Neural sensing in an implantable stimulator device during passive charge recovery
Techniques for sensing neural responses such as Evoked Compound Action Potentials (ECAPs) in an implantable stimulator device are disclosed. A first therapeutic pulse phase is followed by a charge recovery phase that includes at least one high-impedance passive charge recovery duration. The ECAP is sensed during the high-impedance passive charge recovery duration. The time period of the passive charge recovery is lengthened and the high-impedance passive recharge duration entirely overlaps the ECAP (i.e., the neural response duration) at the sensing electrode.
1. A method for operating a stimulator device, the stimulator device comprising a plurality of electrode nodes, each electrode node configured to be coupled to one of a plurality of electrodes configured to contact a patient's tissue, the method comprising:
providing actively-driven stimulation at at least one stimulation node selected from the plurality of electrode nodes, wherein the stimulation comprises at least one pulse comprising at least a first phase;
providing passively-driven passive charge recovery for a passive charge recovery duration,
selecting a first recovery impedance for a high-impedance portion of the passive charge recovery duration and a second recovery impedance for a low-impedance portion of the passive charge recovery duration, wherein the first recovery impedance is greater than the second recovery impedance,
using a variable resistance circuitry of the stimulator device to provide the selected recovery impedances during the passive charge recovery duration,
determining a time at which the neural response will be present at the sensing electrode node and timing the passive charge recovery duration so that the high-impedance passive charge recovery portion will overlap the time at which the neural response will be present the sensing electrode node, and
sensing an evoked neural response evoked by the stimulation during the high-impedance passive charge recovery portion at at least one sensing electrode node selected from the plurality of electrode nodes during the passive charge recovery duration.
2. The method of claim 1 , wherein the variable resistance circuitry comprises a plurality of switching circuits, wherein each of the plurality of switching circuits is coupled with a different one of the electrode nodes and is configured, when selected, to provide variable impedance between its respective electrode node and a common node.
3. The method of claim 2 , wherein the common node comprises a reference voltage selected from the group consisting of a battery voltage, a compliance voltage, a fraction of a compliance voltage, and ground.
4. The method of claim 2 , wherein each of the plurality of switching circuits comprises a plurality of switches wherein the switches are selectable to vary the resistance.
5. The method of claim 2 , wherein plurality of switches comprises a plurality of transistors in parallel.