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sn74cb3q3251: SN74CB3Q3251 - Port A ON capacitance

Part Number: SN74CB3Q3251

HI

Does Cio (on) max (for 3.3V Vcc) of 19pF max apply to both port A and port B.

Finbarr

  • Hi Finbarr,

    Cio (on) comes from both the source and drain capacitance. That means that whichever B port pin is selected would be your source, and the A pin would be your drain. Therefore it is a combination of the two pins.

    Please check these slides for more explanation on Cio to help eliminate any confusion. 

  • Thanks Dakotah

    So if I cascade a nos. of analogue multiplexers I am going to see the cumulative sum of Cd's, Cs's

    And the input capacitance of the load....things have just got a whole lot worse

    Finbarr

  • Finbarr,

    Correct. Think of the mux from a black box perspective. When the switch is on you will seen 19pF of capacitance plus load capacitance. If you have another mux after the first mux that it would be 19pF + 19pF + Cload.

    A basic equation could be as follows:
    N = number of muxs in series

    Total capacitance = (19pF *N) + Cload
  • Thanks again Dakotah

    Sorry for being so pedantic but I still have some uncertainties.

    Let me walk you through my world of understanding, based on the SN74CB3Q3251 being an 8:1 analog multiplexer/Demultiplexer;

    Cio(off) - Port B (input side of multiplexer) - This should roughly correspond to 1 Cs?
    Cio(off) - Port A (COM side of multiplexer) - this should correspond to the sum of 8 Cd's i.e. when OEn is high (all switches off)?
    Cio(on) - Port A and Port B (of selected switch) - correspond to the sum of 8 Cd's and 1 Cs.

    Again apologies for being so "pedantic" but I do need to get a good handle on this.

    Finbarr
  • Finbarr,

    Lets attack this question from an IC level perspective. I am attaching an image from the linked PDF to help clarify.

    When you use this device there are 3 separate capacitance values we will refer to. C io(off) A port, Cio(off) B port, and Cio(on). Lets look at what each value would be for this device (if conditions are met under electrical spec table).


    Cio(off) - Port B (input side of multiplexer) when this channel is not selected - each unselected B port pin  would have a typical value of 3.5pF.
    Cio(off) - Port A (COM side of multiplexer) - when no channel is selected - A port pin  would have a typical value of 19.5pF.
    Cio(on) - Port A and Port B (of selected switch) - would have a typical value of 15pF. Meaning a rough model of the signal path of the selected channel would be a 3ohm resistor followed by a 15pF capacitor to GND.

    I hope this helps!

  • Perfect
    Thanks
    Finbarr