Pulse-density modulation to synthesize stimulation waveforms on an implantable device

1. A neurostimulation system comprising:

an implantable neurostimulator with at least one internal antenna configured to wirelessly receive an input signal via non-inductive coupling;

one or more external components including at least one external antenna configured to transmit the input signal to the internal antenna, the input signal including

electrical energy to operate the implantable neurostimulator;

the implantable neurostimulator further comprising:

a circuit coupled to the at least one internal antenna and configured to obtain the input signal from the internal antenna, the circuit including

an H-bridge electrode driver powered by the electrical energy from the input signal and configured to generate a specific stimulus waveform;

one or more electrodes coupled to the H-bridge electrode driver circuit and configured to apply the specific stimulus waveform to neural tissue; and

a sensor coupled to the one or more electrodes and to the circuit and configured to generate a direct current offset signal by sensing and integrating a direct current across the neural tissue caused by the specific stimulus waveform applied to the neural tissue by the one or more electrodes,

wherein the circuit is configured to generate a pulse density modulation (PDM) encoded signal using the direct current offset signal, and the H-bridge electrode driver is configured to generate the specific stimulus waveform using the PDM-encoded signal.

2. The neurostimulation system of claim 1 , wherein the circuit further includes a memory device that stores a digital representation of the PDM-encoded signal for generating the specific stimulus waveform and the circuit of the implantable neurostimulator is configured to:

rectify the input signal received at the at least one internal antenna through the non-inductive coupling from the at least one external antenna;

extract the electrical energy from the rectified input signal; and

generate the PDM-encoded signal using the digital representation of the PDM-encoded signal from the memory device and the direct current offset signal.

3. The neurostimulation system of claim 2 , wherein the input signal further includes configuration data including parameters for generating the specific stimulus waveform and the circuit of the implantable neurostimulator further comprises an oscillator and a clock and is further configured to:

select, based on the parameters in the configuration data, the digital representation of the PDM-encoded signal stored on the memory device of the circuit; and

control the oscillator to signal the clock to clock the PDM-encoded signal so the specific stimulus waveform is generated by the H-bridge electrode driver and applied at the one or more electrodes using the extracted electrical energy.

4. The neurostimulation system of claim 3 , wherein the memory device of the implantable neurostimulator stores the digital representation of the PDM encoded signal as a bit stream such that the specific stimulus waveform is generated at least in part by using the oscillator and the clock of the circuit to serially output the bit stream.

5. The neurostimulation system of claim 3 , wherein the parameters for generating the specific stimulus waveform comprise a pulse duration and a repetition rate to control the oscillator of the circuit such that the specific stimulus waveform is generated according to the pulse duration and the repetition rate.

6. The neurostimulation system of claim 5 , wherein the oscillator and the clock are coupled to the memory device and are configured to speed up data output from the memory device when the pulse duration is reduced and slow down data output from the memory device when the pulse duration is increased.

7. The neurostimulation system of claim 3 , wherein the parameters for generating the specific stimulus waveform further comprise an amplitude, and wherein the specific stimulus waveform is generated according to the amplitude.

8. The neurostimulation system of claim 2 , wherein the memory device of the implantable neurostimulator comprises a read-only memory device pre-loaded with the digital representation of the PDM encoded signal that is tuned to treating a particular condition for a subject.

9. The neurostimulation system of claim 2 , wherein the implantable neurostimulator further comprises a multiplexing device, and the memory device comprises a plurality of memory devices, wherein each of the memory devices is configured to store a digital representation of a different PDM encoded stimulus waveform signal and the multiplexing device is configured to select one of the memory devices from which the digital representation of the PDM-encoded signal is obtained to generate the PDM-encoded signal.

10. The neurostimulation system of claim 1 , wherein the input signal further comprises polarity information and wherein the circuit of the implantable neurostimulator is further configured to set a polarity state for each of the one or more electrodes in accordance with the polarity information.

11. The neurostimulation system of claim 1 , wherein the sensor and the circuit of the implantable neurostimulator form part of a DC servo loop configured to adjust a DC offset of the specific stimulus waveform.

12. The neurostimulation system of claim 1 , wherein the H-bridge electrode driver is a single-bit driver.