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CD4051B: Interefence/Coupling/Crosstalk issue between Mux input signals

Ritul Patel1
Prodigy 70 points
Part Number: CD4051B
Other Parts Discussed in Thread: SN74LV4051A

Hello,

We are looking to measure two fan rpm by using an 8:1 multiplexer CD4051B (Vcc: 3.3V). The fan is 4-wire pwm-based 12V DC supply. Both fans have separate PWM pins & dedicated 12V DC power rails. Please check the below diagram for Mux connection details.


We faced the random rpm reading of the fan and both fans are slow down. While debugging this issue using DSO, We found that not getting proper output signal (Refer image 1.5.95_BOTH_TACH_5ms) Out/IN while two Tach FB pins (IN/Out0 and IN/out) are connected together. It looks like coupling/interference/crosstalk between two Mux input connections.


If we connect a single Tach FB input connection to Mux input (Refer to image 1.5.95_Tach FB Fan 1 or 1.5.95_Tach FB Fan 2), Fan rpm works perfectly. Also, We verified the PWM Fan input driving signal which looks OK.

Please do the needful and share your thoughts.

  • 0
    Guru 317000 points

    CD4000 devices are designed for high voltages; at 3.3 V, the on-resistance is horrible. Use a low-voltage switch like the SN74LV4051A.

  • 0 in reply to Clemens Ladisch
    Prodigy 70 points

    Hi Clemen,

    Thanks for the quick response.

    End customer has already developed this product. We need to fix this issue without any HW change.

    I checked in the datasheet but was unable to find Rds@3.3V. Please share the typical value.

    Could please explain about how a higher value of Rds which creates this issue? If you have any application note, please share with us.

  • 0 in reply to Ritul Patel1
    Guru 317000 points

    This is from an older version of the datasheet:

    At 5 V, you are coming near the gate threshold voltage of the transmission gate MOSFETs. At 3.3 V, there are no specified values, not even typical ones, but the voltage is even nearer the threshold, and the resistance will be much higher.

    The on-resistance creates voltage dividers with the other resistors in the circuit, and increases noise sensitivity. In this case, I suspect that the fan signals can pull up each other through the pull-up resistors.

    In this circuit, the SN74LV4051A is a pin-to-pin replacement.

  • 0 in reply to Ritul Patel1
    TI__Mastermind 24440 points

    Hello Ritul,

    In addition to what clemens said i think that your design could be seeing noise coupling and crosstalk due to the unused inputs not being terminated to ground. Here is a good FAQ that can help your customer design.

    Also here is a good video to help you learn more about  r on and c on in muxes

    https://www.ti.com/video/series/precision-labs/ti-precision-labs-features-and-parameters.html?keyMatch=

    Pleas let me know if you need any more assistance.

    Regards,

    Kameron