Citation: Choi, S.-W.; Seong, K.; Lee,
S.; Baek, K.-H.; Shim, Y. Noise
Immunity-Enhanced Capacitance
Readout Circuit for Human
Interaction Detection in Human Body
Communication Systems. Electronics
2022, 11, 577. https://doi.org/
10.3390/electronics11040577
Academic Editors: Alvaro
Araujo Pinto and Hacene Fouchal
Received: 21 January 2022
Accepted: 11 February 2022
Published: 14 February 2022
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Article
Noise Immunity-Enhanced Capacitance Readout Circuit for
Human Interaction Detection in Human Body
Communication Systems
Seong-Wook Choi
1
, Kiho Seong
1
, Sukho Lee
2
, Kwang-Hyun Baek
1
and Yong Shim
1,
*
1
School of Electrical and Electronics Engineering, Chung-Ang University, Seoul 06974, Korea;
choiseonguk@cau.ac.kr (S.-W.C.); tjdrlgh@cau.ac.kr (K.S.); kbaek@cau.ac.kr (K.-H.B.)
2
Electronics and Telecommunications Research Institute, Daejeon 34129, Korea; shlee99@etri.re.kr
* Correspondence: yongshim@cau.ac.kr; Tel.: +82-2-820-5483
Abstract:
Recent healthcare systems based on human body communication (HBC) require human
interaction sensors. Due to the conductive properties of the human body, capacitive sensors are
most widely known and are applied to many electronic gadgets for communication. Capacitance
fluctuations due to the fact of human interaction are typically converted to voltage levels using some
analog circuits, and then analog-to-digital converters (ADCs) are used to convert analog voltages into
digital codes for further processing. However, signals detected by human touch naturally contain
large noise, and an active analog filter that consumes a lot of power is required. In addition, the
inclusion of ADCs causes the system to use a large area and amount of power. The proposed structure
adopts a digital-based moving average filter (MAF) that can effectively operate as a low-pass filter
(LPF) instead of a large-area and high-power consumption analog filter. In addition, the proposed
∆
C
detection algorithm can distinguish between human interaction and object interaction. As a result,
two individual digital signals of touch/release and movement can be generated, and the type and
strength of the touch can be effectively expressed without the help of an ADC. The prototype chip
of the proposed capacitive sensing circuit was fabricated with commercial 65 nm CMOS process
technology, and its functionality was fully verified through testing and measurement. The prototype
core occupies an active area of 0.0067 mm
2
, consumes 7.5 uW of power, and has a conversion time
of 105 ms.
Keywords:
capacitance readout circuit; capacitance-to-digital converter (CDC); human body
communication (HBC)
1. Introduction
The future healthcare system needs wearable devices or implantable devices to
monitor human activity. Accordingly, a healthcare system based on human body com-
munication (HBC) has recently been widely studied and utilized in various fields [
1
,
2
].
There are two methods of signal transmission for HBC: galvanic coupling and capacitive
coupling [3–5]
. In galvanic coupling, two pairs of transmitters and receivers are attached
to the skin through which the human body is used as a waveguide that transmits signals
by changing the magnitude of the current. On the other hand, in the case of capacitive
coupling, only two signal electrodes are attached to the skin, and the ground electrodes re-
main floating. This means that the signal needs to travel longer, and the signal experiences
a distant channel environment compared to galvanic coupling. Capacitive coupling per-
forms better than galvanic coupling for frequencies above 60 kHz in multiple aspects such
as transmitting signals at longer distances horizontally and having low-complexity and
low-power consumption characteristics. Thus, the research community generally adopts
capacitive coupling in communication with wearable devices or implantable
devices [3–5]
.
Although capacitive coupling-based communication may potentially provide a better HBC
Electronics 2022, 11, 577. https://doi.org/10.3390/electronics11040577 https://www.mdpi.com/journal/electronics
