A comprehensive study on current source circuits

Abstract

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. It fell to 2.7Ω at and beyond 100KHz. On the other hand, the circuit developed at Leicester produced a more stable output impedance, which was higher, compared to that generated by the former circuit for the majority of the tested bandwidth. The output impedance maintained at 5MΩ until 300KHz. Simulations were run using ideal op-amps, these showed that the GIC circuitry was limiting the bandwidth, -1dB bandwidth for 2KΩ loading was below 1MHz, while the Improved Howland model relied on the parameters of the chosen op-amp and showed no drop off with ideal chips; its output impedance remained at 4MΩ until after 1MHz. Based on the experimental results, the simpler Improved Howland circuit has been considered to be the better circuit for a large frequency range as it is consistent and has room for improvement with better components. © Springer-Verlag 2007.