A custom transmitter chip combines ultra-low-power wireless technology with clever design techniques to realize a tiny swallowable camera capsule for the diagnosis of the gastrointestinal tract. Made by Zarlink Semiconductor, the custom chip is at the heart of the vitamin-sized capsule manufactured by Given Imaging. Patients swallow Given's M2A capsule endoscope, which broadcasts pictures as it passes through the esophagus, stomach, and small intestine. The capsule includes a camera, LEDs, batteries, the custom chip, and an antenna.
The defining characteristic of the radio-frequency (RF) transmitter is its miniscule power consumption to ensure the operating life and image capabilities of the camera capsule as it travels through a patient. The IC's lower power consumption lets the capsule send out more pictures than earlier models, even while it provides new functions that Given requested to support future product enhancements.
The transmitter operates in the 402- to 405-MHz Medical Implantable Communication Service band, which also is used for pacemakers, implantable cardioverter defibrillators, and neurostimulators. Power output in the band is limited to 25 µW.
The transmitter chip embodies a number of rarely used circuit techniques. The designers directly frequency-shift-keyed a voltage-controlled oscillator (VCO). To save space, the design uses the VCO coil as an antenna. (There are only four external components: the coil, a capacitor, a resistor, and a crystal.) Also, digital circuits are shut down when they aren't needed by gating their clock circuits, rather than by cutting power. This arrangement guarantees they are in the correct state when they power-up again.
The semiconductor process plays a role in the chip's low power consumption. The chip is fabricated by X-fab in Erfurt, Germany. X-fab owns the process technology Zarlink developed before it went fabless, so Zarlink had the knowledge to tweak the 0.18-µm process to create new poly and metal-insulator-metal capacitors, which are used in the chip's efficient dc-dc converter. The converter, in turn, enables the chip's RF circuitry to run at 1.8 V, while its digital core uses half that voltage, cutting active power use almost in half.