1. Field of the Invention
The present invention relates generally to stimulating medical devices, and more particularly, to calibration of current sources and sinks in a stimulating medical device.
2. Related Art
The delivery of electrical stimulation has become an established part of medical therapy. Numerous types of medical devices have components positioned on, or implantable in, a recipient's body in order to stimulate a recipient's tissue. Such devices are sometimes referred to herein as stimulating medical devices. Stimulating medical devices commonly include a plurality of electrodes that function as the interface between electronics of the device and the recipient's body tissue. In general terms, current is delivered to the recipient's tissue via the electrodes in order to evoke a response, such as a perception (e.g. for sound perception) or a function (e.g. for limb movement), in the recipient.
FIG. 1 is a schematic diagram illustrating the delivery of current to tissue. In this illustration, an implantable stimulating medical device 110 comprises an implantable component 112, and a multi-electrode system in the form of two platinum electrodes 101. Each of the electrodes are connected to component 112 by insulated wires 104. Component 112 comprises a stimulating current source 102 that provides current to electrode 101A. The current passes through the recipient's tissue 100, including nerve cell 107, and returns to ground 114 within implantable component 112. The return of this current is shown schematically by arrow 103.
At the surface of platinum electrodes 101, chemical reactions take place, changing the electron current from the current source to an ion current 105 in the tissue. A charge 106 remains on the electrode surface, causing an increase in voltage in the tissue. Under normal conditions, these chemical reactions are reversible by a change in the direction of current. That is, a reversal in the direction of current will neutralize the increase in voltage. As such, it is common for the stimulation current to be delivered as biphasic pulses, in such a way that there is no net charge delivered to the tissue. A biphasic pulse includes a positive charge pulse followed by an equal negative charge pulse. In certain circumstances, the current level (amplitude) and periods of both the positive and negative pulses are substantially the same. In other circumstances, one of the pulses is applied over a longer or shorter period, but has lower or greater amplitude, respectively. However, in both circumstances, the total charge remaining in the tissue after delivery of both the positive and negative pulses is substantially zero.
In circumstances using biphasic pulses, if current is allowed to flow in one direction for too long, toxic products can escape and damage or destroy the surrounding tissue. Likewise, if the voltage between two electrodes is allowed to remain elevated for too long, toxic species are irreversibly generated. To ensure that stimulation remains safe, and that no toxic species escape, it is known that the DC and low-frequency (LF) states of the electrodes, sometimes referred to as the DC/LF voltages and the DC/LF currents, must remain within certain bounds. For a typical cochlear implant electrode having an area of about 0.25 mm2, these values are generally a few hundred milli-volts (mV), or tens of nano-amperes (nA). Additionally, the United States Federal Drug Administration (FDA) requires that the magnitude of the current through an electrode, during a 1 ms period, be below 100 nA. The use of charge neutralizing biphasic pulses helps ensure that these requirements are met, but charge errors occur in practice.
In certain stimulating medical devices, separate current source circuits and current sink circuits, referred to simply as current sources and current sinks, respectively, are configured to deliver or receive stimulating current. The sources and sinks each use a Digital-to-Analog Converter (DAC) to control the flow of current.