Texas Instruments FDC2214 Capacitance to Digital - Review

Table of contents

RoadTest: Texas Instruments FDC2214 Capacitance to Digital

Author: dougw

Creation date:

Evaluation Type: Evaluation Boards

Did you receive all parts the manufacturer stated would be included in the package?: True

What other parts do you consider comparable to this product?: I have designed custom circuitry in the past to perform this function

What were the biggest problems encountered?: I had no problems at all - the software worked intuitively and the evaluation module worked flawlessly.

Detailed Review:

FDC2214EVM Road Test

The FDC2214 is a very high performance 4 channel capacitance measuring chip with up to 28 bits of resolution and a noise floor down around 0.3 femtofarads.

It also implements some clever techniques to minimize external EM interferences and noise and it features an I2C interface (@400kb/s) for easy connection to most microcontrollers.

Their innovative technique of creating a resonant tank circuit with the capacitance being measured allows this narrow band oscillation to be largely immune from the influence of other (noise) frequencies.

The evaluation module is actually 4 circuit boards all connected by thin sections of PCB which may be severed without tools to separate the cards.

The first card is a TI MSP430F5528IRGC MCU. (http://www.ti.com/product/MSP430F5528)

This is a 16 bit, 64 pin VQFN MCU (running with a 24 MHz crystal) with integrated USB, 128KB FLASH, 8 KB RAM + 2KB for USB. There is also and assortment of A/D converters and 47 I/O pins, but this MCU card is pretty specifically designed for this EVM as it only has a USB connector, a 7 pin interface to the capacitance card and a reset connector. The capacitance connector is detailed in the next section.

The middle card hosts the FDC2214 capacitance sensing chip. It sports 4 capacitance channel connectors as well as a serial digital interface connector. The FDC2214 converts capacitance to frequency by making the test capacitance dictate the resonant frequency of a tank circuit. To measure the resulting frequency, the chip can use its internal oscillator in low cost applications, which is pretty stable over a range of supply voltages, but it can vary about 0.13% over extreme temperatures. This evaluation module uses an external 40 MHz crystal oscillator, which permits much more stable operation. (Labeled Y2 in the image below)

The chip can measure capacitance from femtofarads to 200 nanofarads very accurately, however I find the description of accuracy and resolution in the datasheet a bit confusing. The reference frequency counter is 16 bits so it sounds like in the time it takes the reference counter to count its maximum count, the capacitance-under-test counter can reach a count 28 bits long. In the best corner case. If I read it right this allows you to trade-off measurement speed for accuracy, but it would still be rare to get a 28 bit reading. So resolution is programmable by setting the reference count. Accuracy will depend on how accurate your reference clock is, which is why an external crystal is suggested for accurate applications.

The bottom of the capacitance sensor card has labels for the digital interface pins.

The first 2 pins are I2C

The ADDR pin (input) selects one of two possible I2C addresses.

The SD pin (input) puts the chip into Shutdown mode.

IntB is a configurable status output.

The last 2 pins are power supply.

Note there are four channels on this card, only 2 channels are connected to the sensors below, the other two are brought out to connectors, allowing test sensors to be easily connected.

 

The last two cards are touch sensors that look like this (still joined together in this picture):

I2C is used to set and read control registers as well as read data. Registers are used for tasks such as configuring the clock, adjusting the gain, selecting which channels to monitor, deglitch filtering, and device modes. I count at least 35 registers accessible via I2C, although 28 registers are just 7 register types times four channels. This arrangement packs a lot of functionality and a lot of flexibility into a very small package. Not long ago the functionality in this chip would have required a whole circuit board, or possibly four cards.

Here is a quick demo of the evaluation module in action:

There are many applications for capacitance sensors, they can be used to detect metals or non-metal substances. They can be used to measure depth, volume, proximity and touch. I sometimes work with proprietary capacitance sensors, so I can't discuss those interesting applications, but as part of this road test I want to demonstrate a few practical applications where this chip might be very useful.

The first sensor I want to demonstrate is a water depth sensor. This chip allows very accurate measurement of water (or other liquids) depth. Shown below is a steel pipe and some 3D printed parts I will use to make a water depth sensor. The sensor is purposely designed to be short so it will fit well in the video.

The following video shows how the sensor is assembled and how it works:

Such a sensor can be easily and very inexpensively made without any fancy parts - but I have a 3D printer, so I used it.

The next sensor I want to demonstrate is proximity sensing of a person. This chip is so sensitive it can detect people a few feet away:

Finally I want to show how a sensor can be easily designed to measure liquid volume, even when the reservoir volume is not directly proportional to depth.

This sensor also uses water to sense water.

Summary

I am very impressed with the performance and versatility of this chip. I think it is a great solution for a wide variety of capacitive sensing applications and it will save designers lots of design time over custom equivalent solutions.

 

Relevant Links:

Texas Instruments FDC2214 Capacitance to Digital Road Test page

http://www.ti.com/product/FDC2214 chip info

FDC2214EVM FDC2214 with Two Capacitive Sensors Evaluation Module | TI.com

Sensing Development Kits | Newark element14 Canada

FDC2214RGHT TEXAS INSTRUMENTS, Capacitive Touch Sensor, 2.7 V, 3.6 V, WQFN, 16 , Newark element14 Canada

TIDA-00464 High Temperature Touch through Glass Reference Design | TI.com

FDC2214 Noise-immune Capacitive Sensing Solution for Proximity Sensing | TI.com Software

http://www.ti.com/lit/an/snoa927/snoa927.pdf Capacitive Sensing Basics

http://www.ti.com/lit/ug/tidu736a/tidu736a.pdf  Liquid Level Sensor

WEBENCH Capacitive Sensing Designer | WEBENCH | TI.com

Anonymous
  • There are several other ways to measure capacitance which are usually simpler, but this method is extremely accurate and extremely sensitive, and the chip makes it pretty simple, relative to the complexity of the method.

    The MCU in this kit is pre-programmed and the PC based test software is provided by the manufacturer.

  • Hi Navadeep,

     

    I've answered elsewhere on another thread, but basically the development board from TI uses the MSP430, but it is possible to snap it off, and use whichever microcontroller you desire. I preferred to use the FRDM board for some of the examples, because it has an ARM chip on it, personal preference.

    You can probably change the code on the MSP430, but I didn't explore that, it wasn't core to the review.

    LC is required for the oscillation to occur within a certain frequency range. If the C varies slightly then the frequency varies slightly too, and is measurable. But the L is still required.

  • Excellent review!

     

    >> What exactly is the purpose of LC tank (oscillator) for sensing? Won't just measuring capacitance change work insted of using LC tank and measurig the change?

     

    >> Are you supposed to program the onboard MSP430 to run this sensor or does it come with ready loaded firmware ? Is it customisable?

     

    >> How does MSP430 onboard connect to computer without USB interphase circuit ? (Like we have debugger part for launchpads)

     

    Kindly help me in understanding the following.Thanks.

  • I attached a zip file with the .stl files in it to the road test. It is the PipeCap.zip file at the end...

  • This is a great review and is exactly what I needed for a capasative fluid level sensor. I don't suppose you have the 3D files kicking around? I could do with those for my own testing.

  • I wouldn't be surprised if the phase of the moon affected it.

  • That sort of brings it into a reality check ...

     

    At that sort of figure the air density must play a part ...

  • lets see ... 1 femto farad is the capacitance between 2 metal plates where each plate is 1 square cm and the separation between plates is .885 meters of air

    so 0.3 femtofarads pushes the separation to 2.95 meters.

    The proper engineering term for this quantity is "negligible" .... except when it isn't.

    There are lots of lay terms like "ridiculously miniscule" and a few others that involve expletives ...

     

    Updated and corrected - thanks to

  • Looks like a very useful device.

    We had to have a discussion about what a femtofarad and the local effects on such a small capacitance.

     

    I'm glad you didn't have a lot of spare time amongst your other challenge ...

     

     

     

    Mark

  • One thing I didn't do yet was measure some real capacitors with known value and compare with a capacitance meter to explore the accuracy. Or possibly try an old tuner (variable capacitor) to explore resolution. I might add this in if nobody else tries it.