A comparison of modified Howland circuits as current generators with current mirror type circuits.

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.

[1]  F J Lidgey,et al.  A high output impedance current source. , 1994, Physiological measurement.

[2]  P. W. Van Der Walt A Wien-bridge oscillator with high-amplitude stability , 1981, IEEE Transactions on Instrumentation and Measurement.

[3]  P J Riu,et al.  A broadband system for multifrequency static imaging in electrical impedance tomography. , 1992, Clinical physics and physiological measurement : an official journal of the Hospital Physicists' Association, Deutsche Gesellschaft fur Medizinische Physik and the European Federation of Organisations for Medical Physics.

[4]  J. Jossinet,et al.  Performance and operation of a set of wide band current generators for EIT , 1994 .

[5]  U. Faust,et al.  An isolated wideband current source used in multifrequency electrical impedance tomography , 1994 .

[6]  J Jossinet,et al.  Bioelectrical spectroscopy from multi-frequency EIT. , 1994, Physiological measurement.

[7]  G.J. Saulnier,et al.  ACT3: a high-speed, high-precision electrical impedance tomograph , 1991, IEEE Transactions on Biomedical Engineering.

[8]  L Tarassenko,et al.  Impedance imaging in the newborn. , 1987, Clinical physics and physiological measurement : an official journal of the Hospital Physicists' Association, Deutsche Gesellschaft fur Medizinische Physik and the European Federation of Organisations for Medical Physics.

[9]  R Bragós,et al.  A wide-band AC-coupled current source for electrical impedance tomography. , 1994, Physiological measurement.

[10]  P. W. van der Walt A Wien-bridge oscillator with high-amplitude stability , 1981 .

[11]  L Lue Aspects of an electrical impedance tomography spectroscopy (EITS) system. , 1995 .

[12]  R Bragós,et al.  A parallel broadband real-time system for electrical impedance tomography. , 1996, Physiological measurement.

[13]  K Lindström,et al.  A current injecting device for electrical impedance tomography. , 1994, Physiological measurement.

[14]  R H Smallwood,et al.  Multi-frequency imaging and modelling of respiratory related electrical impedance changes. , 1994, Physiological measurement.

[15]  J Jossinet,et al.  Active current electrodes for in vivo electrical impedance tomography. , 1994, Physiological measurement.

[16]  D S Holder,et al.  Current approaches to analogue instrumentation design in electrical impedance tomography , 1996, Physiological measurement.

[17]  David S. Holder,et al.  A system for impedance imaging epilepsy in ambulatory human subjects , 1994 .

[18]  B H Brown,et al.  The Sheffield data collection system. , 1987, Clinical physics and physiological measurement : an official journal of the Hospital Physicists' Association, Deutsche Gesellschaft fur Medizinische Physik and the European Federation of Organisations for Medical Physics.