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AMC3330: PCB Layout

Ignacio de Mendizabal
Prodigy 80 points
Part Number: AMC3330

Hello TI team,

I am working on the design of a board to measure tensions, from low voltages to high ones. This board includes the AMC3330DWER. I try to understand the layout recommended in your datasheet, but the fact of passing critical signals under capacitors C4, C5, as well as under C8, C9 is counterintuitive for me since it is possible to induce noise over the tracks, even being in differential mode. Is it not better to pass the critical signals over the bottom layer? Can you please explain the reasons for this kind of layout? 

ACM3330 Recommended layout

Thank you!!

  • +1
    TI__Guru 55152 points

    Hi Ignacio,

    Thank you for your question! 

    The main reason we recommend to design in this way is space savings on the PCB. The input common-mode rejection ratio is typically high enough that the noise induced to the tracks from these capacitors does not have a large impact on the measurement. Similarly, if an opamp is used for differential to single-ended conversion of the outputs as discussed in this document: https://www.ti.com/lit/an/sbaa229/sbaa229.pdf

    If the device will be located in a harsh environment or will undergo EMI testing, please see this document : https://www.ti.com/lit/an/sbaa515/sbaa515.pdf, which also shows an alternative high side layout in figure 4-2 and provides guidance on ferrite bead/CMC selection - if necessary. 

    This excel calculator quantifies the expected accuracy of the AMC3330, which may help during your testing of the board later on: https://www.ti.com/lit/zip/sbar013

    Please let me know if you have additional questions. 

  • +1
    Guru 140200 points

    Hi,

    I'm sure that the design would profit from having local ground planes at the input side and the output side of AMC3330 with ground fills on the top side. A ground plane on the bottom layer in combination with a ground fill on the top layer provide an otimum shielding. By this the majority of electrical field lines will land on the ground planes and ground fills and only a fraction of them will land on signal traces and components. That's how shielding works Relaxed

    At the same time the ground integrity is enhanced by using local ground planes.

    Provided no huge currents flow across the ground plane, using ground planes is the very best remedy against all sorts of EMI.

    Or course, as input to output stray coupling can be an issue here, I would not recommed huge ground planes of arbitrary area at both sides of the AMC3330. Just as large as necessary to have all relevant components covered, just as large as the local ground plane on the output side of AMC3330.

    Kai 

  • 0 in reply to Alexander Smith
    Prodigy 80 points

    Hello Alexander,

    Perfect! Now it is quite clear! Thank you very much for your detailed reply as well as for all the documents you sent!

  • 0 in reply to kai klaas69
    Prodigy 80 points

    Hi Kai,

    Yes, both input and outputs are covered with ground planes, so no problem. Also, I paid attention and I did not make them too big to avoid unwanted induced current. Thank you very much for your reply!

    Ignacio