Cap-to-digital converters with high-noise immunity open doors to new, reliable designs

Capacitive sensing is in widespread use today, in everything from smartphone touchscreens to car-trunk openers. Because we encounter touch sensing in so many environments now, some have found the technology can be finicky at times, not responding when you want it to or responding when you don’t want it to. That’s because ambient RF noise can create an electrical potential that masks a capacitive-touch voltage signal or cause one to be sensed when none is intended.

Texas Instruments’ engineers have found a clever way around this problem with TI’s new FDC2x1x series capacitance-to-digital converter, which they say provides “capacitive sensing with the world’s only noise-immune solution.” Intended for use in applications such as proximity sensing, gesture recognition, level sensing for liquids (including conductive ones such as detergent, soap, and ink), collision avoidance, automotive door and kick sensors, and material-size detection, the devices provide noise immunity of 36 dB, which can result in 60 times better performance in the presence of noise than existing capacitive-sensing solutions. Thus, they are said to enable significantly better capacitive sensing of the human body as well as objects in real-world environments: homes, offices, factories, automobiles, and the outdoors.

Key to this improvement is the fact that the external sensor for the device is not just a capacitor alone, but rather an inductor-capacitor, or tank, circuit excited by converter's internal resonant-circuit driver. Designers who use the converter, by their choice of external components, actually tune the sensor system to ignore other frequencies that could interfere with sensing. It may even be possible to create a practical design that uses variable inductors to tune the sensing circuit on the fly, thus providing “frequency-agile” sensing that adapts to avoid or detect different frequencies in changing environments.

The initial four units in the series offer users a choice of two or four sensing inputs (FDC2x12 or FDC2x14, respectively) with 28- or 12-bit resolution (FDC221x or FDC211x, respectively). The FDC221x is optimized for high resolution at 4.08 ksamples/s, while the FDC211x offers a fast sample rate of up to 13.3 ksamples/s to ease implementation of applications that use fast-moving targets. The very large maximum input capacitance of 250 nF permits use of remote sensors, as well as the tracking of environmental changes over time, temperature, and humidity. The devices support a wide excitation frequency range, especially useful for reliable sensing of conductive liquids.

It should be noted that, in comparison to optical and ultrasonic sensing for the applications previously mentioned, reliable capacitive sensing offers several advantages. These capacitive sensing ICs enable sensing through any nonconductive material, so designers can avoid drilling holes in end equipment, improving aesthetics, lowering production costs, and improving reliability. Then too, capacitive sensing is insensitive to ambient light and can detect the presence of black and dark objects. Further, by allowing for remote sensing, the FDC2x1x series supports multiple distributed sensors and greater functionality at minimum system cost. And due to their noise immunity, they let designers use smaller capacitive sensors and extend the range of proximity sensing.

The 4 x 4 x.0.8-mm devices come in leadless plastic packages and are priced between $2.38 and $4.75 each in 1,000-piece quantities. FDC2214EVM and FDC2114EVM evaluation modules, to let designers quickly and easily evaluate the device’s accuracy, are available at a price of $29.00 in single quantities. A broad range of tools is available to ease design, including a TI Designs’ reference design for a noise-immune capacitive proximity sensor system.