IEEE Circuits and Systems Magazine - Q3 2022 - 30

Table I.
Performance comparison with the commercial recording front-ends.
ADS1299 [27]
4/6/8
24
4
12
1000
40
Power /ch (mW)
e-Estimated
5.1
3.3e
2
45.8
5.9
2 (150 Hz)
Input offset voltage (μV)
Gain (dB)
20 (@Gain=24)
1/2/4/6/8/12/24
8.5 (150 Hz)
# Recording channels
ADC resolution (Bit)
AD8233 [26] HybridVNS
1
-
Input-referred noise
(μVrms)
4.9 (200 Hz)
the standard deviation of the coastline
length (SCL) and the slope (SS) at the
end of each UTS are calculated. Finally,
the thresholds of the coastline length
(THCL) and slope (THS) in the UTW can
be determined by ACL, SCL, AS and SS in
the inter-ictal period, respectively, as
M
TH 11 Sk// (5)
M i
M
CL M ,,ACLii
CL
i 11
and
TH 11 Sd// (6)
M i
M
SSiSi
11
=+ #
==
M ,,
A
i
Taking the thresholds in the time domain as an example,
the thresholds of coastline length and slope of the EEG
in a unit time segment (UTS) can be used for the TESD
[28]. The coastline length is defined as the cumulative
length of the EEG in a unit time window (UTW). A UTW
contains a number of sampling points, and a UTS
contains a number of UTWs. When an epileptic seizure
is going to happen, the coastline length at the ictal period
or pre-ictal period is larger than its average value
at the inter-ictal (or seizure-free) period. However, when
there is interference either with a high amplitude but
slow speed (e.g., due to an electrooculogram or electromyogram)
or with a small amplitude but fast speed, the
coastline length is also large and probably close to the
epileptic seizure detection threshold and thus leading to
a misdiagnosis. To mitigate this, the slope of the EEG in
the UTW is calculated together with the coastline length
to improve the detection accuracy. For example, the amplitude
of the coastline length in a given UTW is [29]
N
ACL / () ()xn xn 1
2
=- -
=
n
where x is the value of the EEG signal, n is a discrete number,
and N is the total number of sampling points in the
UTWs. The magnitude of the slope in the UTW is given by
Eq. (4), where D is the number of sampling points and
DN ,<
xn xn D++1
ma (( ), (),... ()) and mi (( ),
(),... ()) are the maximum and minimum valx
xn xn xn D++1
n xn
ues of the EEG in the Dth sampling points, respectively,
and tD is the time interval between the maximum and
minimum values. As an example, Fig. 9 shows both the
coastline length (ACL) and the slope (AS) as functions of
the UTW in the inter-ictal period when the sampling frequency
is 256 Hz and N is 40 or the UTW is 156 ms. Further,
A =
S
(3)
VII. The Switched-Resistor Array Pulse
Generator (SRA-PG)
A typical pulse generator employing constant-current
stimulation consists of a current-steering digital-to-analog
converter (DAC), a high-voltage output stage and a
charge-balancing network. The high-voltage output stage
generates a biphasic pulse by exploiting either a bidirectional
current source/sink or a unidirectional current
source/sink together with an H-bridge [30]. In the SRA-PG,
the current steering DAC is implemented by a switched resistor
array (S0-Sn, MS1-MSn, and RS1-RSn in Fig. 11(a)).
The high-voltage output stage is implemented by a unidirectional
current source/sink and an H-bridge [31]. In
maxmin++ -+ +
D
30
IEEE CIRCUITS AND SYSTEMS MAGAZINE
(( ), (), ... ())( (),( ), ...(11 ))
xn xn xn Dxnx nxnD
t
(4)
where i is a discrete number, M is the total number of UTW
in the UTS, and both k and d are integers indicating that
the thresholds of the coastline length and slope are set
according to the variation between signals in the ictal (or
pre-ictal) period and the inter-ictal period. For instance,
when the UTS is 780 ms (or
M )5= and the sampling frequency
and UTW are the same as in the previous example,
the mean ACL, AS (both in the inter-ictal period and in the
ictal period), THCL and THS in 25 UTS of one patient (patient
No.1) that we recruited in the Affiliated Hospital of Jiangsu
University are shown in Fig. 10. It can be found the THCL
(with a mean value of 180 μV) and THS (with a mean value of
4.7 μV/ms) were obtained for k 3= and
be seen that both the mean ACL (494 μV) and AS (8 μV/ms)
in the ictal period during the No. 20 UTS were lower than
its THCL (559 μV) and THS (8.9 μV/ms), which leads to a
misdetection (namely there is one false negative out of 25).
The algorithm of the TESD in the time domain is simple, low
cost, and computationally fast while with high accuracy,
and thereby is suitable for real-time closed-loop VNS.
M
=+ #
==
d .2= As also can
THIRD QUARTER 2022
IEEE Circuits and Systems Magazine - Q3 2022

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