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FDC2214 active shield

ttd
Mastermind 8050 points
Other Parts Discussed in Thread: FDC2214, FDC1004, BUF634

 Dear Technical Support Team,

I have a question about FDC2214.

Do you have some information of external amp for shield?

What type of opamp do you recommend?

Datasheet shows below, but it doesn't have the detail.

----------------

in order to minimize interference from external objects, some applications require an additional plate which acts
as a shield. The shield can either be:
• actively driven shield: The shield is a buffered signal of the INxA pin. The signal is buffered by an external
amplifier with a gain of 1.

-------------------

Regards,

ttd

  • 0
    Prodigy 70 points

    I got into problem on my side with the EVM and active shield buffer... 

    There is absolutely no spec on kind of input we need... Bias current seems to be a problem!!!

    And my buffer go into oscillation too!

  • 0
    TI__Expert 6480 points

    Hi ttd,

    We had someone perform an experiment using BUF634 which seemed to work. As long as the bandwidth and slew rate are high enough, any unity gain opamp should do the trick.

    Thanks,

    Rachel

  • 0
    Prodigy 240 points

    FYI: We had to change out design from the FDC2214 to use the FDC1004 to get the shielding to do what we needed.   We needed to measure "change in capacitance", not proximity effects.

  • 0 in reply to Rachel Liao
    Mastermind 8050 points
    Hi Rachel,

    Thank you for your reply.
    What sensor do you use with BUF634?
    Do you use it on INxA like datasheet?
    Does it mean single end configuration on datasheet?

    • actively driven shield: The shield is a buffered signal of the INxA pin. The signal is buffered by an external amplifier with a gain of 1.

    FDC1004 has shield driver pin and it can drive shield cable or layer.
    So I couldn't image sensor(single end of differential) with active shield buffer.

    Regards,
    ttd
  • 0 in reply to ttd
    TI__Expert 6480 points
    Hi ttd,

    The idea of using an active shield with the FDC1004 is given in the following app note: www.ti.com/.../snoa926a.pdf. In terms of using the FDC2x1x series, the idea is the same except that the shield drivers need to be external buffers. You would drive both the sensor and the shield electrodes with the same signal in order to remove the parasitic capacitances to ground.

    Thanks,
    Rachel
  • 0 in reply to Rachel Liao
    Mastermind 8050 points

    Hi Rachel,

     Thank you for you reply.
     Can IN0A and IN0B drive both and shield driver?

    Should we preare two BUF634 and two channel of FDC2214 for sensor and shield driver like below?

    Should both phase be same?

     IN0A and IN0B ⇒　Sensor
     INA1A and IN1B　⇒　Shield

    PS. following is mention for FDC1004 external opamp, but engineer didn't recommend it.

    https://e2e.ti.com/support/sensor/capacitive-sensing/f/987/p/454700/1638320#1638320

     Regards,
     ttd

  • 0 in reply to ttd
    TI__Expert 6480 points
    Hi ttd,

    Yes, either IN0A or IN0B can drive both the sensor and the shield driver and they should be in phase. The FDC1004 already has internal shield drivers and so we would only recommend external shields for the FDC2x1x series.

    Thanks,
    Rachel
  • 0 in reply to Rachel Liao
    Mastermind 8050 points

    Hi Rachel,

    Thank you for your reply.

    Which ways are correct for shield driver ?

    Should I use two channel like case2 or only one channel like case1?

    >Yes, either IN0A or IN0B can drive both the sensor and the shield driver and they should be in phase.

    ⇒Dose it means case2?

    Best Regards,

    ttd

  • 0 in reply to ttd
    TI__Expert 6480 points

    Hi ttd,

    IN0A can drive both the sensor and the shield so technically, you would only need one external opamp per channel:

    Thanks,

    Rachel