TITLE:
High Accurate Howland Current Source: Output Constraints Analysis
AUTHORS:
Pedro Bertemes-Filho, Alexandre Felipe, Volney C. Vincence
KEYWORDS:
Howland Current Source; Electrical Bioimpedance; Probability Density Function; Resistors Mismatching
JOURNAL NAME:
Circuits and Systems,
Vol.4 No.7,
November
8,
2013
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.