Excitation voltages for touch sensors
A method including applying a first excitation voltage to an electrode of a touch sensor which charges a capacitive node associated with the electrode from a first voltage level to a second voltage level that is greater than the first voltage level. A first measurement measures a charge to change from the first voltage level to the second voltage level. A second excitation voltage is applied to the electrode which charges the capacitive node from the second voltage level to a third voltage level that is greater than the second voltage level. The capacitive node is discharged from the third voltage level to a fourth voltage level that is less than the second voltage level. A second measurement measures a charge to change from the third voltage level to the fourth voltage level. A measured charge signal is generated based on the first measurement and the second measurement.
1. An apparatus comprising:
one or more processors; and
one or more memory units coupled to the one or more processors, the one or more memory units collectively storing logic, the logic being executed by the one or more processors so as to cause the one or more processors to perform operations comprising:
applying a first excitation voltage to an electrode of a touch sensor, applying the first excitation voltage to the electrode charges, with a first charge, a capacitive node associated with the electrode from a first voltage level to a second voltage level that is greater than the first voltage level;
performing a first charge measurement to measure the change from the first voltage level to the second voltage level;
applying a second excitation voltage to the electrode of the touch sensor, applying the second excitation voltage to the electrode charging the capacitive node from the second voltage level to a third voltage level that is greater than the second voltage level;
discharging the capacitive node from the third voltage level to a fourth voltage level that is less than the second voltage level;
performing a second charge measurement to measure the change from the third voltage level to the fourth voltage level; and
generating a measured charge signal based on the first charge measurement and the second charge measurement;
wherein the first excitation voltage and the second excitation voltage are each derived from a reference voltage.
2. The apparatus of claim 1 , wherein
generating the measured charge signal based on the first charge measurement and the second charge measurement comprises combining the first charge measurement and the second charge measurement.
3. The apparatus of claim 1 , wherein the operations further comprise:
comparing the measured charge signal to a reference measurement; and
determining that an object is within a detection distance of the touch sensor based on a difference between the measured charge signal and the reference measurement.
4. The apparatus of claim 1 , wherein:
the first charge measurement is a first integration performed over a first time period,
the second charge measurement is a second integration performed over a second time period, and
the first time period and the second time period are equal.
5. The apparatus of claim 1 , wherein applying the second excitation voltage comprises amplifying the first excitation voltage.
6. The apparatus of claim 1 , wherein a signal to noise ratio of the measured charge signal is characterized as follows:
(
V
1
+
N
1
)
-
(
V
2
+
N
2
)
(
N
1
+
N
2
where V1 represents a magnitude of a first voltage change from the first voltage level to the second voltage level, N1 represents a first noise component associated with the first voltage change, V1+N1 represents the first charge measurement, V2 represents a magnitude of a second voltage change from the third voltage level to the fourth voltage level, N2 represents a second noise component associated with the second voltage change, and V2+N2 represents the second charge measurement.
7. The apparatus of claim 1 , wherein a magnitude of the first charge measurement is different than a magnitude of the second charge measurement.
8. The apparatus of claim 1 , wherein the first voltage level and the fourth voltage level are different.