# Wearable 256-Element MUX-Based Linear Array Transducer for Monitoring of Deep Abdominal Muscles
## 2 Materials and Methods
### Multiplexer Development  
The system requires 32 multiplexers with four inputs each. To limit the die size and the number of connections per chip, it was decided to put eight multiplexers on one chip and cascade four chips in one system. Each switch within the multiplexer can be controlled independently, thus not only allowing one input out of four to be selected but also connecting two or more inputs simultaneously. The switches are controlled via a 3-wire serial interface. The input amplitude is limited to less than \(\pm\)0.7 V and is AC coupled into the MUX.  
Figure 1: Comparison of PWC image of CIRS phantom acquired with different transmit schemes to mimic a system with a LV MUX in the RX path. (**A**) A full 128-element 5 MHz linear array is used for both TX and RX. (**B**) even-numbered elements transmitting and odd-numbered ones receiving. (**C**) is the same as (**B**) but with an additional 10 dB gain to mimic the effect of an additional LNA integrated into a MUX ASIC.  
To reduce the output impedance and allow the connection of a longer cable, we decided to combine each MUX channel with a low-noise amplifier (LNA). This also compensates for the signal attenuation by the switch. The circuit was developed using PREMA's 800 nm BiCMOS process. CMOS transistors with a maximum voltage of 2.8 V were used for the transmission gates of the multiplexer, as well as for the shift register with a clock frequency of at least 20 MHz. For the LNA, both CMOS and bipolar circuits were simulated and tested for the highest gain-bandwidth product. Both concepts achieved similar performance data, but finally, a bipolar amplifier was picked for use in the complete circuit. For the voltage regulator supplying the circuit, using DMOS transistors with a breakdown voltage above 15 V proved to be the most adequate choice.  
Metal-metal capacitors with a capacitance of 2.1 fF/\(\upmu\)m\({}^{2}\) were used for the AC coupling of the input signals. The BiCMOS process used for the ASIC consists of 14 mask layers, including triple metal and a high-capacitance layer. It is manufactured in Prema's own wafer fabrication facility in Mainz/Germany. Before designing and producing the complete circuit, several variations of the circuit blocks were produced on multiproject wafers to characterize their performance. This was necessary specifically for the transmission gates and the LNA. Two rounds of multiproject wafers were processed to optimize the performance before being put together to the complete ASIC. For the ASIC, 32 transmission gates were combined with a serial shift register and eight LNAs to one circuit and produced on wafers. One wafer contains several 100 ASICs, which have to be tested after production so that only fully functional ASICs are used in the next step. Selected ASICs were then assembled in QFN packages. Four QFN packages are soldered together with the ultrasonic transducer on one flexible PCB to form a MUX with 128 inputs and 32 outputs, controlled by one serial bus. (see Figures 2 and 3A).  
Figure 2: The system diagram shows one MUX chip (**A**) and a full system consisting of the 256-element probe (with separate transmit and receive elements) and the 32-ch backend electronics (**B**).