MGC3030 datasheet(17/46 Pages) MICROCHIP

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5.3

External Rx Electrodes

Rx electrodes are connected to the MGC3140. An electrode needs to be individually designed following the guide

lines from the "GestIC Design Guide” (DS40001716), for optimal E-field distribution and detection of E-field variations

inflicted by a user.

5.3.1

Electrode Equivalent Circuit

The hand position tracking and gesture recognition capabilities of a GestIC system depend on the electrode design

and their material characteristics.

A simplified equivalent circuit model of a generic GestIC electrode system is illustrated in the following figure:

Figure 5-2. Electrodes Capacitive Equivalent Circuitry Earth Grounded

CRXTX

CTXG

CRXG

System ground

Transmitter signal

Electrode signal

Earth ground

E-field

VTX

System Ground

eRx

eTx

External Electrodes

VRX

VTx

Tx electrode voltage

VRxBuf MGC3140 Rx input voltage

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.

CRxTx Capacitance between receive and transmit electrodes

CRxG

Capacitance of the receive (Rx) electrode to system ground + input capacitance of the MGC3140 receiver

circuit

CTxG

Capacitance of the transmit (Tx) electrode to system ground

eRx

Rx electrode

eTx

Tx electrode

The Rx and Tx electrodes in a GestIC electrode system build a capacitance voltage divider with the capacitances

CRxTx and CRxG which are determined by the electrode design. CTxG 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, CH changes its capacitance. Femtofarad changes are

detected by the MGC3140 receiver. The equivalent circuit formula for the earth-grounded circuitry is described in the

following equation:

Equation 5-1. Electrodes Equivalent Circuit

VRxBuf=VTx× CRxTx

CRxTx+CRxG+CH

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

MGC3140

System Architecture

and its subsidiaries

Advance Information Datasheet

DS40002037C-page 17

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