1.3 Open- Drain Driver

An open-drain driver has an inherent difference in reference coupling between the ‘driven’ and ‘floating’ states. When an open drain driver changes its state, it causes a delta on the touch signal (even without PWM). If LEDs or open-drain circuits are close to the touch sensor or sensor traces (less than 4 mm away) they must be bypassed with a capacitor that has a typical value of 1 nF. An additional capacitor needs to be added closer to the LED to make sure that a sudden surge in current does not create noise in the power supply. The typical value of these capacitors is 100 nF.