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CD74HCT4067: Interchannel interference

fallon zhang
Prodigy 40 points
Part Number: CD74HCT4067

74HCT4067M         5.1V powerd；When the input voltage on channel  Ix  is greater than 4.3V，Select other channels will read 500mV voltage（Increased with Ix input） on  pin1（in fact other channels are float）；Is this phenomenon normal?How can I avoid this problem?

 

  • 0
    TI__Mastermind 45455 points

    Fallon,

    Am I understanding your testing correctly? 

    You have Vcc = 5.1 V

    You place >4.3V on a signal path (I0) for example

    All other paths are floating

    You measure with a  multimeter that there is 500mV on the floating channel (I4) for example

    What is the status of the E and S pins during your testing?  What is on the pin 1 common input/output? 

    If you place a weak pull down on a channel instead of leaving it floating does it make a difference?

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    Prodigy 40 points
    Thank you very much for your reply！
       your understanding is correct；
       my new test  method and test result：
    1：
    VCC=5.1V
    S0、S1、S2、S3、E   3.3Vor0V from MCU（3.3V powerd）
    VIN=4.4V
    2：
    VCC=5.1V
    S0、S1、S2、S3、E   3.3Vor0V from MCU（3.3V powerd）
    VIN=3.3V
    3：our actual circuit

    Thank you,

      fallon

  • 0 in reply to fallon zhang
    TI__Mastermind 45455 points

    Fallon,

    In your screen capture you have DC 4.4 V on I0 (pin 9) and you are showing the yellow scope shot measuring at the I/OCOM (pin 1) correct? 

    All other Ix pins are floating but you are changing the Sx pins every ~20ms to select channel I1, I2, I3 .... Your expectation is that when you change to the other channels you should not see any voltage because you are not driving those pins.  

    When you do the same experiment with DC 3.3V on I0 (pin 9) when you change the Sx pins there is no voltage on the other channels.  correct?  

    Are you able to isolate the CD74HCT4067 and compare the I0 pin 9 voltage to the  I/OCOM voltage pin1 on the same scope? 

    Are you able to compare the I1 pin8 voltage tot he I/OCOM voltage on pin 1? 

    This device is a passive FET switch and will simply pass the voltage present on the Ix pin to the COM pin.  I would like to see if there is noise or voltage on the Ix pins.  It is possible that there is noise on the Ix pins and the switch will pass that noise to the COM pin. 

    Adam

  • 0 in reply to Adam Torma
    Prodigy 40 points

    Adam：

    I have found the problem, our 5.1V power supply is only about 4.3V。When one channel input voltage is close to or exceeds the supply voltage, it will affect other channel data.

    Thank you,

    Fallon