# A 28 nm Bulk CMOS Fully Digital BPSK Demodulator for US-Powered IMDs Downlink Communications
## 2 US-Powered IMDs: A System Overview  
The system represented in Figure 1 is an illustrative description of the US-powered integrated circuit to be implanted in the human brain. It is worth noting that the same IC could be used for any specific biomedical application, such as heart-beat stimulations, gastrointestinal activities manipulation, and other kinds of controls and actuations applied to patients affected by chronic diseases [7; 8; 9; 10]. The IMDs are powered-up, exploiting US-acoustic waves generated from external piezo-transducers (_TX-PIEZO_, in Figure 1) kept under the electrical resonance condition and placed in contact with the skin. Thus, once the acoustic waves reach the depth of a second power-receiving piezo-transducer (_RX-PIEZO_), they can be efficiently rectified and conditioned to power up a storage capacitor, namely C\({}_{\text{STOR}}\), in the Figure mentioned above. In fact, the described system is battery-less and plainly covers the concept of Energy Harvesting, exploiting the electro-acoustic coupling of two small, thick and low-invasive piezoelectric devices. Figure 1: Simplified scheme of a US-powered IMD. As depicted in most of the works presented in the literature on the Power Management Integrated Circuit (PMIC) for Energy Harvesting systems, as reviewed in the Ref. [11], the power conditioning circuit is often made up of an impedance matching network [12], an active AC/DC converter [13; 14], a voltage reference [15] and a low-dropout voltage regulator (LDO) [16] that provides a regulated rectified voltage (i.e., \(V_{DD}=0.9\) V in Figure 1). Moreover, starting from the experimental measurements conducted by the authors, a diode-clamping circuit could be necessary for those cases in which the maximum voltage amplitude raises the maximum allowable voltage for the adopted IC technology. Moreover, a Power-On Reset (POR) circuit, as the one shown in the Ref. [17], is often needed to enable/disable the load circuitry. On the other hand, two further subsections are presented in Figure 1, namely, the demodulator and stimulation circuit, both powered by the PMIC. Indeed, the presence of a communication sub-system for an IMD is mandatory, since it allows to establish data communication between the IMDs and the external world for long-term monitoring and stimulation purposes. In particular, the feature of collecting data and communicating them to the external world, namely Data Up-Link, has revealed a promising solution for bioelectronic medicine that exploits these devices [7]. Furthermore, the ability to act on neurons, nerves, the heart-beat, and gastrointestinal activities, made up through the manipulation of electrical transducers, could optimise therapeutic protocols and help relieve patients' pain [10]. These kinds of stimulations come from the modulation of a powering signal generated from an externally placed unit coupled to the implanted receivers for power/data exchanging. The established communication is also defined as a Data Down-Link. Thus, an incoming-signal demodulator is needed to acquire an external signal to properly drive the above-mentioned electrical transducers. Then, the received information is elaborated into an implanted processing unit (i.e., a Digital Signal Processor, DSP, or a Field Programmable Gate array, FPGA) that works together with a Digital-to-Analog Converter (DAC), allowing for the electrical stimulation of the nerves/brain, as schematically represented in Figure 1.  
This work mainly concerns the design of a US-Data Down-Link embedding the proposed fully digital BPSK demodulator. Indeed, the state-of-the-art presents several modulation schemes, such as the ASK [18; 19], PWM-ASK [20], OOK [19; 21; 22], and OOK-PM [23; 24], while the BPSK demodulation has often been used for RF-powered IMDs [25; 26; 27; 28; 29; 30; 31]. The choice of this modulation scheme arises from three main reasons.