AN ELECTRICALLY ISOLATED BALANCED WIDE-BAND CURRENT SOURCE - BASIC CONSIDERATIONS AND DESIGN

At relatively high frequencies, the application of an alternating curr ent through the body or a body segment results in electromagnetic stra y fields which reduce the amount of current actually injected into the tissue under study. This radiation effect can be reduced by use of a symmetrical configuration current source. The symmetry of such an arra ngement, however, depends on the stray capacitances of the source with respect to surrounding equipment. To minimise these effects, it is re quired that the source is electrically isolated from the surrounding e quipment and the subject under study. In this manner stray capacitance s with respect to elements of the current source are reduced. In such a configuration common mode voltages to the input amplifier of the mea suring system are also reduced. The paper describes design considerati ons and the implementation of a wideband current source capable of inj ecting alternating current in the order of 300 mu A(RMS) into biologic al tissue having impedances up to 1 k Omega. Current stabilisation is obtained by means of a control circuit which measures the actual curre nt passing through the tissue under study. Leakage currents arising fr om shielding and stray capacitances are compensated for. The usable fr equency range is between 4 kHz and 1024 kHz and current stability is b etter than 0.2%. Through the use of a symmetrical, floating circuit a configuration is obtained which substantially reduces stray effects. T he current source is connected to other circuits by means of two isola tion ports: (1) a transformer coupling for the carrier frequency; and (2) an opto-coup(er to transfer a phase reference signal obtained from current measurement. The current amplitude can be modulated by contro lling the reference input to the control loop by means of a third auxi liary isolation port for transfer of the modulating signal.