INV-14068
Battery-powered portable or implantable biopotential and bioimpedance measurement devices are becoming increasingly widespread in the medical diagnostics field. The signal acquisitions of the main biosignal-sensing applications such as electroencephalography (EEG) and electrocardiography (ECG) involve voltage measurements from a few microvolts to several millivolts. Biopotentials are conventionally acquired using electrodes covered with electrolyte gels or solutions to decrease the contact impedance at the skin interface. However, wet-contact measurements cause discomfort and dry out in novel long-term monitoring applications such as in brain-computer interfaces where EEG signals are acquired and analyzed over hours or longer.
As a result, for long-term applications, use of dry electrodes is more suitable. In general, dry electrodes such as inexpensive Ag/AgCl are better suited for long-term monitoring, but their use is associated with increased contact resistances that can be above 1MΩ. This characteristic complicates the measurement of small biopotentials for EEG applications by requiring very high input impedance at the analog front-end amplifier. Nevertheless, a significant problem is that this impedance is affected by parasitic capacitances of the integrated circuit package as well as electrode cable and printed circuit board (PCB) capacitances. For instance, when the goal is to record EEG signals with frequencies up to 100 Hz, a capacitance of 200 pF would limit the input impedance at 100 Hz to approximately 8 MΩ which can cause excessive attenuation such that the EEG signal cannot be measured reliably.
To overcome these limitations of prior art, Northeastern University inventors designed an instrumentation amplifier (IA) that implements a novel input capacitance cancellation mechanism that includes a mechanism for digitally tuning the input impedance of the amplifier with on-chip circuits for more accurate acquisition of biosignals, particularly when dry-contact electrode measurements are performed.
Since this approach does not require an additional amplifier for generating negative capacitance, this method does not consume any extra power. The architecture of the instrumentation amplifier comprises a few differential input stages including an inverting input terminal and a non-inverting input terminal; an output stage driven by the input stage; a direct current feedback loop from the output stage to the input stage; and a negative capacitance generation feedback circuit between the input stage and the direct current feedback loop. The negative capacitance is generated at the input stage to cancel parasitic capacitances at the input terminal and thereby boost input impedance.
The two digitally-programmable capacitors between the input stage and the current feedback loop of the instrumentation amplifier cancel the input capacitances from the electrode cables and printed circuit board at the front end. An on-chip calibration unit is employed to adaptively calibrate the programmable capacitors and improve the input impedance. The calibration system can be activated, for example, whenever electrode cables are changed. Also, this calibration system and method can be implemented on the same chip as the IA.