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

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, 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

V_{R x B u f} = V_{T x} \times \frac{C_{R x T x}}{C_{R x T x} + C_{R x G} + C_{H}}

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. For further information on sensor designs with earth-grounded as well as nonearth-grounded devices, see "GestIC Design Guide” (DS40001716).

A brief overview of the typical values of the electrode capacitances is summarized in the table below:

Table 5-1. Electrode 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

Important: Ideal designs have low C_RxTx and C_RxG to ensure higher sensitivity of the electrode system. Optimal results are achieved with C_RxTx and C_RxG values being in the same range.