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Original question:

TINA/Spice/SN74AVC16T245: Output duty issue of SN74AVC16T245 spice models

TINA/Spice/SN74AVC16T245: SN74AVC16T245 test load consultation

Xiaobin Lu
Prodigy 80 points
Part Number: SN74AVC16T245
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

Dear sir，

The data book of SN74AVC16T245 shows that maximum Data Rate is 100Mbps for level-shifting to 1.2V, 200Mbps for level-shifting to 1.8V. So what's your test load？ When I connect the spice model output with a 22inch transmission line and then terminal with 3pf capacitor . The waveform quality like amplitude and duty at the capacitor side seems very bad running at 100Mbps for weak model.

  • 0
    TI__Guru* 97046 points
    Hi Xiaobin,
    That's not surprising -- a 22 inch transmission line adds massive capacitance and delays/reflections that will distort the signal very significantly. What are your parameters for the t-line?

    The test information for this device is provided in section 8 of the datasheet, Parameter Measurement Information. The load is shown in Figure 11.
  • 0 in reply to Emrys Maier
    Prodigy 80 points

    So the max datarate is under the condition of 15pf loading ?     Our t-line parameters are shown below:

    L0 (H/m) 3.240186e-07

    C0 (F/m)  1.545259e-10

    R0(ohms/m)  2.437451e+00

    G0(S/m)   0

  • 0 in reply to Xiaobin Lu
    TI__Guru* 97046 points

    Hey Xiaobin,

    Yes, 15pF loading is used for testing these devices. They are not intended to directly drive transmission lines.

    Your t-line is adding ~86pF to the output of the device and the impedance mismatch at the input and output of the t-line will result in huge reflections.  

    One work-around for this is to add a series resistor at the output of the AVC device -- a ~33 ohm resistor typically will dampen out ringing and help with the signal integrity.  There's an FAQ that goes into a great deal of detail on this topic here:

     

  • 0 in reply to Emrys Maier
    Prodigy 80 points
    hi Emrys Maier,
    Our t-line is about 45 ohm and output resistor of AVC device of weak corner is larger than 45. So I think 33ohm resistor will not help to realize the impedance matching. From our simulation result of weak corner with the loading of 5inch t-line and 3pf loading , there is no overshoot in 1.2V application. On the contrary, the amplitude already can not reach 1.2V.
    Thank you.