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SN74CBT3251: signal will attenuate after passing through two SN74CBT3251 with frequency up to 15 MHz.

Ken9512
Prodigy 110 points
Part Number: SN74CBT3251
Other Parts Discussed in Thread: MUX506, SN74CBTLV3251, SN74CB3Q3251, TMUX1208

Dear sir,

There is a problem when I use multiplexer SN74CBT3251. The signal source is an 50 MHz optical receiver which convert the optical on/off to voltage high/low.

 

My problem is signal will attenuate after pass through two multiplexers SN74CBT3251 with increasing frequency.

When the clock frequency is up to 15 MHz, I got all low after inverter SN74LVC14AD.

 

Do you have any suggestions? Presently, I want to try another multiplexer SN74HC151PWR.

 

Thanks,

Ken

  • 0
    TI__Mastermind 45370 points
    Ken,

    Passive FET switches like the SN74CBT3251 will attenuate signals due to their on-state resistance and on-state capacitance. If you plot the insertion loss (attenuation) across frequency you will see that the SN74CBT3251 device will have about -3dB of attenuation at ~200MHz.

    This attenuation will occur each time a signal passes through the switch so if there are two switches cascaded together there will be -6dB of attenuation at 200MHz.

    Why are you cascading two SN74CBT3251 devices? Have you thought about a device like the MUX506?
    What is the Vcc of the SN74CBT3251?
    What is the voltage of the signal you are placing on the signal path of the switch?


    Thank you,
    Adam
  • 0 in reply to Adam Torma
    Prodigy 110 points

    Dear Adam,

    Thank you for your reply & suggestions.

    1. In my multiplex system, the design include 45 inputs & 2 outputs. Hence, I need two stages of multiplexers, first stage are 6 mux. & second stage are 2 mux.

    2. Vcc of SN74CBT3251 is 5V

    3. The input signal ideally is 15 MHz & 5V clock. (The input signal will attenuated to around 4V when switched on.)

    sincerely,

    Ken

  • 0 in reply to Ken9512
    TI__Mastermind 45370 points

    Ken,

    The SN74CBTXXXX devices are single N channel FETs which means with a 5V supply they will only be able to pass 4V through the switch.  This is because the n-channel FET needs  ~1 V gate to source voltage (Vgs) to turn on the FET.  Excerpt below from selecting the right signal switch  which explains the different types of FET switch architectures.   

    If you would like to pass 5V signals through the switch you will need to select from a different family of devices that have transmission get architectures (n channel in parallel with p channel FET) like the SN74CBTLV3251 or an n-FET switch with its gate voltage boosted by a charge pump like the SN74CB3Q3251.  These devices will be able to pass signals from 0V to Vcc unlike the simple n-channel FETs.

    We also have a new device TMUX1208 that might interest you in your application as well. 

    Thank you,

    Adam