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DDC232 Input Shielding

Kevin Kirspel
Prodigy 155 points

Other Parts Discussed in Thread: DDC232

I'm trying to shield the analog inputs to the DDC232.  Since the package is so small it's hard to get shielding all the way to the package.  I have attached the layout of the DDC232 to the post.  U15 is the DDC232.  The highlighted pink nets are the photodiode outputs to the DDC232.  There is a solid ground plane underneath all components.  The red polygon that is surrounding the area is where the traces will run.  The shielding stops around 200 mils from the chip (on various traces) due to the congestion around the part.  How critical are the shielding traces in my layout?  The analog trace to shield separation is 10 mils.  Is this too much, not enough?  What about solder mask over the analog traces?  Will the solder mask cause leakage currents?

DDC232 Layout.pdf
  • 0
    TI__Expert 4680 points

    Hi,

    Sorry but I only have a general answer, most of it, you may know...

    In general, shielding increases noise by increasing the input capacitance, so, it is not always a good idea to get it really close to the input traces. I.e., there is no point on getting it too close.

    Nevertheless, guarding (shielding against leakage currents) may be required depending on the levels that your application can tolerate.  For 200us integration times and 150pC type full scales, you can get away without much care about it. But I have seen folks using the device in the smallest full-scale range and with 1s integration times (basically the extreme corner), and there, even the internal ibias of the device creates a noticeable shift on the output (that looks like offset). Those folks have cleaning techniques to deal with the flux, conformal coatings to deal with humidity/moisture and surround with ground (guard) DC lines that are close to the inputs, but I don't think getting that close under the IC will be needed. Notice that there is a barrier of GND balls between the inputs and the supply of the DDC232. Nevertheless, although I have not heard about solder mask issues I found this link:

    http://www.pcbdesign007.com/pages/columns.cgi?clmid=9&artid=21182

    It would be good to ask for resistivity numbers to your vendor (and compare them to those of the rest of the materials below the traces...)

    Best regards,

    Eduardo