Wide Band Howland Bipolar Current Source using AGC Amplifier

Electrical impedance tomography (EIT) is a promising technique for imaging the conductivity distribution within the body from voltage surface measurements. The measuring precision is very important, where inaccuracies from the current source generate distortions in the images. These are particularly caused by stray capacitances which reduce the frequency response of the instrumentation. This paper investigates the performance of a bipolar current source attached to a multiplexer using an Automatic Gain Control (AGC) Amplifier circuit. Output current measurements were made in the frequency range 100 Hz to 1 MHz and also output impedance at 100 kHz, both with and without the AGC Amplifier circuit. The results showed a constant output current with a maximum error of ±2% when using the AGC Amplifier circuit whereas 300% not using it. Also, the output impedance was approximately 10.4 and 1.2 M¿ with and without using the AGC Amplifier circuit, respectively. It can be concluded that the AGC Amplifier circuit may increase significantly the precision of the current measurements and may improve the image quality in EIT systems.