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FDC2212: FDC2212 sensor oscillates amplitude too low.

Jack LEE
Prodigy 80 points
Part Number: FDC2212

Hello, I am using FDC2212 as my cap sensor. But currently the sensor oscillate amplitude is too low and it seems there are great noises. How can I adjust it and what can be the problem? In order to make it bigger, I have adjusted it to single channel mode, and set the HIGH_CURRENT_DRV to 1, so the current can be more than 1.5mA. But for me, it seems the amplitude is still too low. Here is the schematic and Oscilloscope image.(For the up Oscilloscope image, I measure it between two plates. For the down Oscilloscope image, I measure it between the plate and gnd. I think the 1.2-1.8V should be between two plates, am I correct?）

 

Thank you for the help

  • 0
    TI__Genius 17440 points

    Hi Jack,

    Your capacitive load is too large compared to your inductor. If you look at the Csensormax entry in the electrical characteristics table in the datasheet, the maximum capacitance is 250nF with a 1mH inductor and 10kHz sensor oscillation frequency. Your inductor is almost an order of magnitude smaller, so your maximum capacitance will be much smaller as well. 

    I recommend using a sensor capacitor around 33pF instead. This will make your system much more sensitive to shifts in the sensor capacitance as well. 

    Finally, you should measure 1.2 - 1.8 V between INxA or INxB and GND. The sensor oscillation amplitude should be 1.2 - 1.8V, not the peak-to-peak voltage between the two sensor plates. These LDC devices use a similar sensor driver and this document should help you further: http://www.ti.com/lit/an/snoa950/snoa950.pdf

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 80 points

    Thank you for the reply kristin. I use 33pf, as you can see it from the schematic for the channel 0. The image is also for channel 0. Why I couldn’t adjust it to 1.2 v to 1.8v?

    Best regards

  • 0 in reply to Jack LEE
    TI__Genius 17440 points

    Hi Jack,

    Thanks for the clarification, I was looking at channel 1. Do you have long wires or cables between the LC tank and the sensor electrodes? I also suggest bypassing the common-mode choke to see if that could be affecting your measurements.

    Best Regards,