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DS91C176: How to improve the signal quality for DS91C176

Mickey Zhang
Genius 12080 points
Part Number: DS91C176

Hi Team,

The customer is using DS91C176.  The customer uses 100ohm as the matching resistance. But the signal is not fine. Please check the attach.

The Figure 19 of the datasheet shows the relationship between the VOD and the output load. 

Q1: I would like to know what the VOD is. Is it the differential voltage between A pin and B pin or the differential voltage between R pin and RE pin?

Q2: In the figure 1 of the datasheet, the matching resistance is 80 ohm on the ATCA backplane. 

How to select the matching resistance?

Can I refer to the figure 19 of the datasheet to select the matching resistance, for example, if the VOD is 500mV, then the matching is needed 40ohm?

Q3: How to improve the signal quality for DS91C176?

Best Wishes,
Mickey Zhang
Asia Customer Support Center
Texas Instruments

  • 0
    TI__Mastermind 43795 points

    Hi Mickey,

    Q1: The VOD is between the A and B pins.

    Q2: The ATCA backplane specified an 80 Ohm termination on both ends of the backplane clock signals.  In general the termination resistance on backplanes is the same of slightly less than the transmission line impedance of the backplane traces.  In this case the trace impedance is ~ 100 Ohms and the termination resistance is 80 Ohms at both ends.  The termination is needed at both ends since there will be some devices driving the backplane from a location towards the middle of the backplane.

    Q3:  With most backplane systems, the most important feature to control signal integrity is stub length.  The distance from the backplane traces to the drivers and receivers on all the attached cards is called the stub length.  The longer this length, the worse signal integrity you will likely get on the backplane.  The larger reflections in your waveform look like stub length and or termination effects.  Sometimes adding series resistance and/or shunt capacitance can help filter the C176 output edges and reduce the system ringing.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    TI__Genius 12080 points
    Hi Lee,

    Thanks for your reply.
    Q1: Can I refer to the figure 19 of the datasheet to select the matching resistance, for example, if the VOD is 500mV, then the matching is needed 40ohm?
    Q2: Can I recommend an Equalizer or a Retimer to improve the signal quality?
  • 0 in reply to Lee Sledjeski
    TI__Genius 12080 points

    Hi Lee,

    Is there any update reply? VAB is 500mV for the customer's application, what should be the matching resistance?

  • 0 in reply to Mickey Zhang
    TI__Mastermind 43795 points

    Hi Mickey,

    Q1: This Figure is not designed to show the correct load resistance. Figure 19 shows the device output VOD into different load resistances.  At 40 Ohms and above the output amplitude is controlled internally to a relatively constant value of 500mV.  As the load resistane goes below 40 Ohms the output amplitude (VOD) will start to decrease.

    The load resistance should always be set approximately equal to the backplane characteristic impedance.

    Q2:  The waveform reflections cannot be fixed with an equalizer or retimer.  It is possible that adding series damping resistors at every load with reduce the ringing amplitude. 

    Regards,

    Lee