Fig. 3

Diagram of the capacitive electrode. The electrode consists of an analog amplifier and a standard printed circuit board. The sensing plate (diameter 15 cm), along with the analog amplifier OP1, forms the high-impedance front end with a capacitance neutralization circuit. The plate capacitor C _s is formed by the ground and the plate of the electrode connected to the non-inverting input of OP1. Diodes D ₁ and D ₂ provide a resistive path at the amplifier input without essentially shorting the signal to the amplifier. The gain of the first stage is simply set by R ₁ and R ₂. A well-known active shield is used to avoid current leakage at the input. The voltage divider formed by C _s and C _in has a detrimental effect on the gain and channel matching. To address this, R ₅ and R ₆ are adjusted to arrive at values such that the input capacitance is almost completely negated, resulting in an amplifier gain that is essentially invariant with respect to C _s. The corner frequency of the band-pass filter is set between 0.1 Hz and 100 kHz