Common Mode Feedback Analysis for EIT Systems with Distributed Current Sources

The use of differential voltage measurements is widely used in EIT instruments. Instrumentation amplifiers are always affected by common mode voltages at their input. These voltages may have different origins, being the current sources and multiplexers the ones which contribute the most. However, even considering ideal current sources and multiplexers, some amount of common mode voltage will always be present due to the measurement principle. Errors produced by common mode voltages are very difficult to calibrate because they depend on the object being measured. At low frequencies it is easy to design instrumentation amplifiers with high CMRR values, but common mode voltages are also high, mostly because of unbalances in electrode impedances. At higher frequencies common mode voltages are smaller, but the CMRR of the amplifiers also decreases. Common mode feedback (CMF) to reduce common mode voltages has been largely used in EIT systems. There is a good theoretical understanding of CMF for systems using a single current source and a demultiplexer. However there is a new class of instruments which use distributed current sources. In these systems CMF is actually used to change the inherent unbalance among current sources instead of being applied to the output of the current source. We present a theoretical analysis of this new CMF technique and the results obtained when applying it to our latest EIT system