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.

Bioimpedance measurement systems often use the Howland current sources to excite the biological material under study. Usually, difference or instrumentation amplifiers are used to measure the resulting voltage drop on this material. In these circuits, common mode voltage appears as artifacts in the measurement. Most researches on current sources are focused on improving the output impedance, letting other characteristics aside. In this paper, it is made a brief review on the load common mode voltage and output swing of various topologies of Howland current sources. Three circuits are proposed to reduce load common mode voltage and enhance load capability by using a fully differential amplifier as active component. These circuits are equated, simulated and implemented. The three proposed circuits were able to deliver an output current with cut-off frequency (-3dB) higher than 1 MHz for loads as big as 4.7 kΩ. The worst measured load common mode voltage was smaller than 24 mV for one of the circuits and smaller than 8 mV for the other two. Consequently, it could be obtained increases in the Common Mode Rejection Ratio (CMRR) up to 60 dB when compared to the Enhanced Howland Current Source (EHCS).

For Electrical Impedance Spectroscopy (EIS) applications, the Enhanced Howland Current Source (EHCS) is a popular choice as an excitation circuit due to its simplicity, reliability, and safety. However, its output impedance degradation at high frequency leads to errors that are unacceptable for high load impedance applications, such as the ones which use dry or microelectrodes. Based on a proposed mathematical model, this work designed an EHCS circuit which includes an output current buffer and frequency compensation. PSpice simulations were performed as proof of concept, and then the measured data were collected for comparison. For the proposed circuit, called here Load-in-the-Loop Compensated Enhanced Howland Source (LLC-EHCS), the results showed that the output current errors are lower than 1% up to 3.7 MHz over the load range of 560-2200 Ω and 1.2 MHz with 5.6 kΩ. On the other hand, for the case of the standard EHCS circuit, these frequencies are 170 and 80 kHz, respectively. Also, the output linear swing was found to be 3 times higher than the EHCS. It can be concluded that the proposed LLC-EHCS may be widely used as an excitation circuit for high load and wide bandwidth EIS applications.

Electrical Bioimpedance Analysis has been widely used as a non-invasive technique for characterizing tissues. Most systems use a wideband and a high precision instrumentation, specially the current source. The objective of this work is to compare the Howland circuit with three OTA-based floating voltage controlled current sources. The results show that both Current Conveyor and class-AB OTA have a wider output current frequency response and both output impedance is 226 % bigger than the Howland circuit at 1 MHz. The class-AB OTA circuit presents a power consumption of 4.6 mW whereas 1.6 mW for the Current Conveyor. The results might be useful for cell impedance measurements and for very low power bioimpedance applications

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