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CD74HC4067: What should be done with the unused input pins of the multiplexer IC ?

Varghese Thomas Thoppil
Prodigy 70 points
Part Number: CD74HC4067
Other Parts Discussed in Thread: CD74HCT4067

Hi

I am using CD74HC4067 mux and I have some unused input pins. Is it OK to keep these unused input pins floating or should I need to pull these pins either to VCC or GND

  • 0
    Guru 272510 points

    Analog switches allow analog signals, so you can let them float (but connecting to a fixed voltage would not hurt).

  • 0 in reply to Clemens Ladisch
    Prodigy 70 points

    Hi Clemens

    Thank you so much for the reply Sir but I would like to know that whether if I keep these unused pins floating, will that affect the readings on the other used pins ?

    Also I would like to know in which situations we usually pull up or pull down these unused pins to VCC or GND ?

  • 0 in reply to Varghese Thomas Thoppil
    Guru 272510 points

    For the effects of inactive channels, see the specified Switch “OFF” Signal Feedthrough. If you have high radiated noise in your environment and want to prevent the unused pins from picking up high frequencies, ground them.

    You need tie a pin to VCC/GND only if it is a CMOS input; see [FAQ] How does a slow or floating input affect a CMOS device?

  • 0 in reply to Clemens Ladisch
    Prodigy 70 points

    Hi Clemens

    Thanks for the reply but I have some more doubts regarding this

    The specified Switch "OFF" Signal Feedthrough for CD74HC4067 at Vcc of 4.5V is -75 dB.  So what I am guessing about this parameter value is that if the noise level is higher than this value, I should be grounding the unused pins instead of keeping them floating. Is the guessing correct ? if Not please share some ideas on this parameter

    Sir, also you have mentioned that we need to tie the pin to Vcc or GND only if it is a CMOS input. In CD74HC4067, how will I know if the unused input pins are CMOS inputs ?

    From the datasheet I have read that " The CD74HC4067 and CD74HCT4067 devices are digitally controlled analog switches that utilize silicon-gate CMOS technology to achieve operating speeds similar to LSTTL, with the low power consumption of standard CMOS integrated circuits ". So based on the above sentence, it appears that the input pins are cmos inputs. Is it correct ? If Yes then how could we leave the unused input pins floating ?

  • 0 in reply to Varghese Thomas Thoppil
    Guru 272510 points

    That specification means that noise is attenuated by 75 dB. And as shown in figure 3, it's frequency dependent.

    When in doubt, just ground the unused pins of the CD74HC4067; it never hurts.

    CMOS inputs are connected to the gates of two complementary transistors (see the linked FAQ). An analog switch uses CMOS transistors in a different configuration as transmission gates.