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FDC2214: shield implementation

Felix Lange59
Prodigy 40 points
Part Number: FDC2214
Other Parts Discussed in Thread: OPA322

Hello together,

we are working on implementing the FDC2214 for a touch sensitive button application

Regarding the implementation of the shield layer some questions are still open for us:

- We intent to use the FDC2214 sensor outputs in differential mode. Is it necessary (is it an advantage) to have separate shield signals (shield buffer)
  for RX (INA0B) and TX (INA0A) signal? Do we need for each sensor (INA0 to 3) separate shield signals to be driven?
- Are there any recommendations for the implementation of the shield buffer for FDC2214?

Goal for us is a higher sensitivity and raliability for the finger detection.

I hope you can help.

Thanks in advance and regards,

Felix

  • 0
    TI__Genius 17440 points

    Hi Felix,

    Although not necessary, it would be an advantage to use separate shield signals. Your shielding would be most effective if you use a buffered version of each INxA/INxB signal to shield the connected sensor.

    I have asked an amplifiers engineer to recommend a part that should suit your specifications (>10MHz GBW, 5V dual supply, and able to drive a large output capacitance). He recommends the OPA322: 

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 40 points

    Hello Kristin,

    this helped a lot, thank you for your quick answer and sorry for my late reply.

    Regards,

    Felix

  • 0 in reply to Felix Lange59
    Intellectual 420 points
    Hello;

    OPA322's slew rate is 10V/us. So Output voltage wil be distorted.
    Is that proper for 10mHz? I think no.

    FPBW = SR/(2*pi*V)
    So A opamp must be chosen which slew rate is 120V/us.

    Am I right?

    Best Regards.
  • 0 in reply to user5866401
    TI__Genius 17440 points
    Hello,

    SR = Vp*2*pi*f, where Vp is the sensor oscillation amplitude and f is the sensor frequency. The absolute maximum slew rate you could need is 113V/us, but this is assuming that your sensor frequency is at the maximum that the device can support (10MHz) and the sensor is oscillating at its maximum amplitude (1.8V). You should never set your sensor frequency to 10MHz, partly because you will have higher resolution at lower frequencies and partly because any increase in sensor frequency couldn't be measured by the device.

    Though you are right that the OPA322 will not work for all possible sensor configurations, its slew rate should work for many sensors configurations. If you keep your sensor oscillation amplitude at 1.5V and in the upper KHz range, the 10V/us slew rate should work for you. You can change your inductance value to shift the sensor frequency. If for some reason you need to have a higher sensor frequency, please feel free to post again and we can help you select an op amp with a higher slew rate.

    Best Regards,