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SN74CBT3257C: output waveform abnormal

Henry Wang3
Expert 2855 points
Part Number: SN74CBT3257C
Other Parts Discussed in Thread: TMUX1574

Hi Sir,

My customer used a signal generator to catch the output waveform on the SN74CBT3257C without damping resistors.

They found when the A port is connected to the B port, why another B port is not isolated and still catching the output waveform?

Could you explain this failure phenomenon?

Chanel 1 is B2 port

Chanel 4 is B1 port (S1 = L, A port = B1 port)

  • 0
    TI__Genius 10974 points

    Hi Henry,

    All multiplexers can exhibit what is called "crosstalk" when operating at higher frequencies. Essentially, crosstalk is defined as the coupling of an unwanted signal on an OFF channel from an ON channel. How much signal appears on this OFF channel depends on a few factors including the layout of your board and the frequency the signal being passed is at. In this case, 50MHz seems to be enough on the board you are working with to show some coupling on an OFF channel, hence seeing some of the signal appear there.

    Let me know if you have further questions on this.

    Thanks!

    Bryan

  • 0 in reply to Bryan Marshall
    Expert 2855 points

    Hi Sir,

    Thank you for the prompt reply!

    My customer also tested the PI5C3257 on the same board, they found only the SN74CBT3257C has crosstalk phenomenon shown as below figure. [I2C_SCL(Pericom) => PI5C3257, I2C_SCL(TI) => SN74CBT3257C]

    Do you have suggest devices which can to replace it?

    Does the datasheet have any parameters which can identify the Signal isolation capability?

    The parameter of Voutu on the D/S, the MIN is 2V, it means the VOL? the VOH means the 5.5V?

  • 0 in reply to Henry Wang3
    TI__Genius 10974 points

    Hi Henry,

    Our recommended SPI mux is the TMUX1574 which will have much better performance as it is a much higher bandwidth rated device. Furthermore, this device also has a crosstalk specification you can look at in the datasheet at well (application curve).

    Lastly, the specification you are looking at describes the undershoot protection the SN74CBT3257C has. It is basically saying that the output will be clamped to the output voltage - 0.3V when an undershoot event occurs.

    Thanks!

    Bryan

  • 0 in reply to Bryan Marshall
    Expert 2855 points

    Hi Sir,

    Thank you for the prompt reply!

    About the figure.23 Off Isolation and Crosstalk on the TMUX1574 datasheet. Is it mean the attenuation of Channel-to-Channel Crosstalk is better than the attenuation of the off-isolation, right?

    The attenuation of Channel-to-Channel Crosstalk on the adjacent channel (ex: S1A, S1B), it's the same as the figure.23, right?

  • 0 in reply to Henry Wang3
    TI__Genius 10974 points

    Hi Henry,

    In Figure 23, it is showing both the crosstalk and off-isolation in a single graph. I would not necessarily be comparing them directly and saying one is better than the other as they are describing different things. In absolute terms, yes, the Crosstalk has better attenuation performance in relation to off-isolation which is what you should care about in your application where having low crosstalk is important. Lastly, the adjacent channel to channel crosstalk is the same as described in the graph, just showing across the frequency range.

    Thanks!

    Bryan