Abstract
Howland circuits have been widely used as powerful source for exciting tissue over a wide frequency range. When a Howland source is designed, the components are chosen so that the designed source has the desired characteristics. However, the operational amplifier limitations and resistor tolerances cause undesired behaviors. This work proposes to take into account the influence of the random distribution of the resistors in the modified Howland circuit over the frequency range of 10 Hz to 10 MHz. Both output current and impedance of the circuit are deduced either considering or the operational amplifiers parameters. The probability density function due to small changes in the resistors of the circuit was calculated by using the analytical modeling. Results showed that both output current and impedance are very sensitive to the resistors variations. In order to get higher output impedances, high operational amplifier gains are required. The operational amplifier open-loop gain increases as increasing the sensitivity of the output impedance. The analysis done in this work can be used as a powerful co-adjuvant tool when projecting this type of circuit in Spice simulators. This might improve the implementations of practical current sources used in electrical bioimpedance.
Highlights
Because of its simplicity, stability and other advantages, the voltage controlled current source (VCCS) has been widely used in many applications, such as in neuron-stimulation systems [1,2,3], single-electrode capacitive sensors [4], electrical impedance tomography (EIT) systems both for industrial and medical applications [5,6] and bioimpedance analysis (BIA) for tissue characterization [7,8]
In order to improve the stability of the circuit, two single-ended Howland circuits can be set together in order to have a differential output current, which is called as Mirrored Modified Howland Current Source (MMHCS) and it is shown in Figure 6 [17]
Care should be taken when analyzing the results obtained in this work, especially when they are related to practical circuits. It was fully described the modeling of both output current and impedance of the Howland current source by considering the operational amplifier parameters and the mismatching between electrodes
Summary
Stability and other advantages, the voltage controlled current source (VCCS) has been widely used in many applications, such as in neuron-stimulation systems [1,2,3], single-electrode capacitive sensors [4], electrical impedance tomography (EIT) systems both for industrial and medical applications [5,6] and bioimpedance analysis (BIA) for tissue characterization [7,8]. It is been used for exciting tissue for cancer characterization in electrical impedance spectroscopy [9,10,11].
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