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SN74CBT16214C: Maximum frequency

Bhavya Babu
Intellectual 465 points
Part Number: SN74CBT16214C
Other Parts Discussed in Thread: SN74CBT16214, , TS3DV642

Hi,

I need one 1:3 mux for driving CMOS 3.3V signal upto 60 MHz.

Will this part SN74CBT16214 support that much frequency? There is no bandwidth data available in the datasheet.

Regards,

Bhavya

  • 0
    TI__Mastermind 45370 points

    Bhavya,

    The SN74CBTXXXX devices bandwidth is ~200MHz so it should have no issue passing a 3.3V 60MHz signal.  However these devices are passive FET switches and do not have any drive current capability and cannot source or sink current.  What are you trying to do in your system needing a 1:3 mux?  Do you need all 24 channels or would a smaller 1:3 mux be better.

    page 11 of SN74CBT16214C datasheet:

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    Intellectual 465 points
    Hi Adam,

    I want to mux 16 IOs to three devices. There shouldn't be any signal integrity issues and I should be able to test the interface at maximum frequency without much overshoot, undershoot or signal integrity issues.

    Here are some more questions that I have for you:
    1) Will this part support RGMII 125MHz signals as well?
    2) How is it different from an analog switch like TS3DV642? We are considering to replace this TS3DV642
    with SN74CBT16214C.

    Regards,
    Bhavya
  • 0 in reply to Bhavya Babu
    TI__Mastermind 45370 points

    Bhavya,

    The TS3DV642 is one of our highest performing switches with lowest power consumption on the market.  This is a 7.5 GHz switch compared to the SN74CBT16214C 200MHz switch.  These devices will not cause any signal integrity issues since they are both passive FET switches with low Ron and Con parasitics.  The TS3DV642 includes a charge pump on the gate of the FET which will keep the Ron more consistence across the input voltage range while the SN74CBT16214C will is only a FET and will have a little more Ron variation across the input voltages. 

    If you are looking to mux 16 I/Os to 3 different devices and keep the design small and compact I think the 24 channel 1:3 SN74CBT16214C would be a good place to start since it has 200MHz bandwidth to cover your 125MHz RMGII signal.

    Thank you,

    Adam  

       

  • 0 in reply to Adam Torma
    Intellectual 465 points
    Hi Adam,

    Is the 200MHz bandwidth 3db bandwidth of SN74CBT16214C?
    If so at 125MHz what will be the signal loss?

    Regards,
    Bhavya
  • 0 in reply to Bhavya Babu
    TI__Mastermind 45370 points
    Bhavya,

    Yes, the 200MHz is the 3dB point of the SN74CBT16214C. You can expect less than 3dB loss at 125MHz but since this an older device I couldn't find any plots showing how much less.

    If you want a more exact number you could simulate the lowpass filter with the Ron and Con paracitics in the datasheet or we can see if the apps engineer for this device is able to measure the bandwidth of the SN74CBT16214C when he is back from vacation in 2 weeks.

    I'm sorry we don't have as much data on our older devices readily available.

    Thank you,
    Adam
  • 0 in reply to Adam Torma
    Intellectual 465 points
    Hi Adam,

    Eventhough RGMII clock is 125MHz, the data signals are of twice the frequency ie double data rate.
    Will the device SN74CBT16214C support this data rate?

    Please share the bandwidth details.

    Regards,
    Bhavya
  • 0 in reply to Bhavya Babu
    TI__Mastermind 45370 points

    Bhavya,

    If the data rate of your signal is 250 Mbps theroretically a device with 125MHz bandwidth will be able to support that signal.  However in practice we recommend designing with some margin and would like a device with >125MHz bandwidth.  200MHz device will gi ve you some margin compared to the theoretical value.   

    You can see our Frequently asked questions page on the e2e forum for more information on selecting switches based on data rates.

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    Intellectual 465 points
    Hi Adam,

    I saw the following equation as a relation between rise time of a digital signal and bandwidth:
    Rise time in ns = 0.35/(Bandwidth in GHz)

    For RGMII signal the maximum rise time is 0.75nsec.

    Using the above equation , bandwidth requirement = 0.35/0.75 GHz
    =466 MHz

    So does that mean that the minimum bandwidth requirement for this signal is around 500MHz?

    Regards,
    Bhavya
  • 0 in reply to Bhavya Babu
    TI__Mastermind 45370 points
    Bhavya,

    Yes, based on your calculation you would want a device with at least 466 MHz.

    The way we do our recommendation is 3 times the fundamental frequency. If you have a 250Mbps data rate the fundamental frequency would be 125MHz. 3x125MHz = 375MHz. Our calculation is a rough estimate to give some margin over the theoretical minimum frequency needed of 125MHz for 250Mbps signal.

    I would use which ever approximation you feel comfortable with for your system. Personally, I have never designed a complete system but haven't received any complaints from our customers when I recommend using the formula bandwidth needed = (3/2 )x datarate.

    Thank you,
    Adam