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Microelectronics System Design
for Chronic Brain Implants
ECEN 5007
S. Johnson, V. Ganesan
11-07-02
Motivation
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Research in the area of nueronal signaling is constrained by the test electronics readily
available.
Large, inflexible neural probes are implanted into test animals, requiring bulky cabling
configurations to connect to the recording equipment.
Such a test setup hampers the test subject's movement and thus greatly changes the way
in which the test subject would normally interact with its environment.
This change in normal behavior effects the way in which they learn and in turn can skew
desired test results.
A proposed solution is to place the microelectronics used for recording neural signals
onto a small chip, which rides on top of the mechanical probe.
Such a device would be small enough to be implanted, and allow the test animal to
recover from the surgery and soon after interact normally with its environment.
Key System Requirements
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No chronic external wiring - External wiring interferes with the natural movement and
hence behavior of the test subject
The probe must be able to "float" with the tissue in which it is implanted - The brain
moves inside the skull, an implant that is rigid and anchored to the skull does not allow
for such movement, hampering normal function.
The electronics must be integrated with the mechanical probe - Proximity to the
acquired signal helps keep noise low (short transmission path) and aids in a higher
resolution of data.
Low power consumption - Must keep power consumption low to minimize energy
storage (battery) 1mW goal.
Low power dissipation - Must keep power dissipation low to reduce heat load on tissue.
(A rise in temperature of 0.5C causes stress in the tissue disrupting normal function, a
2C rise causes tissue death).
Small size - Chip must be small so as not to interfere with normal function. For this
application the area of tissue is 3mm2, and the mechanical probe and acquisition
electronics must fit within a 1mm2 footprint.
System Description
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Dual Module Approach
Probe Module
Surface Module
Analog Front End
Demultiplexer
Probe
Interfaces
ADC
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Clock
Configuration
& Control
Power
Mgmt
Serial Interface
Multiplexer
LNA IA
Signal
Process
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Signal
Storage
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In itia tion
Sig nal
(wirel ess)
Clock
Rcvr.
Configuration
& Control
Power
Mgmt
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User
In terfa ce
(wired )
Energy
Storage
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For the purposes of this project,
the focus will be the Probe
Module.
The Surface Module is a larger
chip where the majority of
signal processing will take
place. It is attached to the
outside of the scull of the test
animal where constraints of
size, power consumption and
power dissipation are not as
great.
The Surface Module provides
an interface to the researcher to
access data, charge the on-chip
battery cell, and send simple
control commands to the
electronics.
A flexible wire tether bundle
would provide the
interconnections between the
two modules . The Surface
Module is shown only for
completeness of the system.
Signal Acquisition
The signal acquisition portion of the Probe Module is comprised of an analog front end,
a multiplexer, and an ADC. The analog front end interfaces with sixteen probe lines,
and amplifies the incoming neural signals. The first amplification stage is the LNA (low
noise amplifier) and is shown below.
Rail-to-Rail OTA
supply=1.8V
Vsig
Modulator
Selective Amplifier
2nd order Gm-C BPF
(fc tracks fchop)
Modulator
LPF
Vout
LNA (Low Noise Amplifier)
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An AM (amplitude modulation) block which shifts the low frequency, low voltage signal
up in frequency above the flicker noise of the preamp,
A low noise pre-amp
A Selective Amplifier (acting as a bandpass filter with gain, which filters out the low
frequency noise component, leaving the modulated signal component)
A second AM block which demodulates the original signal back to its baseband
frequency.
An LPF filter which filters out the additional, high frequency, modulation components.
The second amplification stage is the Instrumentation Amplifier. It's purpose is to
further amplify the desired neural signal to a level required by the digital processing
electronics, or ADC block.
Low Noise Pre-Amplifier and
Selective Amplifier
• The Selective amplifier is realized by 2 amplifiers
low noise rail-to-rail preamplifier followed by a
2nd order band pass (GmC) filter.
• The pre-amp amplifies both the noise and
modulated input signal. It requires a minimum
corner frequency equal to the chopper frequency,
and a high CMRR
• The Gm-C (bandpass) filter reduces residual offset
from charge injection of input modulator. It
requires a matching oscillator to maintain a
constant Gm.
Low Noise Pre-Amplifier
Magnitude Plot
Gain = 26db
Corner Frequency (-3dB) = 80kHz
Phase Plot
Matching Oscillator
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The Gm-C filters are sensitive to
parasitic capacitances due to lack of
low impedence nodes and this causes
the variation in the time constant of
the filter
Frequency drift causes Gm to shift.
Want a constant Gm value
To get rid of the time constant
variation we track it with on-chip
chopper freqency.
Tracking with off-chip reference
clock will cause more variations due
to temperature and process parameter
changes
Bandpass Filter
Magnitude Plot
Center Frequency = 40kHz
Phase Plot
Multiplexer
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Once the signal is amplified, it is multiplexed with the other amplified probe lines.
The multiplexer's purpose is to sample each of the lines at a minimum of 25kHz.
This satisfies the Nyquist criterion where the sampling rate must be at least twice that of
the signal being sampled.
For this application, the neural signal bandwidth is less than 5kHz. Since the system can
obtain good data from up to sixteen probes at one time, the switching frequency of the
multiplexer must be a minimum of 400kHz to achieve a 25kHz sampling rate for each
line.
The ADC is the final block of the signal acquisition section of the Probe Module. It
digitizes the multiplexed data stream for transport to the Surface Module.
Configuration and Control
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Upon initial power-up and data acquisition, the researcher will determine which probe
signals are useful.
The researcher sends a code to the Configuration and Control block on the Surface
Module, telling it which channels are good.
The "good" channel information is sent as a binary stream to the Configuration and
Control block on the Probe Module. The Control block then powers down the amplifier
stages whose signals are too weak or non-existent, thus saving on power consumption
and lowering power dissipation.
The multiplexer is directed to sample only those lines that are powered. The control
block will be in control of the sequencing of the amplified probe lines, and the
frequency at which they are sampled.
Project Scope
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The Analog Front-End will be composed of at least four input lines from the mechanical
probe.
A minimum signal level for each of these lines is expected to be 100uV. The circuit
begins with all probe amplification lines powered on.
The clocking for the modulator blocks is set to achieve a modulation frequency of
approx. 40kHz.
The Configuration and Control block will be given a command (binary bit stream) that
it must interpret and power down those amplification stages that are not needed, shutting
off particular probe lines. It then relays the sequence of probe lines to be sampled to the
multiplexer.
The switching frequency of the multiplexer is set knowing the number of probe lines to
be sampled (25kHz minimum sampling per line).
The output from the amplified lines is expected to have a 60dB voltage gain (100uV to
100mV). Power consumption for the system is targeted at 1mW.
System Overview - Ideal LNAs
Analog Front End
(Chopper Amplifier Circuitry)
Multiplexer
Probes
Mux switches
and output bus
00
Enable Bits (Shift Register)
Ctr Init
(Pulse)
01
Clk Enable
2 bit
Binary Ctr
Mux switch control logic
100kHz Clk
10
11
Binary Counter
Simulation Results
All LNAs enabled
LNA 2 (400Hz sinewave)
LNA 1 (200Hz sinewave)
LNA 0 (100Hz sinewave)
sw 0 - on
LNA 0
signal
sw 1 - on
LNA 1
signal
sw 3 - on
LNA 3
signal
sw 2 - on
LNA 2
signal
LNA 3 (800Hz sinewave)
Simulation Results
LNAs 0 and 2 enabled
LNA 2 (400Hz sinewave)
LNA 0 (100Hz sinewave)
LNA 3 (800Hz sinewave)
LNA 1 (200Hz sinewave)
Chopper Amplifier
Input
Signal
Amplifier
LPF
Modulator
BPF
Demodulator
Simulated
Noise
Chopper Amplifier
System Overview - CHopper
Simulated
Noise
CHopper
Amplifier
(LNA 0)
Modulator
Clocks
Input
Signal
Simulation Results
LNA 2 (400Hz sinewave)
LNA 1 (200Hz sinewave)
CHopper LNA 0
(4.5kHz sinewave)
All LNAs enabled
LNA 3 (800Hz sinewave)
CHopper LNA
before and after
multiplexer