the Ron will be playing a role. |
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Title: Re: Question on input switch size ? Post by Berti on Aug 30th, 2009, 11:44pm Neglecting linearity and just considering area, the switches should be sizes the same dimensions. Cheers |
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