A Low-Power Blocking-Capacitor-Free Charge-Balanced Electrode-Stimulator Chip With Less Than 6 nA DC Error for 1-mA Full-Scale Stimulation

Abstract

Large dc blocking capacitors are a bottleneck in reducing the size and cost of neural implants. We describe an electrode-stimulator chip that removes the need for large dc blocking capacitors in neural implants by achieving precise charge-balanced stimulation with <6 nA of dc error. For cochlear implant patients, this is well below the industry's safety limit of 25 nA. Charge balance is achieved by dynamic current balancing to reduce the mismatch between the positive and negative phases of current to 0.4%, followed by a shorting phase of at least 1 ms between current pulses to further reduce the charge error. On +6 and -9 V rails in a 0.7-mum AMI high voltage process, the power consumption of a single channel of this chip is 47 muW when biasing power is shared by 16 channels.

Silicon-Integrated High-Density Electrocortical Interfaces

“…Instead of external capacitors, capacitive electrodes made with high-k dielectric coatings have been investigated for safe neural interfaces [88], [100], [101]. Several other techniques for better charge balancing have been demonstrated: 1) shorting electrodes to ground [102]; and 2) utilizing a discharging resistor [94], active current balancing by feedback control [103], [104], generating additional balancing current pulses by monitoring electrode voltages [105], and embedded DAC calibration [93], [106].…”

An experimental study on passive charge balancing

“…the electrode interface increases permanently to large values, strong faradaic currents flow, which are the source of corrosion and toxicity. This effect is usually not modelled, because it must be avoided by long-term charge balancing (Sit and Sarpeshkar, 2007).…”

An energy-efficient, dynamic voltage scaling neural stimulator for a proprioceptive prosthesis

“…Charge balanced stimulation pulses are important for patient safety and preventing electrode degeneration. DC current flows of 100 nA across an electrode have been correlated with tissue damage in animal models [8] and industry targets a DC error of < 25 nA [26]. The stimulator proposed here is capable of stimulating at over 1500 pulses per second, but will in practice only be used to deliver up to 80 stimulations per second per channel (based on maximum observed firing rates of human proprioceptors [37], [38]).…”

“…One solution to this is to use a calibration phase to match a current sink and source prior to stimulation [26], [27]. However, a more elegant solution is to use an H-bridge, which enables the same current source to be used for both phases (thus eliminating mismatch concerns).…”