General Switch/Mux FAQ

Document created by JoeCreech Employee on Feb 19, 2013Last modified by JoeCreech Employee on May 27, 2013
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Link to Switch and Multiplexer basics

The tutorial at the link below provides valuable information on the basic of switches and multiplexers such as:

  • CMOS Switch Basics
  • Error Sources in the basic CMOS Switch
    • Current Leakage
    • Off Isolation/Crosstalk
    • Charge Injection
    • Settling Time
  • Switch application examples
  • High Frequency Switches
  • Parasitic Latchup in CMOS Switches and Muxes
    • Latch-up protection schemes

 

http://www.analog.com/static/imported-files/tutorials/MT-088.pdf

 

 

What do the Pin Cap terms mean and how do I calculate the capacitance between source and drain?

Pin capacitance is specified by Cs(off), which is the source capacitance when the switch channel is not present, Cd(off), which is the drain capacitance when the switch channel is not present, Cin which is the capacitance of the digital interface pins. Cd, Cs (on) is the on capacitance of the switch when the channel is present (switch is in the on state). To calculate an approximate value for the capacitance of the channel subtract (Cs(off)+ Cd(off)) from Cd, Cs (on).

To measure pin capacitance it is important to note that any PCB or trace used in the measurement will have its own inherent capacitance which must be subtracted to get the value of the capacitance due to the switch/multiplexer. Other circuitry attached to the pins will also contribute to the capacitance and should be removed also.

 

How to apply a DC bias to the ADG9xx

See attachment


What are the switch and multiplexer design considerations for hostile environments (exceeding absolute max ratings)?

Care must always be taken by the user to ensure the absolute maximum ratings for each part are not exceeded. For a description of hostile environments and possible ways to mitigate their effects see

http://www.analog.com/library/analogdialogue/archives/45-05/switch_mux.pdf

 

It is always the responsibility of the user to ensure any transient reduction scheme is suitable for the part being used.

 

Are IBIS/SPICE Models available?

The IBIS and Spice models available for ADI parts can be found at:

 

http://www.analog.com/en/tools-software-simulation-models/resources/index.html

 

Or if available on the products webpage.

 

What should the exposed paddles on LFCSP parts be connected to? Are there any special considerations for using LFCSP packages?

If a package has an exposed paddle it should be tied to the lowest potential the part will be operated with or according to the specifications in the datasheet. Should it be connected to any other potential the part may operate but could suffer performance degradation over the lifetime of the part.

Other considerations for LFCSP packages can be found at the link below.

 

http://www.analog.com/static/imported-files/application_notes/AN-1024.pdf

 

What is the noise contribution of CMOS switches?

Most CMOS switches do not contain any noise sources such as clocks and as such can be modeled as resistors in noise calculations. The voltage noise from a MOSFET switch will be predominantly thermal (Johnson) and flicker (1/f).  The thermal noise is due to the channel resistance.  With regards flicker noise you can expect the broadband noise to be very small.

You can make a good approximation of high frequency noise (above the corner frequency) from the Johnson noise equation:


En = sqrt(4kbTR) [V/sqrtHz]


Just to note that current noise in MOSFETs is extremely small.

 

How to reconfigure certain muxes to change from a SPST to SPDT?

Using configurations such as single pole single throw (SPST) switches other multiplexer configurations can be devised using connections on the PCB. For example a quad SPST can be changed into a 4 to 1 multiplexer by connecting the drains of the SPST together. Other configurations are also possible. It should be noted that the specifications for the new configuration will be additions of the single throw specs and any extra loss associated with the PCB routing.

 

How do I calculate settling time of the switch/mux in my system?

Settling time is not specified on ADI switches and multiplexers as it is a complicated measurement due to its susceptibility to the load conditions. Switching times are specified and so using these in conjunction with the application load information settling time can be calculated. For further information please see the tutorial at the link below.

http://www.analog.com/static/imported-files/application_notes/AN-1024.pdf

 

How should I manage unconnected pins?

If pins are unused they should still be connected to a known value. For digital pins all pins should be connected as if they are left to float the part may not behave as described in the data sheet. All source and drain pins should also be connected to a known state as if these pins float to particular voltages the protection diodes may reverse bias causing unwanted current to flow which may be harmful to the part over time. Only pins marked no connect (NC) may be allowed to float.

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