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SN74HCS125-Q1: Max cable length this can drive.

Derek Jackson
Prodigy 10 points
Part Number: SN74HCS125-Q1
Other Parts Discussed in Thread: LMX2595, SN74LVC125A-Q1

Outputs from the SN74HCS125-Q1 will be going off board through a 2m, 100-ohm cable. I cannot add anything on the receiving board which has a LMX2595 receiving the signals.

Question: do I need to add a driver to the outputs of the SN74HCS125-Q1 to ensure a good signal on receiving end, or will the buffer allow enough drive? If any driver is needed, any suggestions which would work best?

Thank you for your suggestions

  • 0
    Guru 264040 points

    A strong driver would require termination at the receiver.

    Your only choice to avoid reflections at the unterminated end is to add source termination. The typical HCS output impedance is about 30 Ω, so add a series resistor of about 70 Ω. The total capacitance of the cable and connectors, together with that 100 Ω, forms a low-pass filter and will limit the maximum data rate you can achieve.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thank you. I am able to add source termination on the receiving end. What would recommend as a driver on the source?

  • 0 in reply to Derek Jackson
    Guru 264040 points

    Source termination must be at the transmitting end (see page 11 of SCAA034).

    If you can add termination at the receiving end, use AC termination. Then you can also use a stronger driver like the SN74LVC125A-Q1.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    That is exactly the information I was looking for. So, I will use the SN74LVC125A-Q1 as the driver, and add the resistor + cap. What is the advantage of using the AC configuration vs Shunt?

  • 0 in reply to Derek Jackson
    TI__Expert 6061 points

    Hi Derek,

    Shunt

    " There are several disadvantages to shunt termination. Usually the line impedance is fairly low (50 Ω to 70 Ω), which requires a resistor of similar value. This causes a heavy dc current drain on the source when in the logic high state and requires a strong line driver to source the current. With the low-impedance resistor pulling to GND, the VOH of the transmission line is lower, reducing the noise immunity at the receiver. Additionally, having a strong pull down on the transmission line might unbalance the rising and falling edges of the signal, causing the falling edge to be faster than the rising edge, resulting in duty-cycle distortion of the signal"

    AC

    "AC termination (see Figure 13) has the precise termination advantages of shunt termination, but reduces the disadvantages of dc current drain and waveform distortion. At each transition of the signal, the capacitor charges up to the voltage level necessary to maintain zero volts across the resistor. During the arrival of the next input transition, the signal drives the full value of the resistor until the capacitor can again recharge." 

    This is from that same app note that Clemens posted.

    Regards,

    Owen

  • 0 in reply to Owen Westfall
    Prodigy 10 points

    Thank you, I missed that even after printing the pages Smiley