Design of an ultra low-frequency, dc-blocked, implantable electromyogram and cortical sensing amplifier

This paper describes the design of an ultra low-frequency, dc-blocked, differential lownoise amplifier (LNA) used for electromyogram and cortical signal detection in an implantable FES or neural prosthetic device. The LNA detects bioelectric signals in the frequency range of 0.035 Hz to 5000 Hz and amplitude range of 10 uV to 100 mV. Large dc voltages of up to 14V can exist across the two sensing electrodes when used for stimulation, requiring the LNA to be dcblocked. A pair of 50 pF dc-blocking capacitors, PFET inputs and a positive feedback mechanism to cancel the parasitic capacitance of the large input PFETs ensure that the low ‘1/f’ noise traits of the LNA are maintained and ultra low-frequency high-pass response is achieved. The LNA, designed and fabricated in a 0.5um CMOS process, occupies an area of 0.36 mm, most of which is due to the two 50 pF capacitors. It operates at 2.7V and draws 2.7uA, providing programmable differential gains between 4 and 120, a maximum bandwidth of 0.035 Hz to 5kHz and an input referred noise voltage of 5.6 uVrms in the frequency range of 10 Hz to 5 kHz.