Abstract
This paper
presents the design of an Application Specific Integrated Circuit (ASIC) for neural recording and stimulation for
applications in the(Peripheral Nervous System (PNS). This ASIC consists of 8
channels stimulator with independent anode-cathode selection, 4 recording
amplifiers, two step-up voltage regulators and a digital control to program and
command the analog parts. The main characteristics of the stimulator are: fully
programmable for stimuli waveform (monophasic, biphasic), wide frequency range
from 7 to 350 Hz with .1 Hz resolution at low frequencies and current range
from 2mA to 5mA with 6
bits resolution in four scales. The characteristics for the recording amplifier
are: very low noise (5nV/ÖHz), high CMRR
(94 dB) as well as gain and bandwidth being digitally programmable. This ASIC has been designed in a high voltage
CMOS technology.
1.
INTRODUCTION
Electrical stimulation and
recording has been an important research activity for decades and is widely
considered as a key question for better understanding, controlling, and
eventually restoring neurological functions using a neuroprostheses (NP). A
generic NP [1] consists of sensors (natural or artificial) used to sense any
physiological measurement such as motor or sensory activity, an electrical
stimulator for stimuli generation and a set of electrodes providing the stimuli
to the nerve or muscle. With these NP a lot of applications have been developed
most of them related with neuromuscular stimulators for the treatment of some
illness like epilepsy, stroke, pain, essential tremor or spinal cord injury
like bladder control, drop foot and grasping [2,3] regaining the lost motor
activity. Although the majority of neuroprostheses are stimulators of the
nervous system, there is an increasing interest in enabling neural signals as
inputs for controlling the stimulators or to provide a feedback in closed loop
systems. Neural signals recorded by any kind of electrode (cuff, sieve, life,)
can be used instead of artificial sensors in
Fig. 1: Simplified blocks diagram
2. METHODS
This system is
part of a wireless system thought to work with different electrode and it is
not fixed to an specific biomedical application. This ASIC does not include the
telemetry module although the digital control has been designed using a general
strategy.
2.1
Stimulator description
The hierarchical methodology
followed in the design of the first version [4] permitted to increase the
number of channels from 4 to 8. Moreover, the initial current range was also
increased with the objective of extending the usefulness to other electrode
types such as cuff or micro needle. This electrical stimulator is based in a
binary weighted current converter with 6 bits resolution giving a current
resolution of 2 mA, the three additional ranges with 5, 20 and 50 mA resolution,
have been implemented with current amplifiers at the output. The stimulator can
be programmed with 16 different waveforms. A waveform is any kind of biphasic
or monopolar stimuli with or without prepulse, train of pulses and/or a
stimulation sequence with different channels, etc. It is possible to select any
anode-cathode combination giving a potential spatial selectivity when using
cuff electrodes.
The same current source is
used for the stimulation and recovery phases such that the charge delivered in
the two phases are very similar and the small differences are only due to the
finite resolution in amplitude and pulse width definition. To avoid that, an
exponential charge recovery process has been implemented short-circuiting the
two electrode terminals.
Moreover, due to
CMOS transistors used as switches at the output to control the current flow,
high current spikes appear at the output when switching them. This stimulator
has been implemented with a specific activation sequence and hardware to avoid these
undesirable current spikes.
This ASIC also includes the control part of two DC-DC
step/up regulators to generate the high voltages needed and optimize the area
for the implantable device reducing the number of external components and only requiring
a 5V power supply. These step/up can be disabled digitally reducing the
power requirements when the stimulator is not used. The step/up voltages
obtained depends on the electrode that is used.
2.2
Recording description
The same circuit includes 4
identical 3 stages ENG amplifiers. Each stage consists of a gain and first
order high pass filter with a zero at the origin [6]. The total input referred
noise is 350 nVrms being the bandwidth 100 Hz to 5 kHz. That makes possible to
record ENG signals mV [6], eliminate ±500 mV DC
component from the electrode and reduce signals outside the band. The low
cut-off frequency can be moved among 4 possible values digitally programmed and
the roll-off is 60 dB/dec of attenuation. The circuit also includes an ADC and
the digital control.
2.3
Internal structure for the control
The digital control has been designed with the main
objective being the reduction in size and power consumption, then its internal
structure has been optimized. Fig 2 represents a simplified schema of the
internal structure. It is based on an internal bus that connects the different
modules and each module can communicate with any other using the bus. The main
blocks are:
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Fig. 2: Internal structure for the stimulator and
recording controls. |
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MainControl: This block controls the access to the bus.
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Ram: This is a ram
of 256 words used to store all the stimulator parameters
-
ComIO: This module
implements the communication between the internal bus and the external world. A
synchronous serial and a parallel connection have been implemented.
-
FreqGen: This module
generates 16 independent frequencies. When a frequency event is generated this
module sends the command, to start the stimulus, to the StimCtrl across an
internal FIFO.
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StimCtrl: This module
generates all the signals that are necessary for the stimulator. It works like
a processor where instructions are the stimuli parameters, for this reason it
is possible to program any kind of stimuli.
-
RecCtrl: It generates
all the control signals required for: amplifier activation, electrode
selection, sampling rate, ADC conversion and storage of digital data in specific
registers for a posterior transmission to the exterior.
Single stimulus or
bursts at the programmed frequency can be generated. The process for a stimulus
generation follows the sequence below:
1.
Module
FreqGen finish the count for a
frequency
2.
Module
FreqGen write in the FIFO the frequency identifier
3.
If
the FIFO is not empty, the StimCtrl reads from the FIFO the pointer to one of the 16
waveforms
4.
StimCtrl reads the initial RAM
address for this waveform
5.
StimCtrl reads the parameter and execute the sequence for
stimulus generation
6.
The
RAM address is increased to the next
one
7.
Step
5 and 6 are repeated as many times as the number of parameters in the waveform
8.
If
the RAM contents is equal 0 then the
waveform is finished and next position in the FIFO is read.
3. RESULTS
This circuit has been implemented in a high
voltage CMOS 0.7mm and used to design
a monolithic opto-coupled stimulator that can be programmed and controlled from
a computer using an USB interface. This system is useful at the laboratory and
acute experiments in animals. It is programmed from the computer to generate
any complex waveform using specific software. In Fig. 3 stimuli pulses through
3 channels, at the top, and the expanded time scale at the bottom are shown.
These stimuli have been programmed with two different frequencies (50 and 100
Hz) highlighting the collisions treatment. Three biphasic waveforms with
prepulse in the stimulation phase and different shape for the recovery phase
have been programmed.
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Fig. 3: Stimulation waveforms programmed at
internal channels Ch0, Ch1 and Ch2 with different parameters. |
This amplifier is an improved version of a
previous work [5] where only one channel was implemented and the full
performances were presented. The most important parameters for this new
prototype are an input referred noise of 350nVrms and a CMRR@1kHz 94dB. In Fig.
4 the bode diagram for a programmed gain of 80 dB and for the four low cut-off
frequencies is shown.
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Fig. 4: Bode for the recording amplifier. |
This ASIC has also been used to implement an
implantable device like that shown in Fig 5. Two stimulation and recording
channels are accessible and has been developed to be used with sieve electrodes
(including a multiplexer for external electrode selection) allowing the access
to any of the available active points in the electrode.
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Fig. 5: Implant for sieve electrodes. The size is 20x20mm2.
Die size 5.9x6.0 mm2 |
4.
CONCLUSIONS
The design of an 8-channels stimulator,
4-channels ENG signals amplifier with filter and two final applications, (a
monolithic stimulator for connecting from a PC and an implantable stimulator
for sieve electrodes) using this ASIC has been presented. The characteristics
of stimulator make it an ideal circuit for an implantable device for studying
and analyse different kind of stimuli in a chronic experiment and also to
verify the electrode and implant evolution along the time.
References
[1] M.R. Popovic, T. Keller, I.P.I. Papas, V. Dietz, M. Morari, Surface-stimulation technology for grasping and walking neuroprostheses. IEEE Engineering in Medicine and Biology Magazine, Vol. 20, pp. 82 – 93, Jan.-Feb. 2001
[2] G. J. Suaning and N. H. Lovell, CMOS neurostimulation ASIC with 100
channels, scalable output, and bi-directional radio-frequency telemetry.
IEEE Trans. Biomed.
[3] S. C. DeMarco,W. Liu, P. R. Singh, G. Lazzi, M. S. Humayun, and J. D. Weiland, An arbitrary waveform stimulus circuit for visual prosthesis using a low-area multibias DAC. IEEE J. Solid-State Circuits, vol. 38, no. 10, pp. 1679–1690, Oct. 2003.
[4] Sacristán J, Lago N, Navarro X and Osés MT. Programmable
stimulation-recording circuitry for cuff and sieve electrodes. 8th
[5] Sacristán J,
Acknowledgements
This work has been supported by ESPRIT CyberHand project IST-2001-35094.