Output Impedance Of Current Source Research Articles

Practical Implementation of a Novel Output Impedance Measurement Technique for EIT System While Attached to a Load.

A novel method for measuring the output impedance of current sources in an EIT system is implemented and tested. The paper shows that the proposed method can be used at the time of operation while the load is attached to the EIT system. the results also show that performance of the system improves when the shunt impedance values from the proposed technique are used to set the adaptive sources as opposed to the shunt impedance values acquired through open circuit measurements.

Mirrored enhanced Howland current source with feedback control.

An impedance spectrum is calculated by the ratio between an injecting current and a resulting measured voltage, which allows the extraction of electrical properties from the material under study. The current source is considered an essential block to deliver a controlled current to a wide range of working loads and large bandwidth. To comply with such requirements, the current source output impedance must be much higher than the load impedance at each discrete frequency within the range. However, stray capacitance from cables and circuitry reduce the output impedance, especially at higher frequencies. We proposed a modified mirrored enhanced Howland current source (MEHCS) by using the feedback technique for a wide frequency range applications on electrical bioimpedance. We implemented four MEHCS circuits [with/without multiplexer (MUX) and with/without feedback], and then the output current and impedance were measured up to 20 MHz. The proposed current source showed an improvement in the frequency response at lower and higher frequencies when compared to the standard circuit. The measured output impedance was 10 times higher in the proposed circuit than in the standard MEHCS. The use of a feedback also increased the bandwidth in almost one decade in low and high frequencies when loaded with a resistor of about 1 kΩ.

Exploration of Low-Power High-SFDR Current-Steering D/A Converter Design Using Steep-Slope Heterojunction Tunnel FETs

Steep-slope heterojunction tunnel field-effect transistor (HTFET) devices promise new opportunities beyond CMOS in low-power high-performance communication applications. In this paper, the circuit design optimization of a low-power 14-bit 1-GS/s current-steering digital-to-analog converter (DAC) using 0.4/0.3 V mixed-supply HTFETs is explored. Based on the device characteristics comparison and circuit analysis, it is shown in this paper that HTFET endorses significant differences in both $I$ – $V$ and $C$ – $V$ due to the steep-slope tunneling mechanism and a nature of vertically fabricated structure. While such differences significantly affect the circuit design corners, this paper gives the device-circuit co-optimization for the HTFET DAC, reaching at higher current source output impedance, less nonlinear switching glitch distortions, and thus superior spectral performance over the Si-CMOS DAC. HTFET device variation is also discussed, and calibration techniques are adopted for the static matching accuracy.

Background Calibration of Pipelined ADCs Via Decision Boundary Gap Estimation

A method of indirect background digital calibration of the dominant static nonlinearities in pipelined analog-to-digital converters (ADC) is presented. The method, called decision boundary gap estimation (DBGE), monitors the output of the ADC to estimate the size of code gaps that result at the decision boundaries of each stage. Code gaps result from such effects as capacitor mismatch, finite opamp gain, finite current source output impedance, comparator offset, and charge injection. DBGE does not require special calibration signals or additional analog hardware and can even reduce the performance requirements of the analog circuitry. The calibration is performed using the input signal and thus requires that the input signal exercise the codes in the vicinity of the decision boundaries of each stage. If it does not exercise these codes, then lack of calibration is less critical because the nonlinearities will not appear in the output signal. DBGE is simple and amenable to hardware and/or software implementations. Simulation results indicate DBGE is highly accurate, robust, and adaptive even in the presence of parameter drift and circuit noise.

ACT3: a high-speed, high-precision electrical impedance tomograph.

This paper presents the design, implementation, and performance of Rensselaer's third-generation Adaptive Current Tomograph, ACT3. This system uses 32 current sources and 32 phase-sensitive voltmeters to make a 32-electrode system that is capable of applying arbitrary spatial patterns of current. The instrumentation provides 16 b precision on both the current values and the real and reactive voltage readings and can collect the data for a single image in 133 ms. Additionally, the instrument is able to automatically calibrate its voltmeters and current sources and adjust the current source output impedance under computer control. The major system components are discussed in detail and performance results are given. Images obtained using stationary agar targets and a moving pendulum in a phantom as well as in vivo resistivity profiles showing human respiration are shown.