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How to implement the switch in a DAC?

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shaq

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Dear all,

How to implement the switch?;
 

lavitaebelle

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Usually MOS transistors in triode region are used for switches. Depending on the configuration, either an NMOS or a PMOS or a combination of both is used.

The length of the MOS is kept minimum to reduce parasitic capacitances. Depending on the application, the Width is adjusted.

Important things to be kept in mind is that the switch has finite on resistance, and this resistance varies in the DAC chain as all switches dont see the same drain and source voltages. This may cause DNL errors.
Also the leakage current has to be kept in mind.
 

shaq

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lavitaebelle said:
Usually MOS transistors in triode region are used for switches. Depending on the configuration, either an NMOS or a PMOS or a combination of both is used.

The length of the MOS is kept minimum to reduce parasitic capacitances. Depending on the application, the Width is adjusted.

Important things to be kept in mind is that the switch has finite on resistance, and this resistance varies in the DAC chain as all switches dont see the same drain and source voltages. This may cause DNL errors.
Also the leakage current has to be kept in mind.

Can you post up your schematic ?

Thanks!!!
 

lavitaebelle

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I dont have any schematic with me. I can tell you how it looks. Replace the ideal switch in your schematic by the MOS. For NMOS the drain is the top, source is bottom connected to current source, gate is connected to VDD.For PMOS source is top, drain is bottom, gate is connected to ground. Well is preferably connected to source to avoid body effect.
(Actually doesnt matter which is top,which is bottom since MOS is a symmetrical device with respect to source and drain)
If you are using a pass gate, i.e combination of NMOS and PMOS, connect them in parallel, NMOS source shorted to PMOS drain at the bottom and PMOS source shorted to NMOS drain at the top. Gate of NMOS to VDD and gate of PMOS to ground.
 

Syukri

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better use transmission gate rathe rather than nMOS switch....no Threshold voltgae drop
 

shaq

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Syukri said:
better use transmission gate rathe rather than nMOS switch....no Threshold voltgae drop

Dear Syukri,

I think TG is a good idea but how to determine the TG's length ?

Is it Lmin ?
 

lavitaebelle

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You cant offhand say use a transmission gate because you have twice the parasitic capacitance on the node when compared to a single MOS. If the signal is a high swing we can go for a transmission gate. So need to know the application before deciding on the switch.

L is usually Lmin to minimize parasitics but again leakage is highest at min length. Need to keep that in mind.
 
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