## 5 Experimental Results  
The internal implant unit is fabricated using UMC 0.13 \(\mathrm{\upmu m}\) CMOS process. The die photo is shown in Figure 10 and the overall chip area is 2.25 mm\({}^{2}\) and the thickness is 0.2 mm. The filtering  
Figure 9: (a) The primary and detection coil (b) The secondary coil (c) The external circuitry. capacitor consists of stacked MOS, MIM and MOM capacitors to maximize the capacitance density. The tuning capacitor is made of MIM capacitors, and the bottom plate is connected to V\({}_{\text{AC2}}\) that is clamped at around V\({}_{\text{DC}}\)/2. To save space, the MOS and MOM capacitors underneath the tuning capacitor are also used as part of the filtering capacitor, and simulations showed no observable effect on the tuning circuit. The chip is glued on the secondary PCB and bonded directly to the coil as shown in Figure 9b. The whole implant unit is then encapsulated by biocompatible silicone. The overall thickness of this implant device is controlled to be less than 1 mm with an outer diameter of 6 mm, as shown in Figure 10. The whole eye implant is much smaller than a contact lens and can be put beneath the eye's conjunctiva easily. The external control unit is implemented using discrete components and a field-programmable gate array (FPGA) as shown in Figure 11. A buzzer is added. When the distance of the two coils is too far apart, the buzzer will beep to alarm the patient to adjust the primary coil to make it closer to the eye implant. This external control unit will be integrated into a single chip in the future work. Figure 11: The external control unit (**a**) top view (**b**) bottom view. Figure 10: The die photo and the eye implant. Figure 12a shows the measured waveform of the 6-bit current DAC with a 5 k\(\Omega\) output load resistor. There is stimulation current when \"En\" = 1. The signal \"Polarity\" is a periodic square wave that changes the current direction. In Figure 12b, it is shown that the output voltage of the rectifier V\({}_{\text{DC}}\) is not very steady. The voltage ripple consists of two components: the first one is the large voltage ripple of around 400 mV that has a frequency of lower than 1 kHz due to power boosting; and the second is the smaller voltage ripple of 50 mV at 80 MHz due to backscattering. After filtering by the 3-V LDO and the 1.2-V LDO, both ripple components are significantly attenuated. The stable 3 V and 1.2 V outputs are then used to supply power to the DAC and the digital controller, respectively. Figure 13 shows the action of power regulation through data backscattering in the non-stimulation time. V\({}_{\text{DC}}\) is periodically detected and the equivalent digital value is backscattered to the external control unit. The external control unit then adjusts the transmitting power such that V\({}_{\text{DC}}\) is regulated to the predefined value of 3.5 V. When the distance between the two coils changes the supply voltage of the PA will be adjusted adaptively. As shown in Figure 14, when the distance changes from 4 mm to 13 mm, the supply voltage of the PA is increased from 280 mV to 900 mV such that V\({}_{\text{DC}}\) is maintained at around 3.5 V.  
Figure 15a shows the transmitted configuration data and the accompanied power boosting during the stimulation time. When new configuration data is set and the primary side attains the status of \"Master\", the data will be transmitted to the secondary side. As shown in Figure 15b, for transmitting a \"1\", the supply voltage of the PA maintains at V\({}_{\text{D1}}\); but for transmitting a \"0\", it will drop temporarily to V\({}_{\text{D2}}\) as discussed in Section 4.2. A short period after the configuration data is transmitted the internal implant unit becomes the master and sends back the acknowledgement bit such that transmission of the configuration data will not be repeated. Figure 12: Measured operation waveform of (**a**) 6-bit current DAC (**b**) rectifier, 3 V LDO and 1.2 V LDO. Figure 14: VDC regulation under the two coils’ distance of (**a**) 4 mm (**b**) 1.3 cm.