In-Column Cross-Talk Suppression in High-Density CMOS-MEAs
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1
Technische Universität Berlin, Chair of Sensor and Actuator Systems, Germany
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2
NMI at the University Tuebingen, Neurochip Research, Germany
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3
Multi Channel Systems MCS GmbH, Germany
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4
Technische Universität Berlin, Chair of Sensor and Actuator Systems, Germany
Motivation
The high-density CMOS-MEA system introduced in [1],
enabling simultaneous stimulation and recording [2], enables neural
imaging at high spatial resolution thanks to its circuit topology in
the readout path. However, this architecture also leads to a kind of
“in-column cross-talk" if many sites belonging to the same column
record highly-correlated signals which may occur under specific
operating conditions. This is e.g. the case during electrical
stimulation applied to a large portion of the entire active chip area
or during system calibration by means of applying a calibration signal
to the bath directly. In this work, we present a simple approach to
model this type of cross-talk and use this model to post-process the
recorded data in order to efficiently suppress cross-talk-related
artifacts.
Materials and Methods
The high-density CMOS-based MEA used here [1, 3]
has 4225 purely capacitively-coupled recording sites, which are
organized in 65 rows and 65 columns as schematically shown in
Fig. 1. Every sensing site consists of a sensor transistor, whose gate
is capacitively coupled to the electrolyte through a metal
electrode at the chip surface covered by a thin high-k dielectric. Local
variations of the electrolyte potential induced by neural activity are
converted into current signals by the sensor transistors. In order to
achieve full imaging capability (readout from all available recording
sites) multiplexing at column level is performed. Hence, only one
column of sensor transistors is active within a given time frame. On
this basis, the simplified small-signal equivalent circuit depicted in
Fig. 2 can be derived. For the sake of simplicity, transistor
parameter variations, channel length modulation-related effects, and
parasitic resistances related to metal interconnects are neglected here.
Resistor r_S models the resistance of the switch
transistor, which connects the sensor transistor's source to bias
voltage V_1. Resistor r_S leads to a so-called source degeneration
which translates into in-column cross-talk.
The following relation applies:
--------> Here Formula #1
with
--------> Here Formula #2
Moreover, g_S = 1/r_S and g_m stands for the sensor transistors'
transconductance. This means that, for k=1...65, output current i_k
depends on the signals recorded by all sensor transistors in the
respective column (v_1 ... v_65 ), i.e. the considered output signal i_k
is affected by cross-talk. It can be shown that matrix A_C is
invertible, hence the original (cross-talk free) recorded signals can
be recovered by multiplication of the recorded data by the inverse
matrix A_C^-1. Since both g_m and g_s are operating point dependent
and may vary from chip to chip, a calibration procedure is developed,
which estimates the parameter y on the basis of a simple and
straight-forward measurement.
Results
Two examples of successful in-column cross-talk artifact suppression
are reported in Figs. 3 and 4, respectively. In both cases, in order to force the
required signals to the different sensor transistors we take advantage
of the programmable stimulation capability of the CMOS-MEA system (for
details, see figure captions and [1, 3]). It is worth to note that both
measurements are performed with the same MEA chip operated in the same
operating point, so that only a one-time determination of matrix A_C^-1 is
required for both signal post-processing procedures.
Conclusion
In this work, in-column cross-talk effects are investigated of the
high-density CMOS-based MEA suggested in [1, 3]. A post-processing
method for compensation of related cross-talk artifacts is proposed as
well. Two exemplary experiments are reported, where successful
compensation of cross-talk-related artifacts is achieved.
References
[1] Bertotti, G. et al., A CMOS-based sensor array for in-vitro neural tissue interfacing with 4225
recording sites and 1024 stimulation sites, IEEE Biomedical Circuits and Systems Conference (BioCAS),
pp. 304-307, 2014.
[2] Velychko, D. et al., Simultaneous stimulation and recording of retinal action potentials using
capacitively coupled high-density CMOS-based MEAs, 9th International Meeting on Substrate-
Integrated Microelectrode Arrays , pp. 78-79, 2014.
[3] URL: http://www.multichannelsystems.com/
Figure Legend
Fig. 1: Transistor-level representation and topology of
the recording circuitry of the CMOS-MEA with high spatial resolution
from [1, 3].
Fig. 2: Small-signal equivalent circuit of the array of sensor
transistors. Note, that only the
65 transistors from an active column contribute to the output currents
considered in the small-signal domain.
Fig. 3: Example 1 of applied in-column cross-talk compensation. By
means of electrical stimulation via the CMOS-MEA itself, all recording
transistors inside the hatched region highlighted in the inset of the
figure are forced to sense a sinusoidal signal. The recording site
marked in red is outside the stimulation region and should therefore
record only noise. Stimulation signal: 1Vpp @ 25Hz.
Fig. 4: Example 2 of in-column cross-talk
compensation. Stimulation signals: Area #1, 0.5Vpp @ 8.3Hz, Area #2,
1Vpp @ 25Hz.
Acknowledgements
Support and funding of this project by the German Ministry of
Education and Research, Projektträger Jülich, and Projektträger VDI
Technologiezentrum GmbH is gratefully acknowledged (references 0315636A and 1312038).
Keywords:
Electrical Stimulation,
Signal analysis,
CMOS-MEA,
Cross-talk
Conference:
MEA Meeting 2016 |
10th International Meeting on Substrate-Integrated Electrode Arrays, Reutlingen, Germany, 28 Jun - 1 Jul, 2016.
Presentation Type:
Poster Presentation
Topic:
MEA Meeting 2016
Citation:
Bertotti
G,
Zeck
G,
Boven
K and
Thewes
R
(2016). In-Column Cross-Talk Suppression in High-Density CMOS-MEAs.
Front. Neurosci.
Conference Abstract:
MEA Meeting 2016 |
10th International Meeting on Substrate-Integrated Electrode Arrays.
doi: 10.3389/conf.fnins.2016.93.00038
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Received:
22 Jun 2016;
Published Online:
24 Jun 2016.
*
Correspondence:
Dr. Roland Thewes, Technische Universität Berlin, Chair of Sensor and Actuator Systems, Berlin, Germany, roland.thewes@tu-berlin.de