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TM4C1294NCPDT: Regarding hardware design around chassis ground

Sheldon He
Expert 8585 points

Part Number: TM4C1294NCPDT

Hi expert,

My customer is designing TM4C and have some questions regarding hardware desgin for chassis ground here.

In spmu365c.pdf, there are 1nF, 4.7nF capacitor and a 1Mohm resistor. 

Q1: For these three values mentioned above, are they coming for general test verification and selected with experience or are they coming form any theoriotical calcualtion? (If there are any calculation, please provide to us)

Q2: For package of the resistor, shaw we use DIP package instead of a surface mount one? Please provide us with some comments here,

Thanks

Sheldon 

  • 0
    TI__Guru*** 135355 points

    Hello Sheldon,

    Sheldon He said:
    In spmu365c.pdf, there are 1nF, 4.7nF capacitor and a 1Mohm resistor. 

    First off, need to separate these slightly. The 1000pF cap displayed is not for the Chassis ground, that is for the "Bob-Smith" termination for the center tap of the isolated windings. The 1000pF value comes out of our System Design Guidelines documentation, see Section 4.1.2 on Page 34: https://www.ti.com/lit/pdf/spma056

    With that explained, the remaining values in question here are the parallel pair of 4700pF and 1MOhm.

    Sheldon He said:
    Q1: For these three values mentioned above, are they coming for general test verification and selected with experience or are they coming form any theoriotical calcualtion? (If there are any calculation, please provide to us)

    These are best practice values based on our experience for best EMC and noise isolation on our EVM boards. The 1MOhm seems to be at least somewhat somewhat standard as it comes up with these designs in general.

    While it doesn't address those components specifically, here is another resource that discusses chassis ground design: https://www.ti.com/lit/an/snla107a/snla107a.pdf

    Sheldon He said:
    Q2: For package of the resistor, shaw we use DIP package instead of a surface mount one? Please provide us with some comments here,

    I am not sure where you are seeing this, the resistor is a surface mount both on my hardware and on the LaunchPad BOM: ERJ-8GEYJ105V