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ADS52J90EVM: discrepancies with design files and ADS52J90 datasheet.

Kurtis Nishimura
Prodigy 10 points
Part Number: ADS52J90EVM
Other Parts Discussed in Thread: ADS52J90,

Hello,

We have a few questions about the ADS52J90, specifically related to discrepancies between data sheet recommendations and what seems to be implemented on the ADS52J90EVM evaluation board:
1) The most striking thing is that there seem to be few or no bypass capacitors placed near the ADS52J90 for any of the power input pins, which seems in direct contradiction with the advice from the datasheet, section 11.1, which suggests: 
"Bypass all supply pins with 0.1-μF ceramic chip capacitors (size 0603 or smaller). In order to minimize the lead and trace inductance, the capacitors must be located as close to the supply pins as possible. Where double-sided component mounting is allowed, these capacitors are best placed directly under the package. In addition, larger bipolar decoupling capacitors (2.2 µF to 10 μF, effective at lower frequencies) can also be used on the main supply pins. These components can be placed on the PCB in close proximity (< 0.5 inch or 12.7 mm) to the device itself."
Can anyone comment on why/how the evaluation board gets away without these components?  We're presently following the datasheet guidelines but obviously these components take up a lot of valuable real estate, so if they are somehow unnecessary, or at least not as many are required as suggested in the datasheet, that would be great to know.
2) In the ADS52J90EVM, the input LVDS clock to the device is AC coupled using 0.1 uF capacitors, but the datasheet recommends a 10 nF AC coupling capacitor in Figure 82.  Is there any compelling reason to use one or the other, here?  We're presently following the EVM design as opposed to the datasheet.
3) Similar to 2, the ADS52J90EVM uses a 100 ohm termination resistor on the input LVDS clock, but this is not called out in the datasheet as far as I can tell.  This does not appear in Figure 82, but by Figure 80 it does appear that the inputs are rather high impedance.  What is the official recommendation here?
4) More a remark than a question, but we noted that the schematic symbol obtained through the ADC page:  http://www.ti.com/product/ADS52J90/pinout-quality  has some pin names that are inconsistent with the datasheet (namely the power pins, pins D6, B6, E5).  These are not a major issue, but it does appear that the "official" files are in need of an update.
Thanks for your help!
-Kurtis Nishimura
  • 0
    TI__Expert 4145 points

    Hi Kurtis,

    Bypassing supply pins with bypass capacitors is generally used to reduce current loop area and the inductance in current return path. It is commonly used to reduce losses in a power supply besides other benefits. The ADS52J90EVM plugs into a wall socket so power efficiency is not a consideration in its design.

    As far as clock termination is concerned, your clock source and logic level determine the type of termination circuit to use. Most clock buffers and clock distribution ICs recommend specific termination circuits for their devices. The datasheet just shows a generic AC coupling capacitor and is not meant to depict clock termination for all possible clock formats/logic levels.

    Regarding the schematic symbol, please use the datasheet pin map, names and mechanical data to create an updated symbol as the device datasheet has gone through multiple revisions since that symbol was created.

    Sincerely,

    Olu