Study on Wide-band Voltage Controlled Current Source for Electrical Impedance Tomography

Electrical Impedance Tomography is applied to acquire the impedance characteristic, which is correlative with the physiological and pathological status of a body, and reconstruct the images of impedance distribution. Current injection with voltage measurement phantom is widely adopted in the application of biomedical detection. Voltage controlled current source (VCCS) develops an important part and its performance directly affects the precision of the system. In this paper we analyze and simulate the driving capability and operating bandwidth of the improved Howland current source. By using a wide-band, low noise amplifier, OPA843, we implement the current excitation circuit, which performs well. The maxim output impedance achieves 103.41Mohm under frequency from 1KHz to 100KHz. When driving a pure resistive load of 2Kohm, the -3dB bandwidth is 63MHz and decreases to 10.7MHz with resistive load of 10Kohm. When the output impedance decreases due to the stray capacitance, a general impedance converter is used in parallel with Howland. The simulation and test results show that the voltage controlled current source performs stably and provides great output impedance and superior driving bandwidth, which is suitable for EIT application.