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Digital Pot: MOS as a logic switch

ctayers

Virtuoso IC6.1.5.500.9

For a class I need to build a digital potentiometer.  To do this Ive seen schematics where a MOSFET is used as a logic gate.  I attached an image really similar to my schematic.  I can get the gate control wires to see VDD, but Im not sure that I know what MOS to use (virtuoso symbol, then simulate), or how to size it.   When I apply VH=10 VDC, I would think if it was working correctly you would see a corresponding voltage on VW that works as if a voltage divider was activated at the MOS that is turned "on".  However VW seems stuck at 0.5V or so.

Can anyone help please?

Parents

  • 1. I tried your suggestion for the Potentiometer.  That works to increase device size and lower the input voltage to 100mV (probably I can reduce the device size now that I know what the problem was).

    2. Im testing VW at open circuit currently.  In the future I think we will use a buffer op amp (Which we design) to help with output impedance.

    3.  VL is grounded

    4.  Would you have a quick link to such an example of a transfer gate? Those would come in very useful to build relay-like switches.  Basically I want to be able to turn a solid-state switch on and off with another voltage signal.  My NMOS W=15u, L=6u currently.  AMI05 process.

    This has been very helpful!

Reply

  • 1. I tried your suggestion for the Potentiometer.  That works to increase device size and lower the input voltage to 100mV (probably I can reduce the device size now that I know what the problem was).

    2. Im testing VW at open circuit currently.  In the future I think we will use a buffer op amp (Which we design) to help with output impedance.

    3.  VL is grounded

    4.  Would you have a quick link to such an example of a transfer gate? Those would come in very useful to build relay-like switches.  Basically I want to be able to turn a solid-state switch on and off with another voltage signal.  My NMOS W=15u, L=6u currently.  AMI05 process.

    This has been very helpful!

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