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FDC2214: active shield with an external op amp

Mabel
Prodigy 70 points
Part Number: FDC2214
Other Parts Discussed in Thread: FDC1004

Hi,

I'm doing a capacitive sensing project with FDC2214 and trying to design a circuit for an active shield with an op-amp.

In this advice, https://e2e.ti.com/support/sensors/f/1023/t/641992 the input signal to drive the op-amp is the FDC2214 signal after the LC tank.

However, in  FDC2214 datasheet, 10.1.2 Shield, it says that the shield is a buffered signal of the INxA pin. It seems that the input signal to drive the op-amp is the FDC2214 signal before the LC tank. And I guess this means the following circuit.

Which method is correct to design an active shield circuit with an op-amp? And what's the difference between these two methods?

Thank you,

Mabel

  • 0
    TI__Expert 3435 points

    Hi Mabel,

    As explained in the previous E2E, We recommend the FDC1004 device which has the shield drivers integrated into the device if you are looking for active shielding.

    After going through the E2E forum we were not successful using an opamp for active shield using FDC2214 devices. Let me know if FDC1004 works for your application.

    Regards

    Sharath.

  • 0 in reply to Sharath Vallamkonda
    Prodigy 70 points

    Dear Sharath,

    I know that FDC1004 has shield drivers, but we have to use FDC2214 for our application.

    I re-designed the op-amp circuit by adding resistance to the circuit in the first and second figures. Although the sensor performance was slightly better with the first figure method, both of them worked well.

    The following figure is the signal from FDC2214 (red) and the output signal from op-amp(yellow).

      

    As the FDC2214 datasheet suggests, there must be a method for the active shield with an external op-amp.

    Then, in FDC1004, to drive the SHLD pin with the same voltage as a CINx, how are they related?

    Thank you,

    Mabel 

  • 0 in reply to Mabel
    TI__Expert 3435 points

    Hi Mabel,

    To reduce the interference from the external objects certain applications require additional sensor which acts as a shield when driven with the same excitation voltage as CIN. We recommended this method with FDC221x devices using external op-amp. The FDC1004 device, a later generation device integrates this feature in it. 

    As you achieved a very good shield signal with an opamp, you can expect better performance for environmental disturbances. 

    Regards

    Sharath.