SAN FRANCISCO, Calif. Engineers from Medtronic disclosed a novel amplifier that is a key component in an implantable "brain radio" the company is developing to monitor and control nervous disorders. Overcoming noise generated from today's standard semiconductor processes was one of the biggest hurdles, according to a paper presented at the International Solid State Circuits Conference here Tuesday (Feb. 13).
"We want to measure the average activity of thousands of brain cells," said Tim Denison, a senior principal engineer at Medtronic Neurological Technologies, who presented the ISSCC paper. "Essentially we want to build a brain radio that we can tune to the particular frequencies of the patient," he added.
The Medtronic system is part of a broad category of neural stimulators, implantable devices that send electrical pulses to areas of the brain to control diseases such as Parkinson's disease, epilepsy and depression. The paper gave a rare glimpse look inside a next-generation device, revealing design struggles in power management and signal sensitivity.
"We find there is quite a lot of 'popcorn' noise in digital and mixed-signal circuits due to poor annealing," Denison said. "In reality we often find ourselves measuring the circuits having a seizure, and we are not even sure what is going on with the patient," he quipped.
Medtronic developed a chopper-stabilized amplifier with a noise efficiency factor of 3.6 to 4.5 to compensate for the silicon noise. The amplifier is more efficient than some disclosed in previous research papers, albeit it required a slightly larger die size.
The design was especially challenging because it had a limited power budget. The entire device runs off an implanted controller with a 1.8V battery that delivers 1.5 microAmps. Saving power was a paramount part of the design because the battery is only changed every seven years via a surgical procedure.
"I haven't seen very many successful chopper amps running at 2 microW," said Denison.
The device has a broad range of applications beyond the monitoring of brain waves. It could also be used for pressure and telemetry sensors as well as other bridge devices, Denison said.
The Medtronic engineer demonstrated a "brain clapper" that monitored alpha waves to tell when a subject's eyes were open or closed. By opening and closing his eye in a demo video, Denison was able to turn a light on and off in his lab.
"You won't see Medtronic marketing a 'brain clapper' anytime soon,but you will see us apply this technology to a variety of diseases," he joked, referring to the popular light switch activated by clapping hands.