Analog Integrated Current Drivers for Bioimpedance Applications: A Review <s> 12 <s> Bioimpedance measurements are performed in a variety of medical applications including cancer detection in tissue. Such applications require wideband (typically 1 MHz) accurate ac current drivers with high output impedance and low distortion. This paper presents an integrated current driver that fulfills these requirements. The circuit uses negative feedback to accurately set the output current amplitude into the load. It was fabricated in a 0.35-μm complementary metal–oxide–semiconductor (CMOS) process technology, occupies a core area of 0.4 mm2 , and operates from ±2.5-V power supplies. For a maximum output current of 1 mAp-p, the measured total harmonic distortion is below 0.1%, and the variability of the output current with respect to the load is below 0.5% up to 800 kHz increasing to 0.86% at 1 MHz. The current driver was tested for the detection of cancer sites from postoperative human colon specimens. The circuit is intended for use in active electrode applications. <s> BIB001 </s> Analog Integrated Current Drivers for Bioimpedance Applications: A Review <s> 12 <s> This paper describes the design of an active electrode integrated circuit (IC) for a wearable electrical impedance tomography (EIT) system required for real time monitoring of neonatal lung function. The IC comprises a wideband high power current driver (up to 6 mAp-p output current), a low noise voltage amplifier and two shape sensor buffers. The IC has been designed in a 0.35-μm CMOS technology. It operates from ±9 V power supplies and occupies a total die area of 5 mm2. Post-layout simulations are presented. <s> BIB002 </s> Analog Integrated Current Drivers for Bioimpedance Applications: A Review <s> 12 <s> A high specification, wearable, electrical impedance tomography (EIT) system with 32 active electrodes is presented. Each electrode has an application specific integrated circuit (ASIC) mounted on a flexible printed circuit board, which is then wrapped inside a disposable fabric cover containing silver-coated electrodes to form the wearable belt. It is connected to a central hub that operates all the 32 ASICs. Each ASIC comprises a high-performance current driver capable of up to 6 mAp-p output, a voltage buffer for EIT and heart rate signal recording as well as contact impedance monitoring, and a sensor buffer that provides multi-parameter sensing. The ASIC was designed in a CMOS 0.35- $\mu \text{m}$ high-voltage process technology. It operates from ±9 V power supplies and occupies a total die area of 3.9 mm2. The EIT system has a bandwidth of 500 kHz and employs two parallel data acquisition channels to achieve a frame rate of 107 frames/s, the fastest wearable EIT system reported to date. Measured results show that the system has a measurement accuracy of 98.88% and a minimum EIT detectability of 0.86 $\Omega $ /frame. Its successful operation in capturing EIT lung respiration and heart rate biosignals from a volunteer is demonstrated. <s> BIB003
transistors of the corresponding transconductor provided as common mode feedback for differential output transconductor. This quiescent resistance is dependent upon device dimensions. As a result, in the presence of any fabrication inaccuracies or device mismatches, the output DC level cannot be accurately defined and results in a large DC offset. In BIB001 , a similar architecture has been used in a 0.35 μm standard CMOS technology that operates with a ±2.5 V supply. The schematic of the current driver is shown in Figure 10 . . Alternative current driver topology (optimized for supply voltage of ±2.5V) BIB001 . The circuit employs a pre-amplification stage by utilizing a differential difference transconductance amplifier (DDTA 1 , DDTA 2 ) followed by a transconductance stage (Gm 1 , Gm 2 ) performing the voltage to current conversion. The output current is sensed by two on-chip resistors (R s1 , R s2 ), whose voltage is fed back via four single to single-ended buffers (B 1 -B 4 ) into the negative terminal of the preamplifier, thus establishing a negative feedback loop BIB001 . The output compliance of the current driver is 4 V, with a maximum output current capability of 1 mA p-p . To overcome the DC offset issue in the previous design, an auxiliary common mode voltage at the input of the transconductor is defined. The current driver can deliver a maximum output current of 500 µA p with an accuracy of 0.44% in the frequency range of up to 800 kHz, and a THD of less than 0.26% was achieved for an output current of 1 mA p-p BIB001 . The output impedance is 1 MΩ at 500 kHz, and approximately 360 kΩ at 1 MHz. The input/output phase delay at 1 MHz is 9.5 • BIB001 . Although performance was improved for low power consumption, the issue of instability is still present, which necessitates a large on-chip compensation capacitor. Moreover, due to the presence of a dominant pole at high frequency, which results in the output impedance acting as a capacitor rather than resistor, the generated phase error must be minimized by an added compensation scheme to reduce the effect of the capacitance at the output. The proposed current driver in BIB002 BIB003 is part of an active electrode IC for wearable Electrical Impedance Tomography (EIT). The current driver in Figure 11 has been designed in a 0.35 µm CMOS technology. It operates from ±9 V power supplies and occupies a total die area of 5 mm 2 . The current driver is composed of a differential difference transconductance amplifier (DDTA), followed by a wideband operational transconductance amplifier (OTA). In this manner, the DDTA measures the output current through the sensor resistor, R f , and forms a linear feedback. In this EIT system, one current driver is configured as a current source and the other as a current sink, both working together to form a complete current path. The transconductance and output impedance are given respectively by: where A loop is the overall open loop gain of the current driver and R load is the load. where r oOTA is the output impedance I out and A DDTA is the open loop gain of the DDTA. 