## 2 System Concept  
This work's concept of a custom near-field interface is illustrated in Figure 2 and incorporates the following particular features:  
* Separate coils are used for data and energy transfer. Hence, the data resonators can be optimized for a low quality factor (1 to 10) to realize a high data link bandwidth in the same order of magnitude as the carrier frequency, in line with established theory on the ringout behavior of LC resonators [13; 19]. * To avoid interference in the data link and detuning of the energy link, coupling of energy and data coils, even being located on the same substrate, must be avoided by suitable geometrical design of the coils. Hence, novel coil topologies are presented and analyzed. * Selecting the operational frequency of the data interface to be lower than that of the energy link allows for extracting the data signal with simple low-pass instead of bandpass filters. Moreover, harmonics from the power transfer interface cannot fall into the communication band. In this work, near-field communication will cover the 13.56 MHz industrial scientific and medical (ISM) band, while the 40.68 MHz ISM band is utilized for wireless power transfer. * Active signal generation is applied, i.e., both implant and reader unit feature the same transceiver unit to realize a symmetric data link with equal communication capabilities for both devices. This enables the application in closed-loop systems with high bandwidth requirements in both communication directions. Moreover, realizing a broadband communication from reader to implant increases channel availability in the given half-duplex system. * Modulation and demodulation circuits shall be realized as purely digital circuits to enable a compact implementation. This means that the modulated carrier is generated as a rectangular wave in a digital circuit; a digital class-D output stage successively applies the signal to the analog filtering and data coil stages, which extract the modulated sinusoidal signal. In the receiver, the sinusoidal signal is converted back to a rectangular wave by a comparator stage, and is then demodulated by a purely digital circuit. While comparator-based receivers processing rectangular signals [20; 21] and single-bit sampling receivers processing time-discretized samples of a rectangular signal [22] were conceived for biomedical near-field communication, the reliability of their custom demodulation stages in terms of the bit error rate is analyzed neither by simulation nor by measurement. Furthermore, their applicability is limited, as they rely on custom CMOS circuits and frequency bands not reserved for industrial, scientific, and medical (ISM) purposes. The unique contribution of this work is to analyze and evaluate a transceiver that adapts the complex and energy-intensive operations of mixing, low-pass filtering and and multi-bit quantization to the domain of single-bit sampling receivers: Low-power digital circuits discretize the analog data signal with respect to amplitude and time and demodulate the samples by applying digital operations of downconversion and filtering. The transceiver shall be implemented based on commercially available and small-scale electronic components, such as recent ultra-low-power field-programmable gate arrays, and adapted to an ISM frequency band to enhance its applicability. In particular, the objective is to create a near-field communication interface using a 13.56 MHz carrier with a symbol rate of 6.78 MHz, so that only two carrier cycles will encode a single symbol. As the digital receiver will only allow for constant-envelope modulation [23] and as on-off keying has the potential drawback of ringouts still being detected as an active carrier, binary phase-shift keying will be used as a modulation technique. The expected raw data rate is thus 6.78 MBit/s.