Howland Current Source Research Articles

A COMPARATIVE STUDY OF CURRENT SOURCES USED IN BIOIMPEDANCE MEASUREMENT SYSTEMS

The current source circuit is one of the most important parts in the bioelectrical impedance devices and Bio Impedance Tomography (BIT). There are many types of circuits like the Howland Current Source, Improved Howland Current Source, General Impedance Convertor, Wien Bridge circuit etc. This paper presents the comparative analysis of the available current sources and study of the best suited ones for the bioimpedance measurement and its various applications.

A fully parallel multi-frequency EIT system with flexible electrode configuration: KHU Mark2

We report the development of a new multi-frequency electrical impedance tomography (EIT) system called the KHU Mark2. It is descended from the KHU Mark1 in terms of technical details such as digital waveform generation, Howland current source with multiple generalized impedance converters and digital phase-sensitive demodulators. New features include flexible electrode configurations to accommodate application-specific requirements, multiple independent current sources and voltmeters for fully parallel operations, improved data acquisition speeds for faster frame rates and compact mechanical design. Given an electrode configuration, we can design an analog backplane in such a way that both current injections and voltage measurements can be done without using any switch. The KHU Mark2 is based on an impedance measurement module (IMM) comprising a current source and a voltmeter. Using multiple IMMs, we can construct a multi-channel system with 16, 32 or 64 channels, for example. Adopting a pipeline structure, it has the maximum data acquisition speed of 100 scans s−1 with the potential to detect fast physiological changes during respiration and cardiac activity. Measuring both in-phase and quadrature components of trans-impedances at multiple frequencies simultaneously, the KHU Mark2 is apt at spectroscopic EIT imaging. In this paper, we describe its design, construction, calibration and performance evaluation. It has about 84 dB signal-to-noise ratio and 0.5% reciprocity error. Time-difference images of an admittivity phantom are presented showing spectroscopic admittivity images. Future application studies using the KHU Mark2 are briefly discussed.

Comparison of three current sources for single-electrode capacitance measurement

The capacitance of a single electrode is usually measured by injecting a current to the electrode and measuring the resultant voltage on the electrode. In this case, a voltage-controlled current source with a high bandwidth is needed because the impedance is inversely proportional to the excitation frequency. In this design note, three different current sources are discussed: (1) the Howland current source, (2) a modified Howland current source, and (3) a dual op-amp current source. The principle and dynamic performances are presented and compared. Simulation and experimental results show that although the Howland current source has the lowest (i.e., worst) output impedance, its output is the most stable among the three current sources when the frequency changes. Therefore, it is suitable for single-electrode capacitance measurement. Initial tests have proven the feasibility of single-electrode capacitance sensor with the Howland current source.

Low-noise current excitation sub-system for medical EIT

Electrical impedance tomography (EIT) has many potential applications in medicine. The sensitivity and spatial resolution of EIT can be enhanced significantly by increasing the measurement signal-to-noise ratio (SNR), to which a significant contribution is made by the SNR of the current excitation sub-system. In this paper we present a prototype EIT current excitation sub-system with 80 dB SNR, exploiting both digital and analogue techniques. It uses digital waveform synthesis, a 16-bit DAC and subsequent reconstruction filter, to drive an enhanced Howland current source. Detailed analysis and testing of the current sub-system are presented. Its output impedance is 10 MΩ for different load impedances, varying slowly over the frequency range from 10 kHz up to 4 MHz.

Noise analysis of a Howland current source

We present a detailed noise analysis of a Howland current source that can supply up to 25 mA output current with a current gain of approximately 20. A modified topology is also considered which includes an additional compensation resistor at the non-inverting terminal of the amplifier to balance the input impedance when the amplifier is used with current gain. A lowest measured current noise of 13 pA/√Hz (for f > 40 Hz) is obtained for the balanced configuration. We find that the addition of a compensation resistor improves the thermal stability of the amplifier and this agrees well with theoretical predictions. Some care must be taken when matching the resistance ratios of the Howland current source as matching at the level of 1% results in a theoretical output impedance of approximately 22 kΩ for our design. The analytical expressions developed in this work should allow noise analysis to be performed for general resistance combinations.