Datasheet

2012-2013 Microchip Technology Inc. Advance Information DS40001667C-page 11
MGC3130
3.3.1 ELECTRODE EQUIVALENT
CIRCUIT
The hand Position Tracking and Gesture Recognition
capabilities of a GestIC system depends on the
electrodes design and their material characteristics.
A simplified equivalent circuit model of a generic
GestIC electrode system is illustrated in Figure 3-2.
FIGURE 3-2: ELECTRODES CAPACITIVE EQUIVALENT CIRCUITRY EARTH GROUNDED
•V
TX
: Tx electrode voltage
•V
RXBUf
: MGC3130 Rx input voltage
•C
H
: Capacitance between receive electrode and
hand (earth ground). The user’s hand can always
be considered as earth-grounded due to the
comparable large size of the human body.
•C
RXTX
: Capacitance between receive and transmit
electrodes
•C
RXG
: Capacitance of the receive (Rx) electrode
to system ground + input capacitance of the
MGC3130 receiver circuit
•C
TxG
: Capacitance of the transmit (Tx) electrode
to system ground
•e
Rx
: Rx electrode
•e
Tx
: Tx electrode
The Rx and Tx electrodes in a GestIC electrode system
build a capacitance voltage divider with the
capacitances C
RxTx
and C
RxG
which are determined by
the electrode design. C
TxG
represents the Tx electrode
capacitance to system ground driven by the Tx signal.
The Rx electrode measures the potential of the
generated E-field. If a conductive object (e.g., a hand)
approaches the Rx electrode, C
H
changes its
capacitance. This minuscule change in the femtofarad
range is detected by the MGC3130 receiver.
The equivalent circuit formula for the earth-grounded
circuitry is described in Equation 3-1.
EQUATION 3-1: ELECTRODES
EQUIVALENT CIRCUIT
A common example of an earth-grounded device is a
notebook, even with no ground connection via power
supply or ethernet connection. Due to its larger form
factor, it presents a high earth-ground capacitance in
the range of 50 pF and thus, it can be assumed as an
earth-grounded GestIC system.
A brief overview of the typical values of the electrodes
capacitances is summarized in Tab l e 3-1.
TABLE 3-1: ELECTRODES
CAPACITANCES TYPICAL
VALUES
Capacity Typical Value
C
RXTX
10...30 pF
C
TXG
10...1000 pF
C
RXG
10...30 pF
C
H
<1 pF
V
RxBuf
V
Tx
C
RxTx
C
RxTx
C
RxG
C
H
++
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