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DS280BR820: Signal-detect Parameters Vsdat and Vsddt

Daniel Jackson
Intellectual 575 points
Part Number: DS280BR820

The DS280BR820 datasheet (September, 2016) indicates on p.9 that the Vsdat AC signal detect assert (ON) differential voltage threshold level is the "Minimum input peak-to-peak amplitude level at device pins  required to assert signal detect. 25.78125 Gbps with PRBS7 pattern and 20 dB loss channel."  The Vsddt parameter is its de-assert (OFF) companion parameter, similarly worded.  Both make reference to the level "at the device pins", yet also specify a 20dB loss channel.  If the amplitude of importance is that amplitude present at the device pins, what difference does it make what characteristics the channel might have?  For that matter, what difference do the precise datarate and signal pattern make, since the signal detect circuitry is presumably sensitive mostly/only to the (lower-frequency) behavior of the energy content of the signal?

  • 0
    TI__Guru*** 146195 points

    an engineer is assigned for this issue and will reply soon

  • 0
    TI__Mastermind 43795 points

    Hi Daniel,

    There is also some sensitivity to the data pattern itself.  In this case 8B/10B or PRBS7 have less very low frequency information then patterns like 128B/130B or PRBS31. By placing the measurement and specification at the receiver pins it removes channel attenuation and transmit amplitude variables from our specification. 

    This is a relatively easy simulation to run based off the minimum transmit amplitude which would be driving the channel.  Then the pass-fail determination is simply the waveform envelope at the DS280BR820 input.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    Intellectual 575 points

    Lee,

    a) Do patterns such as 128B/130B or PRBS31 tend to cause the apparent signal level to shift up versus down (compared with patterns such as 8B/10B and PRBS7) relative to the programmed signal detect threshold?

    b) The Programmer's Guide in Table 21 indicates the lowest-selectable signal detect assertion threshold to be 110..305mVppd.  With the Datasheet indicating the level is to be measured at the device pins, and also specifying use of a 20dB loss channel, that would appear to require use of a signal source having an amplitude of no less than 1100..3050mVppd, which would be either questionable, impractical or impossible.

  • 0 in reply to Daniel Jackson
    TI__Mastermind 43795 points

    Hi Daniel,

    Sorry for the delay.

    a. All of these patterns are DC balanced.  The signal level should stay balanced for all of them.  The 8b10b and PRBS7 patterns will have less low frequency content.  This will decrease the maximum amplitude at the DS280 input pins.

    b. 

    Table 21 in the programming guide has an Assert Range of 110 - 305 mV

    Table 22 in the programming guide has a Deassert Range of 45 - 145 mV

    This gives some hysteresis between the turn-on and turn-off thresholds to ensure smooth operation of the function.

    The 20dB channel is referenced at ~ 13 GHz. For the low frequency components of the signal the loss will be much less than 20dB.  Typical generator amplitudes are 400 - 1200mV peak to peak.

    Regards,

    Lee