Analog Integrated Current Drivers for Bioimpedance Applications: A Review <s> Closed Loop Non-Linear Feedback Integrated Current Drivers <s> This paper describes an alternative sinusoidal current driver suitable for bioimpedance applications where high frequency operation is required. The circuit is based on a transconductor and provides current outputs with low phase error for frequencies around its pole frequency. This extends the upper frequency operational limit of the current driver. Multifrequency currents can be generated where each individual frequency is phase corrected. Analysis of the circuit is presented together with simulation and experimental results which demonstrate the proof of concept for both single and dual frequency current drivers. Measurements on a discrete test version of the circuit demonstrate a phase reduction from 25 $^{\circ}$ to 4 $^{\circ}$ at 3 MHz for 2 mAp-p output current. The output impedance of the current driver is essentially constant at about 1.1 M $\Omega$ over a frequency range of 100 kHz to 5 MHz due to the introduction of the phase compensation. The compensation provides a bandwidth increase of a factor of about six for a residual phase delay of 4 $^{\circ}$ . <s> BIB001 </s> Analog Integrated Current Drivers for Bioimpedance Applications: A Review <s> Closed Loop Non-Linear Feedback Integrated Current Drivers <s> A wideband, CMOS current driver for bioimpedance measurement applications has been designed employing nonlinear feedback. With the introduction of phase compensation, the circuit is able to operate at frequencies higher than the pole frequency of the output transconductor with minimum phase delay. Moreover, it isolates the poles required for stability from the high frequency characteristics of the output transconductor. The circuit has been simulated in a 0.35-μm CMOS technology and operates from ±2.5 V power supplies. Simulations show that for a 1 mAp-p output current, the phase delay is less than 1° for frequencies up to 3 MHz, rising to 1.5° at 5 MHz. Dual frequency currents to the load are demonstrated. <s> BIB002 </s> Analog Integrated Current Drivers for Bioimpedance Applications: A Review <s> Closed Loop Non-Linear Feedback Integrated Current Drivers <s> The variation of tissue’s electrical properties, both dielectric and conductive characteristics, through a frequency spectrum is characterised as bioimpedance spectroscopy. In electrical impedance spectroscopy (EIS) of biological tissues, the overall impedance measurement is performed by applying AC signals, either a current signal or a voltage signal, to the surface of the tissue or cell via one pair of electrodes and recording the resulting signals via a separate or the same pair, which are then converted to impedance via a demodulation process. Considering current stimulus, a wide band current driver up to 3 MHz is desired so that it can penetrate to the nucleolus of the cell and provide accurate information about the morphology and physiology of the cell. This thesis intended to design a novel wide band current driver with phase compensation scheme for tetra-polar EIS applications. It presented the measurement result and experimental validation of the proposed current driver which provide superior performance to the existing state of the art current drivers in terms of bandwidth, output impedance and phase error. The integrated fully differential non-linear feedback current driver is fabricated in 0.35 μm CMOS technology and occupies a silicon area of 1.2 mm2 . With the introduction of automatic phase compensation circuit, the current driver is able to operate at frequencies close to the pole frequency of the output transconductor with phase errors as low as 3o at 3 MHz Frequency. Another important aspect of the phase compensation is that the output impedance of the current driver remains (largely) constant throughout the entire range of operational frequencies (100 kHz – 3 MHz). The current driver can deliver a maximum output current of 1.8 mAp-p from a supply of ± 2.5 V. For an output current of 1 mAp-p the accuracy is 0.24%, 0.43% at 1 and 2 MHz respectively. With active compensation, the phase error at the output of the current driver reduces to 1o at 1 MHz and 3o at 3MHz. The current driver can be configured to operate with multi-frequency (multi-sine) by connecting in parallel extra current drivers (with their corresponding phase compensation). <s> BIB003 </s> Analog Integrated Current Drivers for Bioimpedance Applications: A Review <s> Closed Loop Non-Linear Feedback Integrated Current Drivers <s> In a conventional sinewave current driver for electrical impedance spectroscopy, as the frequency is increased the input/output phase delay of the current driver increases due to limited bandwidth. The required maximum phase delays of ${\mu }\text{m}$ CMOS process technology, which provides a phase error reduction from 22° to 3° at 3 MHz. Its core occupies a silicon area of 1.2 mm2. It operates from a ±2.5 V power supply and can deliver output currents up to 1.8 mAp-p at 3 MHz. <s> BIB004
The current driver in BIB002 is based on non-linear feedback, as illustrated in Figure 14 . The system has been designed in a 0.35 µm CMOS technology with ±2.5 V supply voltages. The system has the advantage of the dominant pole not affecting its high frequency operation, as opposed to other linear feedback current drivers discussed earlier. However, the transient response is longer due to low frequency dominant pole of the lowpass filters BIB002 . The amplitude of the differential current was set by differential DC voltages of ±V cont and the frequency of operation was dictated by the frequency of differential sinωt and cosωt input signals at of multipliers M X2 and M X4 , and the corresponding square waves at M X1 and M X3 . The differential voltage across R s was converted to DC voltage through the switch multipliers M X1 and M X3 and four RC low pass filters (LPF) to extract the DC component BIB004 . It resulted in an output voltage of (2/π) V p /2 where V p was the peak voltage of the AC signal across R s . The resulting DC voltages were compared to differential control voltages and amplified by A 1 in the current driver and compared to zero and amplified by A 2 in the compensation circuit. The signal was then modulated back to AC via analogue multipliers M X2 and M X4 and current amplifiers AI 1 and AI 2 . In the compensation circuit, the same circuit as the current driver an at 90 o phase shift with respect to main driver at cosωt, where (ω) is the desired frequency of operation of the current driver BIB002 , which considerably reduces the phase delay of I drive to beyond the pole frequencies of M X2 , M X4 . The compensation circuit (lower circuit) senses the amplitude of the out-of-phase component of the voltage across R s and generates an amplified version of it. The outputs of both circuits were added and feedback nearly canceled out the out-of-phase components BIB002 . The detailed analysis of the phase compensation is provided in BIB001 . In this system configuration, there are two differential outputs of the multipliers M X2 and M X4 : one drives the load Z L and the other is used for feedback via R s . As the presence of R s does not enhance the output resistance of the system, the high output resistance was attained by using high swing cascodes. On the other hand, since the dominant pole is placed at very low frequencies, the capacitor effects at higher frequencies are not a limiting factor in the output impedance measurements, and the system achieves an output of 1 MΩ over the entire bandwidth of operation BIB003 . The loop gain of the current driver is analogous to the linear feedback system BIB002 with an additional 2/π factor as a result of AC-DC conversion at the LPF. If AGRs>>1 then I drive(peak) = πV cont. R s BIB004 where A is the voltage gain of amplifiers A 1,2 , G is the transconductance gain of multipliers M x2,4 and current amplifiers AI 1,2 , and R s is the sense resistance. The measurement results of the current driver shows the current amplitude without compensation varies by 1.8% for 1mA p-p , while the phase error starts to increase from 500 kHz and reaches 20 • at 3 MHz BIB004 BIB003 . With compensation, the maximum output of 1 mA p-p has an accuracy of 0.24% and 1.3% at 1 and 2.5 MHz, respectively. With compensation, the phase error reduces to 1 • at 1 MHz and 3 • at 3MHz, regardless of the current amplitude BIB004 . The measured THD of the current driver for a current of 1 mA p-p was 0.25% at 200 kHz, rising to 0.4% at 500 kHz. Although the system provides superior results in terms of output impedance, phase error, and bandwidth with respect to other linear systems, the design is more challenging and occupies more area.  Where A is the voltage gain of amplifiers A1,2, G is the transconductance gain of multipliers Mx2,4 and current amplifiers AI1,2, and Rs is the sense resistance. The measurement results of the current driver shows the current amplitude without compensation varies by 1.8% for 1mAp-p , while the phase error starts to increase from 500 kHz and reaches 20° at 3 MHz BIB004 BIB003 . With compensation, the maximum output of 1 mAp-p has an accuracy of 0.24% and 1.3% at 1 and 2.5 MHz, respectively. With compensation, the phase error reduces to 1° at 1 MHz and 3° at 3MHz, regardless of the current amplitude BIB004 . The measured THD of the current driver for a current of 1 mAp-p was 0.25% at 200 kHz, rising to 0.4% at 500 kHz. Although the system provides superior results in terms of output impedance, phase error, and bandwidth with respect to other linear systems, the design is more challenging and occupies more area.