|Product Performed to Expectations:||10|
|Specifications were sufficient to design with:||10|
|Demo Software was of good quality:||10|
|Demo was easy to use:||10|
|Support materials were available:||10|
|The price to performance ratio was good:||10|
|TotalScore:||60 / 60|
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.
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.
Texas Instruments FDC2214 Capacitance to Digital Road Test page
http://www.ti.com/product/FDC2214 chip info
http://www.ti.com/lit/an/snoa927/snoa927.pdf Capacitive Sensing Basics
http://www.ti.com/lit/ug/tidu736a/tidu736a.pdf Liquid Level Sensor