Study of precision constant current sources with resistor load and a cole-cole load for multi-frequency in electrical impedance tomography

The current source is one of the most important circuits in Electrical Impedance Tomography (EIT) systems. It should be able to supply low current (1 mA) at 10 kHz–1 MHz and can support load between 100 Ω–10 kΩ. Moreover, it must have output impedance greater than 100 kD. In this study, we evaluated an Enhanced Howland (EH) and Triple Op-Amps (TOA) current sources with two different op-amps namely LM741 and AD844. The constant current was measured at 1 kHz to 1 MHz with two different loads (Resistor and Cole-Cole load). The results showed that the constant and high output impedance current sources for multi-frequency could be fulfilled both EH and TOA current sources prepared with op-amp AD844 and a Cole-Cole load. The constant current can be achieved up to 100 kHz for the op-amp AD844 while op-amp LM741 is only able to achieve up to 10 kHz. Meanwhile, measured output impedance showed that the EH current source is better than TOA current source. The output impedance of the EH current source was 1.18 MD at 1 kHz and was higher than 600 kD at 500 kHz, while the output impedance of the TOA current source was 1.16 MD at 1 kHz and was higher than 300 kD at 500 kHz. Otherwise, the op-amp LM741 is only able to produce output impedance below 200 kD at 10 kHz for both EH and TOA.