Capacitance compensation STMPE321
6/40 Doc ID 15791 Rev 3
2 Capacitance compensation
The STMPE321 is capable to measuring up to 5.1pF in capacitance difference between the
reference point (Zref) and the individual channels. In the case where PCB connection
between the sensor pads and the device is too long, the "REFERENCE DELAY" register is
able to shift the reference by up to 5.1pF, allowing the TOUCH channels to measure added
capacitance 5.1pF with offset of 5.1pF, as shown in following diagram.
In the case where this is still not enough to compensate for the capacitance on sensor lines
(due to very long sensor trace), an external capacitor of up to 30 pF could be connected at
the A_Ref pin.
This would further shift up the dynamic range of the capacitance measurement.
Figure 4. Capacitance compensation
The sensed capacitance is accessible to the host through the IMPEDANCE registers.
!-6
)NCREASINGCAPACITANCE
P&
P&
P&
P&
P&
P&
2EFERENCEDELAYREGISTER
$YNAMICRANGEP&
2EFERENCEDELAYREGISTERP&
$YNAMICRANGEP&
2EFERENCEDELAYP&
!2EFP&
$YNAMICRANGEP&P&
