Analog Integrated Current Drivers for Bioimpedance Applications: A Review <s> 5 <s> A high-swing, high-performance CMOS telescopic operational amplifier is described. The high swing of the op-amp is achieved by employing the tail and current source transistors in the deep linear region. The resulting degradation in differential gain, common-mode rejection ratio (CMRR), and other amplifier characteristics are compensated by applying regulated-cascode differential gain enhancement and a replica-tail feedback technique. A prototype of the op-amp has been built in a 0.81-/spl mu/m CMOS process. Operating from a power supply of 3.3 V, it achieves a differential swing of /spl plusmn/2.15 V, a differential gain of 90 dB, unity-gain frequency of 90 MHz, and >50-dB CMRR. It is shown, analytically and through simulations, that the operational amplifier maintains its high CMRR even at high frequencies. <s> BIB001 </s> Analog Integrated Current Drivers for Bioimpedance Applications: A Review <s> 5 <s> In this paper, the design of a specific integrated circuit for the measurement of tissue impedances is presented. The circuit will be part of a multi-micro-sensor system intended to be used in cardiac surgery for sensing biomedical parameters in living bodies. Myocardium tissue impedance is one of these parameters which allows ischemia detection. The designed chip will be used in a four-electrode based setup where the effect of electrode interfaces are cancelled by design. The chip includes a circuit to generate the stimulus signals (sinusoidal current) and the circuitry to measure the magnitude and phase of the tissue impedance. Several integrated circuits have been designed, fabricated and tested, in a 0.8-/spl mu/m CMOS process, working at 3 V of power supply. Some of them including building blocks, and other with the whole measurement system. Experimental tests have shown the circuit feasibility giving expected results for both in-vitro and in-vivo test conditions. <s> BIB002 </s> Analog Integrated Current Drivers for Bioimpedance Applications: A Review <s> 5 <s> Multi-frequency electrical impedance tomography (MF-EIT) systems require current sources that are accurate over a wide frequency range (1 MHz) and with large load impedance variations. The most commonly employed current source design in EIT systems is the modified Howland circuit (MHC). The MHC requires tight matching of resistors to achieve high output impedance and may suffer from instability over a wide frequency range in an integrated solution. In this paper, we introduce a new integrated current source design in CMOS technology and compare its performance with the MHC. The new integrated design has advantages over the MHC in terms of power consumption and area. The output current and the output impedance of both circuits were determined through simulations and measurements over the frequency range of 10 kHz to 1 MHz. For frequencies up to 1 MHz, the measured maximum variation of the output current for the integrated current source is 0.8% whereas for the MHC the corresponding value is 1.5%. Although the integrated current source has an output impedance greater than 1 MOmega up to 1 MHz in simulations, in practice, the impedance is greater than 160 kOmega up to 1 MHz due to the presence of stray capacitance. <s> BIB003 </s> Analog Integrated Current Drivers for Bioimpedance Applications: A Review <s> 5 <s> Bioelectrical impedance measurements are routinely used in many clinical applications such as lung function monitoring, brain imaging, body segment monitoring and tissue characterization. Advantages of the method include low-cost, simplicity and non-invasiveness. One bioimpedance imaging technique of particular interest is Electrical Impedance Tomography (EIT). EIT works by reconstructing the differences in electrical conductivity inside a body. The design of instrumentation for bioimpedance measurements is very challenging, particularly for applications requiring wideband operation (e.g., imaging of cancer biomarkers). Such applications require wideband current drivers that can deliver accurate and stable sinusoidal current (AC) to the electrode-tissue load. Many current drivers for bioimpedance measurements are based on discrete electronics, mostly using the modified Howland circuit (MHC). However, the accuracy and performance of the MHC depends on the performance of the operational amplifiers used and on the degree of matching of its resistors, making the approach not attractive for integration. In addition, because it uses both positive and negative feedback it can suffer from stability problems and since it employs voltage feedback it suffers from bandwidth limitations. ::: This thesis is concerned with the design implementation and experimental evaluation of novel current drivers for wideband, fully-integrated, bioimpedance instrumentation for EIT applications. Three new current driver circuits are introduced based on the operational transconductance amplifier (OTA) approach, and implemented in sub-micron CMOS technology. The described integrated current drivers overcome many of the limitations of discrete current drivers, i.e., limited precision at high frequencies, stability, and low-bandwidth. Two new open-loop current drivers implemented in 0.35-μm CMOS technology are presented and their performance is analyzed and simulated. One of the current drivers was fabricated as an integrated circuit and its performance was experimentally verified using RC loads, a saline tank electrode phantom and electrodes on the forearm. In order to further improve its performance, a novel negative impedance converter (NIC) circuit is introduced which compensates for the stray capacitance at the output node due to the cable connecting the current driver to the electrodes. Finally, a new current driver in 0.6-μm CMOS technology is presented using feedback to provide high output impedance from DC to over1 MHz. Simulation results including Monte-Carlo analysis are presented to show the operation of the circuit. <s> BIB004 </s> Analog Integrated Current Drivers for Bioimpedance Applications: A Review <s> 5 <s> A multi-modal spectroscopy IC combining the impedance spectroscopy (IMPS) and the multi-wavelength near-infrared spectroscopy (mNIRS) is proposed for high precision non-invasive glucose level estimation. A frequency sweep (10kHz - 76kHz) sinusoidal oscillator (FSSO) is proposed for high resolution (500 steps) for IMPS. The output voltage swing of the FSSO is stabilized by an adaptive gain control (AGC). The measurement results of the IMPS and mNIRS are combined by an artificial neural network (ANN) in external smart device so that mean absolute relative difference (mARD) is enhanced to 8.3% from 15% of IMPS, 15% - 20% of mNIRS. A proposed 12.5mm 2 0.18μm CMOS chip consumes peak power of 38mW at 1.5V. <s> BIB005
In this architecture, the offset resulting from the mismatch of the cascode current mirrors is eliminated by use of choppers. In processes with small supply voltages, cascodes can be a limiting factor in the output swing. In this system, although the presence of triple cascode increases the output impedance, it severely limits the output current amplitudes, as each cascode consumes at least an effective gate driving voltage. Any transistor in the signal path translates into a pole in the transfer function. Achieving a high phase margin is not an easy task BIB001 . The on-chip resistors are prone to mismatch. Using them as a biasing technique to drive the cascodes may not be an optimum option, as the variation from fabrication may cause some transistors to go in to the triode region and reduce the output impedance of the driver. When high frequency operation is considered, all the parasitic capacitors from triple cascode, in addition to stray capacitors, will result in phase imbalance between the input and output. A sinusoidal current driver based on ramp integrator and fabricated in 0.8 μm process with 3 V supply was discussed in BIB002 . The block diagram of the current driver is illustrated in Figure 4 . The output is fed to a voltage to current buffer that is used to define the limits of the current amplitude and to reduce the loading effects. The system works at three distinct frequencies of 1, 8, and 16 kHz through a 2-bit digital control capacitor bank denoted by letter S. The maximum output current is 7 μAp. The oscillation frequency is defined as BIB002  where VI is a constant reference voltage generated on-chip, CI is the integrator capacitor, gm is the transconductance of the OTA, and VH-VL is the allowable voltage swing for the integrator output BIB002 . A Gm-C ramp integrator, along with a second order band pass filter, was used to produce sinusoidal output voltages. A major drawback of this system is that it performs in open loop configuration. Hence, even a small input voltage can saturate the system. Moreover, the output current is directly affected by internal parameters of the system. The output impedance is defined by the output impedance of the buffer. Since the operating frequency is quite low, the capacitor effects of the limited bandwidth OTA in buffer are not a major issue. Good matching between the oscillation frequency of the ramp generator and band pass filter is a major task. As was mentioned in BIB002 , as the frequency increased, the amplitude of the current decreased, which resulted in inaccurate current measurements. No information about measured THD and output impedance is provided. A high Q band pass filter is neces- A G m -C ramp integrator, along with a second order band pass filter, was used to produce sinusoidal output voltages. A major drawback of this system is that it performs in open loop configuration. Hence, even a small input voltage can saturate the system. Moreover, the output current is directly affected by internal parameters of the system. The output impedance is defined by the output impedance of the buffer. Since the operating frequency is quite low, the capacitor effects of the limited bandwidth OTA in buffer are not a major issue. Good matching between the oscillation frequency of the ramp generator and band pass filter is a major task. As was mentioned in BIB002 , as the frequency increased, the amplitude of the current decreased, which resulted in inaccurate current measurements. No information about measured THD and output impedance is provided. A high Q band pass filter is necessary for better synchronization and a higher order or a smaller bandwidth is likely required to suppress the even harmonics from the ramp generator to an acceptable level, in order to have a clean sinusoidal input to the buffer. The current driver in  provides a current source and current sink from a 3.3 V supply. The schematic of the current driver is shown in Figure 5 . The amplitude of the current is determined by a 5-bit current DAC, which can be programmed from 16 to 512 µA. A timing control block generates different frequencies in the range of DC-500 Hz from a nominal 16 kHz clock that can be calibrated through a 3-bit capacitor bank  . The output impedance of the system is: where A o is the open loop gain of feedback amplifier, g m2 is the transconductance of M2, and r o1 and r o2 are the output resistance of transistor M1 and M2, respectively. The measured output impedance is 1.8 GΩ. With a load of 5 kΩ, a current amplitude of 512 µA was observed. The same principle for the open loop gain systems applies here. Moreover, the current amplitude and frequency of operation are dictated by DAC and capacitor bank, respectively. Any offset from the DAC will alter the current accuracy. Sensors 2019, 19, x FOR PEER REVIEW 6 of 19 6 sary for better synchronization and a higher order or a smaller bandwidth is likely required to suppress the even harmonics from the ramp generator to an acceptable level, in order to have a clean sinusoidal input to the buffer. The current driver in  provides a current source and current sink from a 3.3 V supply. The schematic of the current driver is shown in Figure 5 . The amplitude of the current is determined by a 5-bit current DAC, which can be programmed from 16 to 512 μA. A timing control block generates different frequencies in the range of DC-500 Hz from a nominal 16 kHz clock that can be calibrated through a 3-bit capacitor bank  . The output impedance of the system is: where Ao is the open loop gain of feedback amplifier, gm2 is the transconductance of M2, and ro1 and ro2 are the output resistance of transistor M1 and M2, respectively. The measured output impedance is 1.8 GΩ. With a load of 5 kΩ, a current amplitude of 512 μA was observed. The same principle for the open loop gain systems applies here. Moreover, the current amplitude and frequency of operation are dictated by DAC and capacitor bank, respectively. Any offset from the DAC will alter the current accuracy. The current driver shown in Figure 6 can be utilized as a driver for EIT systems. Transistors M1A, M1B, M2A, and M2B form the voltage to current converter, and transistors M3A, M3B, M4A, M4B, M5A, and M5B form an active inductive load BIB003 . Transistors in active load configuration are categorized as M3A and M4A to form cascode load, and M5A and M0A to form a source follower to provide the feedback control of the gate voltage transistor M3A BIB003 . The current driver shown in Figure 6 can be utilized as a driver for EIT systems. Transistors M1A, M1B, M2A, and M2B form the voltage to current converter, and transistors M3A, M3B, M4A, M4B, M5A, and M5B form an active inductive load BIB003 . Transistors in active load configuration are categorized as M3A and M4A to form cascode load, and M5A and M0A to form a source follower to provide the feedback control of the gate voltage transistor M3A BIB003 . . Open loop wideband current driver BIB003 . The high output impedance was achieved through the active load design by inserting a zero at low frequencies rather than using cascode stages. The transconductance of the current driver circuit is: Figure 6 . Open loop wideband current driver BIB003 . The high output impedance was achieved through the active load design by inserting a zero at low frequencies rather than using cascode stages. The transconductance of the current driver circuit is: where g m is the small signal transconductance of input transistors M2A-B, which are linearized by source degeneration triode transistors formed by M1A-B and represented by R. It is important to note that the transconductance of the current driver is directly proportional to g m of the transistors M2A-B, which is a design parameter for input transistors. Although the use of source degeneration is to linearize the transconductance, due to the small value of g m R, proper accuracy cannot be achieved. Even with the assumption of a large g m R, the transconductance will still be 1/R, which is a function of the drain current in the transistor. The output impedance of the circuit is BIB003 Z o = r o4 1 + jω C g m5 1 + jω C g m2 r o2 g m5 BIB005 As a result, the output impedance at low frequencies is equal to r o4 , which is the small signal output resistance of transistor M4. At high frequencies this can be varied by the location of zero and pole, which are BIB004 : Another limitation of such a system that performs in open loop is large gain variations, where even a small input voltage can saturate the system. Hence, the allowable input voltage range is basically defined by the transistors' saturation voltage ±2 √ V od , where the overdrive voltage (V od ), K n W plays an important role, as it can be a serious limitation for the maximum output current amplitude. To overcome this obstacle, either the bias current should be increased or the transistor sizes should be decreased. Increasing the bias current would significantly compromise the output resistance, since, as discussed earlier, the low frequency output resistance is defined by r o4 , which should be in the range of MΩ to ensure accuracy. It is also known that r o = 1/λI D , which dictates the fact that it would be a difficult task to achieve current in the range of mA. Considering the second option, which relates to reduction of transistors sizes, will result in small transconductance for input pairs. As mentioned earlier, although the introduction of a zero in the output impedance of the system improves its high frequency performance, the low frequency output impedance is degraded to some extent. The addition of a large 100 pF on-chip capacitor resulted in a simulated output impedance of 2.5 MΩ at 1 MHz BIB004 . However, the low frequency output impedance was only 375 kΩ. Thus, an improvement is observed in terms of high frequency operation, with reduced accuracy in low frequencies. The measurement results for loads from 100 Ω to 1 kΩ and operating frequency range of 10 kHz to 1 MHz provided 0.8% current variations where the maximum operating current was 500 µA p-p BIB003 . The output impedance at 10 kHz was >1 MΩ, however, it was reduced to >160 kHz at 1 MΩ frequency. The THD of the maximum output current was 42 dB (0.79%), measured at 600 kHz BIB003 . The effect of stray capacitors, especially at the output, not only degraded the output impedance, but also produced significant phase shift, especially at high frequencies. The input/output phase delay was 3.6 • at 1 MHz BIB003 . 