Voltage controlled current sources: Dependence of the current phase on the load resistance.

H. Märtin*, E.Gersing**

* Institute of Electronics, Tallinn Technical University, Ehitajate tee 5,
EE-0026 Tallinn, Estonia

** Zentrum Physiologie und Pathophysiologie der Universität, Humboldtallee 23,
D-37073 Göttingen, Germany

In electrical impedance tomography, voltage controlled current sources are widely used for current injection into the body under investigation. As far as imaging is based on the real part or the magnitude of impedance, the application of such current sources is suitable for frequencies up to some 100 kHz. Howevwer, using the imaginary part or the phase of impedance, structures may appear in the image wich do not correlate with the structures of the object itself.

Therefore, we investigated the behaviour of active current source circuits, for example, the Howland source, depending on load resistance and frequency in the range of 1 to 100 kHz. The internal resistance of the current sources had been adjusted to about 10 MOhm. The load resistance was varied between 0.1 KOhm and 5.6 kOhm. The measurements were carried out by means of a "hp 4194A Gain/Phase Analyzer".

Using an operational amplifier LF356 which has a GBW of 4.5 MHz, a phase difference of 7.7 degree was found at a frequency of 30 kHz and a load resistance of 3.3 kOhm. There is a linear dependence of the phase load resistance up to 5.6 kOhm. At a frequency of 100 kHz a phase difference of 25 degree was found under the same conditions. The measuring current was 1.4 mA. The phase diference was nearly independent of the magnitude of the current generated.

A faster amplifier, the AD843, with a GBW of 34 MHz, exhibited smaller phase difference: 1.8 degree at 3.3 kOhm and 30 kHz, and 6.3 degree at 100 kHz. For tomography based on imaginary part or phase of impedance, the properties of voltage controlled current sources even with an operational amplifier as fast as the AD843 are not sufficient. Therefore, it is necessary to measure magnitude and phase of the injected current even though such circuitry is more complicated.