This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

MSP430FR2533: Radiated immunity mutual capacitance sensors

Part Number: MSP430FR2533

In the CapTIVate technology guide is stated Class A immunity with mutual capacitance buttons is expected to work at 3 VRMS.

We would like to make a solution based on mutual capacitance buttons which has to be fully functional at 10VRMS.

Is there any experience if this is reachable.

We did some testing with the CapTIVate EVM and adding a ground plane improved the immunity from radiation.

I hope someone can share their experiences making mutual capacitance more immune or confirm 10VRMS immunity is actually reached  with mutual capacitance buttons.

  • Hi Tom,

    I've asked out CapTIvate experts to look into this for you. They'll be getting back with a response soon. Thank you for your patience.

    Best regards,
    Caleb Overbay
  • Hi Tom,

    Thank you for evaluating the MSP430FR2533 device.  Let me start by clarifying whether you are testing conducted immunity or radiated immunity.  You post mentions radiated immunity, but the stress levels (3Vrms, 10Vrms) are conducted immunity stress level specifications.  Typically conducted immunity stress levels are specified in RMS volts, whereas radiated immunity tests are specified in electric field strength (volts per meter).  Typically for a capacitive sensing interface, the more difficult test to pass is conducted immunity for a line-powered application.  The distinction between the tests is based on the frequencies being tested, as in the 100kHz-10's of MHz range the antennas needed would be prohibitively large- so the noise is coupled to the equipment under test via its power cabling directly.  So, the better question is are you testing frequencies below 80 MHz via a coupling network, or above 80 MHz via an antenna?

    From a conducted immunity point of view, the 3Vrms guidance is based on IEC 61000-4-6 with noise coupled directly to the MCU DC power supply via a coupling decoupling network.  10Vrms is achievable if you have a decent quality power supply that helps to block common mode current.  The difference in performance between direct DC and AC supply injection points can be quite significant.  In addition to the power supply, other factors such as layout and electrode design do come into play.

    I recommend that you review our EMC reference design- there is a lot of really good material in there, and a lot of detailed test data that illustrates how mutual-mode noise immunity varies with different power supply topologies.  Please let me know if you have any questions and I would be happy to answer them.

    If you are asking about radiated emissions specifically and not conducted emissions, then the question is whether the design can tolerate a 10 V/m electric field in the higher frequency range.  CapTIvate, by nature of its charge transfer mechanism, looks a lot like a low pass filter.  The electrode impedance combined with the IO capacitance forms a low-pass filter that attenuates higher frequency noise.  Additionally, we have frequency hopping features that improve performance.  Ultimately, a PCB layout designed for noise immunity is critical.  This is where the addition of ground shielding as you mentioned becomes important.  If you review the above mentioned reference design, you will find a 32-button mutual capacitance layout that utilizes solid ground planes for shielding.  This adds parasitic capacitance, but drastically improves noise immunity.  In addition, we populate discrete 68pF capacitors between each RX and circuit ground near the MCU to reduce the susceptibility band in frequency.

    Many of the principles discussed in the document above for conducted immunity also apply to radiated immunity.


**Attention** This is a public forum