Abstract
Common mode voltages are frequently a problem in electrical impedance tomography (EIT) and other bioimpedance applications. To reduce their amplitude common mode feedback is employed. Formalised analyses of both current and voltage feedback is presented in this paper for current drives. Common mode effects due to imbalances caused by the current drives, the electrode connections to the body load and the introduction of the body impedance to ground are considered. Frequency selective narrowband common mode feedback previously proposed to provide feedback stability is examined. As a step towards multifrequency applications the use of narrowband feedback is experimentally demonstrated for two simultaneous current drives. Measured results using standard available components show a reduction of 62 dB for current feedback and 31 dB for voltage feedback. Frequencies ranged from 50 kHz to 1 MHz.
Highlights
The established method of electrical impedence tomography (EIT) (Boone and Holder 1996, Bayford 2006) uses an array of sensing electrodes as shown in figure 1(a)
When there are a large number of electrodes the voltages across the distant sense electrodes due to the current drive can be very small
The problem of large common mode voltages at higher frequencies in bioimpedance applications can be suppressed by adding narrowband common mode feedback
Summary
The established method of electrical impedence tomography (EIT) (Boone and Holder 1996, Bayford 2006) uses an array of sensing electrodes as shown in figure 1(a). The body is a nonuniform conducting medium. Current is injected into two electrodes and the voltage across pairs of electrodes is detected and amplified. Algorithms using the resulting voltage measurements deduce the disposition of different components in the body (Bayford 2006, Adler et al 2009). Measurements from a current drive with simultaneous multiple frequencies are employed (McEwan et al 2007). Increasing the number of electrodes and employing drives with more than one sinewave frequency enable better definition of the body components. When there are a large number of electrodes the voltages across the distant sense electrodes due to the current drive can be very small. The amplitude of the differential voltages depends on the application, but voltages down to about 0.05 mV can be expected
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