Question for using MOSFETs as switches

[nathalie92]

How can one calculate the switch off resistance of a MOSFET used as an analogue switch?

What kind of circuitry would one need if this switch off resistance is to be increased?

Thanks

 

[erikl]

How can one calculate the switch off resistance of a MOSFET used as an analogue switch?

Simulate it within its real application environment.

What kind of circuitry would one need if this switch off resistance is to be increased?

Use lower width and/or length.

 

[crutschow]

Is this a discrete MOSFET or one that's part of an IC you are designing?

 

[nathalie92]

Is this a discrete MOSFET or one that's part of an IC you are designing?

I am designing a circuit which will contain discrete MOSFETs as my switches. However, since my sensor has an input impedance of 300MOhms, the swithces which typically have an off resistance in the MOhms region will interfere with my potential sensed by the sensor, hence I need to design a circuit that will increase this off resistance. Any ideas, how I can stack the MOSFETs?? Thanks

 

[FvM]

the swithces which typically have an off resistance in the MOhms region will interfere with my potential sensed by the sensor

Are you referring to specific switch data or is this just made up out of thin air? You'll find discrete MOSFET switches with roff > Gohm.

 

[nathalie92]

Are you referring to specific switch data or is this just made up out of thin air? You'll find discrete MOSFET switches with roff > Gohm.

I;m referrring to specific switch data...roff is not quoted in the MOSFET datasheet,hence I used Idss to calculate it ..From were do u find such MOSFET swtitches with roff > Gohm? What does one look out for in the datasheets?

Thnx

 

[FvM]

The datasheet should show Id leakage current versus Vds.