FDC2214: Settling time, inductor size and EVM LED function

Part Number: FDC2214

The initial testing with the FDC2214 went well and we're moving forward with some proof-of-concept prototypes with a lash-up of EVBs for the next stage of development.

We're using them in differential mode as shown in figure 55 of the datasheet.

A few questions:

    1. Inductor size is a concern, and we would like to find some physically smaller parts than the Bourns CMH322522-180KL on the EVB. Does your application team have any suggestions? We are oscillating at 5.2 - 5.6MHz, so there appears to be room to reduce the inductance value and still stay below the 8.75MHz recommendation for a Fin divider value of 1. The sensor capacitance is quite low, total capacitance with the sensor is ~52 - 56pF.
    1. I'm seeing the drive current setting drift over time, it varies from 9 to 14 when using the 'Detect Idrive init' function in the EVB GUI. Is this normal? Do most customers just pick a value in the midrange, or is some sort of auto-tuning recommended? We are trying to pick up very small capacitance changes, full span is about 0.05pF.
    1. What is the function of the LEDs on the EVB? There doesn't seem to be any description in the EVM user's guide. Green seems to mean that all is well. When we moved to differential sensing, when powering up an EVB, it comes up with the red LED illuminated. If we unplug our sensor and power up, the LED is green, and it stays green after connecting the sensor. Any idea what is going on here? Is it related to the EVB powering up with the default settings and they're causing an overflow or error condition at initial start?

Best Regards, Mark P

  • Hello Mark,

    We tested a few types of inductors and the CMH322522 inductors were the best shielded ones we found. For these surface mount shielded inductors, the noise performance is material and manufacturing-process dependent. The customer can verify the noise performance of a particular type of inductor by soldering it to the EVM and checking the peak-to-peak noise of the data stream. 

    The initial idrive is not so important. We recommend setting bits 12 and 10 in Register 0x1A - CONFIG to b01 (turning off auto-amplitude correction), and setting the current drive so that the oscillation amplitude of the LC resonator is around 1V to 1.8V.

    The LED indicates if the oscillation frequency is close to the default of the EVM. It is not related to any error condition.