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Mahavir

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How to design Analog multiplxer?
Mar 10^th, 2009, 10:47am

Hi all,
As a part of my dissertation, I need an anlog mux that will pass different DC voltage levels based on the select line inputs. These are fixed dc levels. I used a very simple conept of pass transistor logic with decoder on top to turn on particular switch as shown in diagram. The AS in diagram means Analog Switch.But i am facing problem in output stage.
How can I make the signal flow unidirectional? Is pass transistor logic for Analog Switch suitable one?
Also is there any other way to design the analog mux?

Plz reply,
Thanks in advance.

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HdrChopper Community Fellow Offline Posts: 493	Re: How to design Analog multiplxer? Reply #1 - Mar 10^th, 2009, 6:29pm what is wrong with using TX gates in your desing?
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ywguo Community Fellow Offline Posts: 943 Shanghai, PRC	Re: How to design Analog multiplxer? Reply #2 - Mar 12^th, 2009, 2:35am Mahavir, Quote: How can I make the signal flow unidirectional? Is pass transistor logic for Analog Switch suitable one? What is "unidirectional" here? What's your problem? Yawei
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subgold

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Re: How to design Analog multiplxer?
Reply #3 - Mar 16^th, 2009, 8:03am

why you need to force the mux to be unidirectional? what kind of following and preceeding stages does the mux have? if unidirection is really necessary, an output buffer seems to be enough?

whether the switch is suitable depends on the voltage levels of the input, output and the switching signal. If the switch can always be completely turned on, then it should be ok.

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Mahavir

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Re: How to design Analog multiplxer?
Reply #4 - Mar 22^nd, 2009, 2:17pm

Hi All,

Sorry for late reply.
Actually, the inputs to the transmission gate are constant dc voltage levels. I figured out what my problem is. The transistors in tx gate are not getting turned on at same instant. and so one of the transistor is causing drop which of equal amount as threshold. I think i need to design a inverter fork. Currently I am working on the possible width ratios.

But problem remains. Since dc voltage levels are inputs and some of them are smaller than threshold of transistor, what effect will they cause? Since all outputs are connected together at one node, what will happen when a particular tx gate turns off and other turns on?

Need help
Plz reply. �

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ywguo

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Re: How to design Analog multiplxer?
Reply #5 - Mar 24^th, 2009, 9:01pm

Quote:

But problem remains. Since dc voltage levels are inputs and some of them are smaller than threshold of transistor, what effect will they cause? Since all outputs are connected together at one node, what will happen when a particular tx gate turns off and other turns on?

Since the tx gates are composed of NMOS and PMOS, the dc voltage levels go to the output without any loss even if they are smaller than the threshold of transistors.
The input signals will conflict if only one tx gate off and others on.

Yawei

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