Hongwei Bian and Wuqiang Yang
Modified Howland current source, Dual op-amp current source, Output impedance
This paper deals with two current sources, a modified Howland current source (MHCS) and a dual op-amp current source (DOCS), which are used in electrical impedance tomography (EIT) systems. To estimate the output impedance of the two current source circuits accurately, an approximated non-linear op-amp model is employed. In this model, the input impedance and the open-loop gain of the op-amps are considered to be non-ideal, but the output impedance of op-amps is approximated to be zero. Approximated analytical formulas for both MHCS and DOCS are deduced. For comparison purposes, the general analytical solution based on the non-ideal op-amp model, and the simplified formula based on the simplified ideal model are derived. Simulation has been carried out for both MHCS and DOCS, using µA741 and LM6365 for each circuit. It has been verified that the approximated formulas can provide sufficiently accurate results. The analysis approach can also be used for other similar op-amp circuits.
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