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FDC2214: Soft robotic sensor capacitive sensing application

Preston Peranich
Prodigy 40 points
Part Number: FDC2214
Other Parts Discussed in Thread: LDC1614

Hi, I am developing a solution to measure the capacitance of a StretchSense soft robotic sensor (SRS) (see: https://stretchsensed.wpengine.com/wp-content/uploads/2019/03/SSD18-Datasheet.pdf). I had a couple questions about the FDC2214 for use in this application. 

1: In the FDC2214 datasheet, there is a clause under the "Do's and Don'ts" section that states that hot-swapping sensors should not occur. I would like to retain the ability to connect and disconnect the SRS without powering down the IC (the SRS is connected with standard 0.1" pitch headers). Does this classify as hot-swapping? Are there any workarounds?

2: To ensure we can sense all values of capacitance, I would like the design to be capable of sensing 200pF-2000pF. Is this a reasonable range for the FDC2214? Would properly selecting the LC resonator components enable this range? What resolution would we expect for such a range?

Thanks,

Preston Peranich

  • 0
    TI__Mastermind 23810 points

    Preston,

    1. Hot-swapping in this context means that the sensor connections should not be disturbed while the FDC2214 is exciting the sensor. If you can control the timing of connecting/disconnecting the SRS, then you can put the FDC2214 into sleep mode without completely turning off power.

    2. It should be able to support that range of capacitance. Please note that the most successful applications for the FDC2214 involve relative measurements. You may find the accuracy less than desired, even though the resolution may be acceptable. You should be able to test your application pretty easily using the EVM before you get too far into the design process. Please see this app note that discusses resolution for the LDC1614. The principles are identical for the FDC2214.

    Also, please use an external oscillator for this application (the internal oscillator will add quite a bit of noise).

  • 0 in reply to Clancy Soehren
    Prodigy 40 points

    Hi Clancy, thanks for the response! One more question: we intend to read the capacitance of the sensor at a rate of up to 300Hz. Is this possible with the FDC2214?

    Best,

    Preston

  • 0 in reply to Preston Peranich
    TI__Mastermind 23810 points

    Yes, it is possible for a 300Hz update rate. Keep in mind though that there is a correlation between increasing the sample time and decreasing the noise. Sampling that fast will limit the effective resolution. The app note I linked to mentions this. 

    Increasing RCOUNT lengthens the conversion time which improves resolution but decreases the sampling rate.