Hardware trimming

AN4316

20/26

DocID024847 Rev 2

6

Hardware trimming

6.1

Cs trimming

The Cs capacitance is a key parameter for sensitivity. For touchkey sensors, the Cs value is

usually comprised between 8.7nF to 22nF. For linear and rotary touch sensors, the value is

usually comprised between 47nF and 100nF. These values are given as reference for an

electrode fitting a human finger tip size across a few millimeters dielectric panel.

The signal delta for a touchkey is usually above 20 while it’s around 100 for linear and rotary

touch sensors. These values are given for a normalized test finger.

6.2

Shield adjustment

The efficiency of the shield depends on the waveforms matching between the shielded

channel and the shield burst pulses. The parameters to adapt the shield waveform are Cs

and Rs. These parameters adjustment is performed in 2 steps:

1.

Active shield Cs trimming

The burst envelop of the channels belonging to a same bank should end at the same

voltage level (VIH):

Figure 12. Active shield Cs trimming