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TXB0304: The internal circuit of the TXB0304 chip passes through a 1K resistor and then outputs it, causing an impedance mismatch in the high-speed circuit.

bl z
Prodigy 10 points
Part Number: TXB0304
Other Parts Discussed in Thread: TXU0304, TXU0104, TXU0204

The internal circuit output of the TXB0304 chip needs to pass through a 1K resistor, which will cause a resistor-impedance mismatch during high-speed signal conversion, causing serious reflections. If a 50R resistor is connected to the load terminal, the load voltage will be very small due to the voltage division relationship. During the circuit test, due to the presence of the 1K resistor, there was indeed serious signal reflection and ringing. The TXB0304 chip seems not to be used in high-speed signal conversion, which is inconsistent with the description in the data sheet.

  • 0
    TI__Mastermind 24980 points

    Hi Bl,

    Note that the device selected is recommended for a specific niche of applications similar to section 8 of the data sheet.

    However, transmission line effects leads to impedance matching with the recommendation for series damping resistors at the output. 
    Help see FAQ] What happens when I connect a logic device's output to a 50 ohm transmission line? for more information. 

    Furthermore, if your signals per channel are in fact not auto bidirectional (within the niche of the referenced section 8), the TXU device family can be a better recommendation. Please see TXU0104 / TXU0204 / TXU0304, thanks. 

    Best Regards,

    Michael. 

  • 0 in reply to Michael Ikwuyum
    Prodigy 10 points

    Hi Michael,

    According to section 6.6 of the data sheet, the data rate of the chip TXB0304 can reach 100Mbps, but its output drive strength is a 1K resistor, which means that it is not feasible to connect a damping resistor in series at the output end, and the signal source is still mismatched. Signal reflections will still bounce back and forth on the transmission line.

    Best Regards,

    BL

  • 0 in reply to bl z
    Guru 256450 points

    The TXB has edge accelerators that short the output resistor when switching, so its output impedance is much lower.

    The TXB must not be used with long traces or cables that have a noticeable capacitance because this would prevent the edge accelerators from working. Furthermore, such capacitances can cause ringing, which might be amplified by the edge accelerators and result in oscillations.

    Most logic devices are capable of driving a line with a characteristic impedance of 50 Ω, but they will not have enough drive strength for an actual 50 Ω resistor. What is the actual problem you're trying to solve? What voltage drop do you require over the 50 Ω resistor?

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    I want to connect the output signal of TXB0304 to the oscilloscope through the RF connection cable for testing, but the signal reflection is very serious during the test. When the terminal resistor of the oscilloscope is set to a 50R resistor, its voltage is only 75mV.

  • 0 in reply to bl z
    Guru 256450 points

    What is the purpose of your testing? Neither the oscilloscope cable nor the termination have characteristics similar to a real circuit.

    Use a high-impedance input with as small a capacitance as possible.