• State TI Thinks Resolved
  • Locked Locked
  • Replies 10 replies
  • Answers 3 answers
  • Subscribers 32 subscribers
  • Views 379 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

FDC2212: cannot pass the R/S project for EMC

Cherry Zhou
Mastermind 22235 points
Part Number: FDC2212
Other Parts Discussed in Thread: FDC1004, TINA-TI

Hi team,

Here's an issue from the customer may need your help:

Signals captured in the 80 to 200 MHz band are subject to interference and cannot be used. According to the demo board manual, two capacitors close to the chip pins can be adjusted for interference immunity:

1) Is it required to adjust the values of the 4 capacitors together when adjusting the capacitors?

2) Regarding the above frequency band, is it required to be adjusted up or down from the original 18 pF? And are there recommended values?

Could you help check this case? Thanks.

Best Regards,

Cherry

  • 0
    TI__Mastermind 37605 points

    Cherry,

    What is the sensor frequency?
    Are they using our EVM, or their own PCB?

    Just so you know, we are recommending the FDC1004 over the FDC2xxx.
    The FDC1004 supports active shields, which helps manage EMI.
    The FDC2xxx family does not have this feature, which causes additional EMI challenges in many applications.

    regards,
    John

  • 0 in reply to John Miller - Sensing
    TI__Mastermind 22235 points

    Hello John,

    Thanks for your support.

    John Miller - Sensing said:
    What is the sensor frequency?

    Sensor frequency is 6.5MHz, external clock is 40MHz.

    John Miller - Sensing said:
    Are they using our EVM, or their own PCB?

    They are using their own PCB.

    Thanks and regards,

    Cherry

  • 0 in reply to Cherry Zhou
    TI__Mastermind 37605 points

    Cherry,

    Thank you for the additional information.
    Can the customer provide a schematic and layout of their PCB, along with pictures or diagrams of any additional shielding they are using?

    As you might already know, the E2E will be down from Sept. 28 to Oct. 2, so this thread will be on hold during this time.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    TI__Genius 10295 points

    Hi John,

     

    The remaining layers corresponding to the PCB are all GND.

    Thanks,

    Annie

  • 0 in reply to Annie Liu
    TI__Mastermind 37605 points

    Annie,

    Thank you for the layout and schematic.
    I have an idea that might help, but it will take a day or two before I can report any results.
    As you might know the E2E will go off-line later today.
    I will close this thread for now, and we can communicate over email from here on.

    regards,
    John

  • 0 in reply to John Miller - Sensing
    TI__Mastermind 37605 points

    Apologies Annie. 
    I will email Cherry with the updates and she can pass them on to you.

    John

  • 0 in reply to John Miller - Sensing
    TI__Mastermind 22235 points

    Hi John,

    May I know is there any feedback from your side since I haven't got your email? 

    Thanks and regards,

    Cherry

  • 0 in reply to Cherry Zhou
    TI__Mastermind 37605 points

    Apologies Cherry.
    I did not follow through with an email as I promised.
    Would you prefer to continue on this thread or switch to private E2E messages?
    Regards,
    John

  • 0 in reply to John Miller - Sensing
    TI__Mastermind 22235 points

    Hi John,

    Would you like to continue on this thread since we could point the end customer to our discussion? 

    Thanks and regards,

    Cherry

  • 0 in reply to Cherry Zhou
    TI__Mastermind 37605 points

    Cherry,

    As mentioned earlier in the thread, many of our customers have struggled to get to production with the FDC2xxx devices because of their sensitivity to EMI and a lack of options for shielding.

    For this reason, we recommend using the FDC1004 which has active shields and also has a non-resonant switch-cap front end.

    The best option - other than increased shielding - is to reduce your sensor frequency so frequencies above that are attenuated more.

    Some simulations of your sensor circuit are summarized in the plot below as an example.
    The blue curve shows the results of an AC sim of the circuit from your schematic.
    The simulation shows a resonant frequency of about 5MHz, which is a little lower than you mentioned, but will serve as a point of comparison.
    The orange curve results from increasing C15 from 33pF to 135pF. This causes the resonant frequency to decrease from 5MHz to about 3MHz. 

    The thing to notice is the level at 8MHz and above decreases 11dB, which would reduce EMI in the range of 8MHz - 200MHz by at least that amount.
    You can also accomplish this by increasing the values of C13, C14, C21, C22 to 120pF (ΔC = 102pF). 
    Reducing the sensor frequency further would get you more EMI attenuation.

    The TINA-TI schematic of your circuit is attached.
    Please let me know if you have any questions.

    Regards,
    John

    SensorCircuit.TSC