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SN74CBT16233 I am in doubt what to do with the inputs of the unused switches.

user5124012
Prodigy 70 points
Other Parts Discussed in Thread: SN74CBT16233

Hi,

I'm designing a circuit using the SN74CBT16233 switch. I have three unused switches in my design and I am in doubt what to do with the inputs of the unused switches. The datasheet says that the CONTROL inputs should always be either high or low, which is clear and I made sure off.

However, this doesn't seem to mean that the switch inputs need to be grounded.

The datasheet refers to the TI document "Implications of Slow or Floating CMOS Inputs" so I read it.

This states the following:
"For CBT or CBTLV devices, this applies to the control inputs.|

This is another hint that it shouldn't be needed to hook up inputs to ground (or Vcc).

Now, in the above FAQ you say this:
"-Switch channel inputs can either be left floating or terminated to GND (which can help reduce signal reflections). "

Now...this only creates more doubt.

For my application, saving a few traces on the PCB to the unused inputs would be extremely helpful for my design, but of course I do _not_ want any problems caused by the floating inputs.

Can they interfere with the signals in the used switches ?

I am designing something for a vintage computer system. (Clock) signals are max. 3.5Mhz.

  • 0
    TI__Intellectual 2900 points

    Hi

    Technically, floating the analog IO pins should not channge any functionality of SN74CBT16233. However, grounding the unused analog input/output pins is a good design practice. For example, if any transient over voltage event occured on these unused pins, the ground trace can quickly help discharge the extra amount of current instead of triggering on the chip's internal ESD mechanism. Floating pins can build up charge/voltage on the pin, which brings more reliability concern. Therefore, grounding all these unused pins is highly recommended.

  • 0 in reply to Fan.Wang
    Prodigy 70 points

    O lordy....I wanted less traces on my PCB, not more ;)

    So even ground the outputs ? I knew that inputs grounding was good design conduct (at least I remember that from electronics school about 30 years ago ;)) but even the outputs...mmm...
    OK I need this board to be reliable and it would be foolish not to listen to advice from the manufacturer so I'll stick to grounding all unused pins.

    Thanks for the quick and clear reply :) !

  • 0 in reply to user5124012
    TI__Intellectual 2900 points
    The unused output pins are also better to be grounded as well. The reason is that for a simple passive MOS switch, it is designed symtrically. In other words, inputs and outputs are interchangeable.
  • 0 in reply to Fan.Wang
    Prodigy 70 points

    Ah yes of course, so stupid of me !!! :)

    Will ground them all :)

    Thanks again !

  • 0 in reply to user5124012
    Prodigy 70 points

    Just wanted to report that the design I made works flawlessly...I grounded all unused in- and outputs as suggested. Works rock-stable, thanks again for the help !