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I have built a capacitance measuring system using the Texas Instruments FDC2214EVM evaluation module. The system is used to monitor the location of the user's hand in an enclosed space, where capacitance is mapped on to displacement.

Operating Principle

The FDC2214 device measures capacitance by determining the resonant frequency (40MHz or less -- in my case, roughly 5-10MHz) of an LC tank circuit connected to a capacitive sensor. As the sensor capacitance varies due to proximity of the user's hand, so does the resonant frequency of the circuit. Because all other parameters of the LC tank circuit are known, the resonant frequency of the circuit can be used to calculate the sensor capacitance, and therefore displacement.

System Configuration

System Block Diagram

Four capacitive sensors are connected to the FDC2214EVM -- one for each of the evaluation module's four inputs. Each sensor is a 5cm x 5cm piece of copper tape mounted on an acrylic plate. Each sensor is connected to the FDC2214EVM using 1m of coaxial cable.

At the FDC2214EVM-end of the coaxial cable, two jumper wires (small segments of 22759/32-22 wire terminated with Mini-PV connectors) are soldered to the conductor and shield, respectively, and plugged into female headers on the FDC2214EVM for sensor input and ground, respectively.

FDC2214EVM-end of the coaxial cable -- terminated with jumper wires.

FDC2214EVM with female headers. Ignore the torn-out SMD inductors -- this was an old test setup.

At the Sensor-end of the coaxial cable, one jumper wire is soldered to the conductor and plugged into an inline Mini-PV receptacle. On the other side of this inline Mini-PV receptacle is a wire that is soldered to the copper sensor.

Sensor-end of coaxial cable. Mini-PV plug-receptacle assembly and coaxial jumper wire are partially obscured/not shown. The jumper wire is the same as those used to connect the FDC2214EVM input and ground, as shown in the first image of this post.

Question

I am considering replacing the Mini-PV inline plug-receptacle assembly between each coaxial cable and each sensor. I am concerned that the use of these connectors might lead to reliability issues, because of:

  1. Changes in connection quality after many mating cycles
  2. Loosening of the connection due to mechanical disturbance

Ideally, the chosen connectors should not affect the noise-level of the capacitance signal. If a connector introduces a constant offset capacitance, this is manageable, as the offset can simply be removed during data processing.

I have decided that, because the capacitive sensors will be removed and stored each time the system is used, the chosen connectors should:

  1. Be crimp connectors, which are, to my understanding, more tolerant of mechanical strain.
  2. Be latched, to prevent loosening due to tugging, etc.

I am exploring the option of using Molex Mini-Fit Jr connectors (see my associated Digikey cart: https://www.digikey.com/short/ztdzv4) instead of the Mini-PV connectors. They meet both of the requirements above, and also have a relatively cheap manufacturer-approved crimp tool, unlike many other connectors. That being said, this sort of connector is often used for power supplies and the like, which is very different than my application.

I am curious to know if anyone has any advice regarding which type of connector would be best for this capacitive sensing application.

Thank you, Dylan

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    \$\begingroup\$ A picture would help and give me no excuse in not trying to decipher the words. Draw a system picture showing connectors. It’ll help loads and might allow you to see ways of avoiding some connectors. \$\endgroup\$
    – Andy aka
    Jan 28, 2020 at 19:11
  • \$\begingroup\$ @Andyaka I've added some pictures. Thanks for the input. \$\endgroup\$
    – Dylan B.
    Jan 28, 2020 at 19:46
  • \$\begingroup\$ So, just in case you can, decide what you can get rid of. \$\endgroup\$
    – Andy aka
    Jan 28, 2020 at 20:28
  • \$\begingroup\$ @Andyaka The coaxial cable should be removable, since I am still in the process of comparing different cable options and sensor geometries. So, for the time being, I can't avoid the need for connectors. My goal is to maximize the reliability of the connections so that I can meaningfully evaluate my cable and sensor alternatives. Once I've settled on a design, I will likely remove the connectors entirely. \$\endgroup\$
    – Dylan B.
    Jan 29, 2020 at 13:05

1 Answer 1

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  • The mating cycles seldom affected the touch performance in our studies where we were working with the TI captivate sensors.
  • Rhe mating cycles wil definitely I created the contact resistance of the foil or the whole sensor connection collectively
  • The variation in the resistance will not significantly impact the quality of the capacitive sensing application in terms of sensitivity
  • Go for a gold plated connector (FPC or even plug receptacle type) for almost no variation in the sensitivity
  • According to my data, change in temperature and humidity will have significantly higher effects than the small change in the contact resistance
  • I would suggest to note down the variation when the same experiment is repeated multiple times. Later, you can manually increase the series resistance by a few 10s lot Ohms and see the variation.
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