A Comprehensive Study on Current Source Circuits

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A current source is one of the key components in an EIT system, particularly in EIT based breast imaging which is called electrical impedance mammography (EIM). It is also a critical source of systematic errors due to limited output impedance and associated frequency bandwidth. One method for increasing the output impedance is a Generalised Impedance Converter (GIC). This paper investigates two Howland-based, voltage-to-current conversion (V/I) circuits; one is a simple, optimized current source developed by the Leicester Group while the other includes a GIC. This investigation evaluates both circuits using PSpice, measuring output impedances at 1KΩ loading along with -3dB and -1dB bandwidths; these are of clinical interest, as EIT needs to cover a wide range of frequencies. It was expected that the GIC would increase the output impedance with required frequency bandwidth. The results show that, the GIC has improved the output impedance, reaching 120MOhm. However, the high output impedance was only for a limited bandwidth.