title: A comparison of modified Howland circuits as current generators with current mirror type circuits.
abstract: Multi-frequency electrical impedance tomography (EIT) systems require stable voltage controlled current generators that will work over a wide frequency range and with a large variation in load impedance. In this paper we compare the performance of two commonly used designs: the first is a modified Howland circuit whilst the second is based on a current mirror. The output current and the output impedance of both circuits were determined through PSPICE simulation and through measurement. Both circuits were stable over the frequency ranges 1 kHz to 1 MHz. The maximum variation of output current with frequency for the modified Howland circuit was 2.0% and for the circuit based on a current mirror 1.6%. The output impedance for both circuits was greater than 100 kohms for frequencies up to 100 kHz. However, neither circuit achieved this output impedance at 1 MHz. Comparing the results from the two circuits suggests that there is little to choose between them in terms of a practical implementation.
authors.affiliation: Department of Medical Physics and Clinical Engineering, Royal Hallamshire Hospital, University of Sheffield, UK. mpp98pb@sheffield.ac.uk
authors.forename: P
authors.initials: P
authors.lastname: Bertemes-Filho
authors.forename: B H
authors.initials: BH
authors.lastname: Brown
authors.forename: A J
authors.initials: AJ
authors.lastname: Wilson
mesh_terms: D001698:Biomedical Engineering; D017097:Electric Impedance; D004566:Electrodes; D004582:Electronics, Medical; D004867:Equipment Design; D005069:Evaluation Studies as Topic; D006801:Humans; D014054:Tomography / Q000295:instrumentation* / Q000379:methods