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FDC1004 behavior for sheild

ttd
Mastermind 8580 points
Other Parts Discussed in Thread: FDC1004

Dear Technical suport Team,

I have more questions about FDC1004.

1.)
Shield and Cin are following waveform when customer scope it.
Could you please teach what behavior from "A" to "D" ?
I'd like to know behavior of each stage.

I think that from "A" to "C" means charging and "D" means that FDC1004 draws a charge.
But if it is correct operation, I don't know why it has 2step behavior(1.2V stage and 2.4V stage).

2.)
Currently, shield layer capacitance may be over 400pF.
So it seems that shield doesn't effect.
Do you have any idea to improve it?
One of idea, how is driving shield with OPAMP?
If you already have investigated it, could you tell me what OPAMP you used for?
I am searching OPAMP(voltage follower＝Gain 0dB) that can drive around 1000pF.

Best Regards,
y.i

  • 0
    TI__Expert 7400 points
    Hi:

    Sorry but I cannot discuss this since this is part of the internal architecture and behavior of the device which is proprietary. All I can say about this is that it is based on a switch cap architecture and the common mode reference voltage is 1.2V. Charging and discharging of the cap sensor will happen during the different voltage levels. Please refer to a switch cap circuit for a basic operation.

    If the shield sensor is over 400pF, you will see rounded off edges of the waveform will you probe it. The shield becomes ineffective after 400pF, so the only way to use the shield is to make sure that the capacitance load is <400pF with enough margin. Driving it with a discrete op amp is not recommended because it will not be as effective as the internal drivers due to phase delay but also BW of most op amps. Since the waveform is a pseudo square wave, the edges are very high frequency components of which most op amps may not be able to support.

    Thanks
    -David Wang
    Capacitive Sensing Applications