IEEE Solid-States Circuits Magazine - Fall 2019 - 32

clamp is a conventional method used
to record intracellular signals from a
single ion channel, but it is limited to
recording only a single channel [106].
Hence, a planar-type microelectrode
array (MEA) was developed to measure multiple channels simultaneously
from a brain slice or neuron culture.
These arrays typically consist of nontoxic and anticorrosive metals such
as Au, Ir, Pt, and titanium nitride, and
the metal electrode is a few tens of
micrometers in size [107]. Similar to in
vivo microelectrodes, the number of
microelectrodes and recording channels has continuously increased over
time. For in vitro recording systems,

ADC

ADC

ADC

the largest number of microelectrodes
reported to date is 59,760 [33] while
32,768 is the maximum recordingchannel count [36].

High-Density Neural
Interface: Circuits
In either the in vivo or in vitro recording system, the typical neural signals
picked up by the microelectrodes
are action potential and local field
potential, which have amplitudes of
a few tens of microvolts to several
millivolts and a bandwidth up to several kHz. To acquire such weak signals with sufficient fidelity, properly
designed recording circuits should

ADC

MUX

MUX

ADC

ADC

(b)

m

m

m

m

ADC

Row Decoder

(a)

ADC

ADC

(c)
Front-End Amplifier

ADC

(d)
Recorded Electrode

Programmable-Gain Amplifier

Switch

Inactive Electrode
m

Local Memory

FIGURE 3: The various routing configurations that connect microelectrodes to recordingchannel circuits. (a) A direct wiring structure with dedicated recording-channel circuits for
each channel, (b) a direct wiring structure with a portion of the recording circuits shared
among multiple channels, (c) a switch matrix structure with flexible routing, and (d) an
active pixel-array structure with full scan capability. MUX: multiplexer.

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IEEE SOLID-STATE CIRCUITS MAGAZINE

be connected to the microelectrodes
for amplification, filtering, and digitization, with the primary goal of
minimizing low-frequency noise.
As the density of MEA and the number of microelectrodes in the array
increase, the area and power consumed by the recording-channel
circuits also increase, and the overhead for routing connection paths
between the microelectrodes and the
circuits eventually grows to become
a bottleneck in scaling up the recording system. To mitigate these issues,
great efforts have recently been
exerted to develop more advanced
neural recording circuits that consume lower power [122]-[124] and
smaller areas [125]-[127] and feature better routing methods that
minimize the routing overhead
while maintaining the capability of
recording from a large number of
microelectrodes in the array [8]-[39],
[85]-[104], [108]-[121].
Various routing configurations
have been proposed to connect the
microelectrodes and the recording
circuits (Figure 3). The most straightforward way is to connect each micro
electrode with its dedicated recording channel, which consists of a frontend amplifier and an analog-to-digital
converter (ADC) [8]-[12] [Figure 3(a)].
In such a simple configuration, in creasing the number and density of
microelectrodes is significantly limited due to the excessive number
of recording circuits and amount of
routing overhead incurred. If a portion of the recording circuits is shared
between multiple channels using
multiplexers, the area (and power
as well, in some cases) required for
the circuits can be reduced [13]-[27]
[Figure 3(b)]. This is especially advantageous for in vivo recording, where
the size of the overall recording system is a critical factor.
To reduce the routing overhead
while providing good and flexible
coverage over the microelectrodes
in the array, the structure based on the
switch matrix can be introduced [28]-
[33] [Figure 3(c)]. In this structure, a
fabric of switches is devoted to creating
IEEE Solid-States Circuits Magazine - Fall 2019

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