# A Low-Cost Electronic System for Human-Body Communication
## 3 System Description
### Differential Current Driver  
There are two electrical current types: direct current (DC) and alternating current (AC). Direct current travels in one direction and does not change. Alternating current can switch directions or vary in magnitude, usually at a specific frequency. In galvanic coupling, an alternating current is injected into the skin by the differential current driver (Figure 1). The skin acts as a communication medium and carries the signal throughout the body. The differential current driver component is responsible for coupling data frames to the communication medium, converting the message bits into current pulses at the electrodes. The proposal is inspired by the circuit proposed by [PERSON] [10], which can be miniaturized as an application specific integrated circuit (ASIC), becoming a relevant candidate to be part of a bio-sensor. Our work focuses on designing a lightweight circuit embedded in wearable devices, with constrained computational resources and energy. This circuit is optimized for high-speed communication and, finally, it supports active interference rejection. We adopt a baseband signaling scheme to make possible the use of low-cost, general-purpose microcontrollers. Baseband transmissions typically use digital signaling over a single wire (the body). The digital signal in baseband transmission occupies the entire network medium's bandwidth to transmit a single data signal. Baseband communication is bidirectional, allowing devices to send and receive data through a single cable. However, data transmission and reception cannot occur on the same wire at the same time. The circuit produces the output current pulses according to the input combinations, as shown in Table 2.  
Due to the on-off nature of digital devices, our circuit supports several coding schemes such as non-return to zero (NRZ), alternated mark-inversion (AMI), Manchester, and high-density bipolar 3 (HDB3), which represent digital data through variations in amplitude and duration in a carrier wave. We use the NRZ scheme for simplicity and because it consumes less power [15]. As in Figure 1, the differential current driver component presents the following elements: (i) voltage level converter, (ii) polarity inverter, (iii) fixed current sources, (iv) controlled current sinks, and (v) DC-DC converter. We describe each element next. Voltage level converter: it converts the 0V or 5V input voltage to symmetric output voltage, \(\pm 1\) V, driving the controlled current sinks when ON-OFF input is active (0V). This circuit translates \begin{table}
\begin{tabular}{c c c} \hline \hline
**Input Voltage (IN)** & **ON-OFF** & **Output Current** \\ \hline - & 5 V & 0 mA \\ 0 V & 0 V & \(+1\) mA \\ 5 V & 0 V & \(-1\) mA \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Differential current driver IN/OUT reference values. signals from one logic level or voltage domain to another, allowing compatibility between integrated circuits (ICs) with different voltage requirements. Polarity inverter: it changes the pulse polarity. As polarities applied to controlled current sinks are opposed, they are never active at the same time. Therefore, the signal output represents the characteristics of the different described baseband modulation schemes. Fixed current sources: when the ON-OFF input is active (0V), they supply a constant current (1 mA), drained by the controlled current sinks or injected in the connected electrode, depending on the sink state. Thus, a constant current source is a valuable component because it can supply steady current even if there are changes in resistance, and even a wide variance in the resistance. This is essential because the circuit needs a steady current supply, without fluctuations. Controlled current sinks: they are active when the ON-OFF input is 0V, and the other inputs are positive. In such a case, they drain a fixed current equal to 2 mA. Otherwise, the drained current is zero. By activating the current sink, the connected electrode demands an additional 1 mA current.