Evaluation of the quality of electrode contact with a skin surface

1. An electrode contact quality evaluation device configured to evaluate a connection between at least one electrode and a skin surface, the electrode contact quality evaluation device comprising:

a current source that generates a test current;

an injection electrode selection circuit that supplies the test current to a selected injection electrode through a selected injection lead;

a driven return circuit that generates a return current having opposite polarity to the test current;

a return electrode selection circuit that supplies the return current to a selected return electrode through a selected return lead;

a sense electrode selection circuit that supplies a signal obtained from one or more selected sense electrodes through one or more selected sense leads, to the driven return circuit;

differential measurement circuitry that measures differences between signals present at the injection lead and the one or more sense leads and between signals present at the one or more sense leads and the return lead; and

a processing device that evaluates the differences and generates an output representing a quality of at least one connection between at least one of the electrodes and the skin surface.

2. The electrode contact quality evaluation device of claim 1 , wherein the driven return circuit generates the return current the average of which has a magnitude substantially equal to and of opposite polarity to the test current.

3. The electrode contact quality evaluation device of claim 1 , wherein the differential measurement circuitry comprises at least a pair of differential amplifiers, a first of the differential amplifiers electrically coupled to the selected injection electrode and the one or more selected sense electrodes, and a second of the differential amplifiers electrically coupled to the one or more selected sense electrodes and the selected return electrode.

4. The electrode contact quality evaluation device of claim 3 , wherein outputs of the differential amplifiers are converted from analog signals to digital signals by an analog to digital converter, and wherein the digital signals are provided to the processing device.

5. The electrode contact quality evaluation device of claim 1 , wherein the driven return circuit comprises at least an inverting amplifier and is electrically coupled between the one or more selected sense electrodes and the selected return electrode.

6. The electrode contact quality evaluation device of claim 1 , wherein the current source is a square wave generator.

7. A method of evaluating the quality of electrode contact with a skin surface, the method comprising:

checking for and determining that at least three electrodes are in physical contact with a skin surface using electrode contact detection circuitry;

evaluating and determining a quality of an electrical connection between at least one of the electrodes and the skin surface using electrode contact quality evaluation circuitry, the evaluating and determining including:

selecting at least one of the electrodes as an injection electrode, at least another of the electrodes as a sense electrode, and at least another of the electrodes as a return electrode;

applying a test signal to the injection electrode;

applying a return signal of opposite polarity to the return electrode;

determining a voltage drop of the test signal and of the return signal; and

computing at least one contact impedance for one of the injection electrode or the return electrode; and

inhibiting instrument operations if the quality of the electrical connection is determined to be insufficient.

8. The method of claim 7 , wherein evaluating and determining the quality of the electrical connection comprises computing a contact impedance for the injection electrode and a contact impedance for the return electrode with a single test operation.