Fully-integrated Gm-C anti-aliasing filter for sense system of implantable cardiac devices

Implantable medical devices such as pacemakers, internal cardi-ac defibrillator (ICD), and cardiac resynchronization therapy (CRT) devices have conducting leads that are used to sense the heart signals and to pace into the heart muscle in order to artificially stimulate it. The sensing system in these devices monitor patient´s heart signals continually; therefore, it must always remain powered-up. As a result lower power consumption for this block is very critical. Additionally device sense systems have bandpass anti-aliasing filters with lower-pole frequency at sub-Hz to be able to attenuate unwanted heart-repolarization signal. To be able to implement such a low-frequency pole, today´s medical devices use off-chip microfarad-range hybrid capacitors along-with internal integrated circuit resistors. Depending on the device type, often nine to twelve medically-graded off-chip capacitors are placed on the hybrid substrate. These capacitors are major source of four drawbacks: (1) critical reliability, (2) precious real-estate space area (limiting the device size), (3) external spurious noise pickup, and (4) additional cost. This paper presents a continu-ous Gm-C filter that realizes the identical cardiac sense-system anti-aliasing filter transfer function in a fully integrated form with no external capacitors. To realize sub-HZ pole-frequency using pico-Farad (pF) range integrated-circuit capacitors, the circuit combines three techniques to reduce the transconductance of the operational amplifier into pico-Siemens (pS or pA/V) regime: active lineariza-tion, current cancellation, and series-parallel active load. Further-more, because current consumption is a critical performance parame-ter for implantable cardiac device, each filter is designed to consume less than 20nA. Finally effects of amplifier non-idealities such as linearity, noise, offset, and leakage in the filter are addressed. The filter can be fabricated in any mixed-signal 0.18μm process.