High-Power CMOS Current Driver With Accurate Transconductance for Electrical Impedance Tomography

Abstract

Current drivers are fundamental circuits in bioimpedance measurements including electrical impedance tomography (EIT). In the case of EIT, the current driver is required to have a large output impedance to guarantee high current accuracy over a wide range of load impedance values. This paper presents an integrated current driver which meets these requirements and is capable of delivering large sinusoidal currents to the load. The current driver employs a differential architecture and negative feedback, the latter allowing the output current to be accurately set by the ratio of the input voltage to a resistor value. The circuit was fabricated in a 0.6- μm high-voltage CMOS process technology and its core occupies a silicon area of 0.64 mm (2) . It operates from a ± 9 V power supply and can deliver output currents up to 5 mA p-p. The accuracy of the maximum output current is within 0.41% up to 500 kHz, reducing to 0.47% at 1 MHz with a total harmonic distortion of 0.69%. The output impedance is 665 k Ω at 100 kHz and 372 k Ω at 500 kHz.

Design of a CMOS active electrode IC for wearable electrical impedance tomography systems

“…As shown in Figs 6 and 7, the current driver transconductance is 1.99 mA/V up to 500 kHz with a phase delay of -4.32ᵒ. The output current amplitude can reach 6 mA p-p , compared to only 2mA p-p in [8]. At 500 kHz the output impedance is 1.7 MΩ (excluding the effects of off chip stray capacitance).…”

“…In addition, both these designs have a relatively low operational bandwidth (100 kHz in [7] and 256 kHz in [6]) and low output impedance. A high voltage current driver IC is reported in [8] but the design is not suitable for a parallel EIT system using active electrodes.…”

A High Frame Rate Wearable EIT System Using Active Electrode ASICs for Lung Respiration and Heart Rate Monitoring

“…For current drive, the parasitic capacitance due to the cables and switch network ranging up to hundreds of pF is reduced to only a few pF. This allows the current driver to maintain a high output impedance even at high frequencies [12], [16]. This not only increases the measurement accuracy but also increases the system operating bandwidth.…”

“…Although some of the more recognizable systems have gone through several updates [7], [8] EIT is still far from ready for daily clinical practice due to image qualities affected by errors in the hardware as well as stray capacitance influence [9] caused by the impractical patient electrode interfaces [10], [11]. Methods investigated to improve system performance include: 1) current drivers with high output impedance [12]; 2) fully-parallel read-out channels to reduce delays in EIT scans and improve image frame rate (this is not only more suitable for monitoring neonates whose breathing is twice as fast as adults, but also can capture rapid physiological changes such as epilepsy [13], [14]); 3) stray capacitance cancellation circuits aiming to calibrate out the stray capacitance [15], [16]; which finally evolves to the most practical, 4) active electrode approach that reduces the effect of parasitic capacitance in the cables and switch networks by placing the electronics in direct contact with the electrodes [17].…”

A CMOS current driver with built-in common-mode signal reduction capability for EIT

“…Current drivers based on push-pull current mirrors controlled by a voltage input are reported in [4]- [6] for biomedical application ICs. Several custom current driver ICs using feedback have also been reported [7], [8], offering better performance than open-loop systems. However, the source and sink sides of those drivers are relatively independent, and the matching is heavily dependent on the feedback resistor and the input voltage.…”