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FDC1004EVM: FDC1004EVM hardware and configuration problems

Brian Casagrande
Prodigy 10 points

Part Number: FDC1004EVM
Other Parts Discussed in Thread: FDC1004

Good morning,

I am using the FDC1004EVM and have encountered the following issues. I would like to understand how to resolve them:

  • Through the USB port of my PC, I am introducing interference (ground loop, etc.) that causes the measurement to fluctuate by approximately ±0.05 pF. This occurs when the PC is connected to the charger, while when it is running only on battery, the fluctuation is around ±0.002 pF.
  • We have developed our own sensor with a diameter of 10 mm to measure distance from a surface. In addition to the connection length, which we have already found to be very influential, we noticed that when CIN1 or CIN4 (LEFT or RIGHT sensor) starts detecting capacitance and its value increases, the capacitance readings on all other CIN inputs also change. Therefore, when connecting 2, 3, or 4 sensors, we are no longer able to obtain realistic values. The EVM configuration is with each CIN set as single-ended (SHLD1 shorted to SHLD2). Our sensor uses CIN and GND, with a shield on the PCB. Currently, sensors LEFT and RIGHT are still connected to CIN1 and CIN4 (they do not use GND).

The intended application is to connect 3 capacitive sensors to the EVM and perform position measurements in the range of 0 to 400 µm.

Thank you in advance for your support.

  • 0
    TI__Intellectual 2660 points

    Hi Brian,

    I would be happy to assist you with your issues.

    For your first issue I would recommend the following,

    • USB Isolator: Use an isolated USB hub or USB isolator (e.g., based on digital isolators) between your PC and the FDC1004EVM. This breaks the ground loop path entirely.
    • Ferrite Beads: Place ferrite beads on the USB cable to attenuate high-frequency common-mode noise.
    • Battery-Powered Operation: As you've already observed, running the PC on battery eliminates the issue. For production or bench validation, a battery-powered USB source or isolated supply is the cleanest solution.
    • Active Shielding: The FDC1004 integrates a shield driver capable of driving up to 400 pF, which provides some immunity to environmental EMI. Ensure the shield driver outputs (SHLD1/SHLD2) are properly utilized.

    As for your second issue, I believe the root cause is inadequate shielding of the sensor connections. I would recommend the following,

    • Use shielded cables for all sensor connections. Connect the shield conductor of each cable to the FDC1004's SHLD1/SHLD2 pins, not to a static PCB ground. This engages the active shield driver and prevents the sensor's electric field from coupling into neighboring channels.
    • Do not leave sensor shields floating or connected only to GND. Your current configuration (PCB shield present but sensors not using GND, with SHLD1 shorted to SHLD2) means the shield is not being actively driven. The active shield driver replicates the CIN signal on the shield conductor, which suppresses parasitic coupling and cable capacitance effects

    • When shielded twisted-pair cables are added, the nominal capacitance per channel will increase. Typically readings increased from ~10 pF to ~23 pF depending on the length of the wire. The FDC1004 supports up to 100 pF of parasitic capacitance offset (CAPDAC), so this is well within the device's compensation range. Configure the CAPDAC registers for each channel to null out the baseline.

    • Ensure adequate trace spacing on your sensor PCB. Signal traces for each CIN channel should be routed with sufficient spacing and surrounded by driven shield traces to prevent parallel capacitive coupling between channels
    • I would also recommend reading, "Capacitive Sensing: Ins and Outs of Active Shielding".

    I hope this information is able to help resolve your issue. If you require any further assistance, please let me know and I would be happy to help.

    Best Regards,

    Chase Girard