For self capacitive sensing arrangements, capacitance is formed
between the sensor pad and the ground. When an object, such as
a finger or stylus, comes into close proximity with the sensor, the
capacitance begins to increase. With mutual differential sensing
the capacitance is formed between the sensor and excitation pads.
In contrast to self -capacitive sensing, when a finger (or some other object) comes closer to it, the capacitance decreases.
Self-capacitive sensing has two capacitances (signified by S1
and S2 in Figure 2). When connected in parallel, S1 and S2 are
added together. Then total area S becomes S = S1 + S2. S increases, with the consequence that capacitance (C) is also raised. Under
the condition that a constant current is applied, the gradient of
voltage decreases proportionally as C increases. When a material
is inserted, S will decrease and hence, C decreases.
When the user touches the sensing pad, C is added to parasitic
capacitance (Cp). The total capacitance C, which is C = Cp + C,
V = (i/C)t
The time (t), to charge the capacitance (C) until the voltage
reaches to Vt, is changed by the amount of C.
t = (Vt/i)C
From this, touch events can be determined and accurately
quantified by measuring the time to charge (t) (Figure 3).
Mutual differential sensing offers better touch performance
than self-capacitive sensing. The basic principle of this is when a
finger comes closer to the sensor input pad (CinX) the line of elec-
tric force emitted from the excitation pad (Cdrv) is terminated. It
reduces the number of lines of electric force, with the capacitance
between Cdrv and CinX decreasing to some degree (this is signi-
fied by ΔC).
The change ΔC is detected and converted into voltage, Vout,
which relates linearly to ΔC. It is possible to detect touch events
by measuring Vout. When it exceeds threshold voltage (Vt), then
a registered touch event takes place.
The CV amplifier realizes the CV conversion through a state
change of two-phase (Phase 1, Phase 2) that are synchronized to
Cdrv signal. Figure 5 shows the circuits of CV amplifier when the
finger is not touching the sensor pad. Since sensor capacitance
(C1) does not change (C1 = C), reallocation of charge among C1,
C2, and Cf does not occur between in Phase 1 and in Phase 2. As
the result, amplifier output is also unchanged, it remains zero (Vf
Since a part of the lines of electric force are terminated by a
finger, sensor capacitance C1 decreases (C-ΔC). Because of this
change, reallocation of charge among C1, C2 and Cf occurs between in Phase1 and in Phase2. As the result, amplifier output
Vf = (ΔC/Cf)VDD
The upshot of mutual differential sensing is that detection of
the changes in the capacitive field is quicker and more precise,
Figure 2: Simplified model of self-capacitive sensing.
Figure 3: Quantifying a touch event using time of charge.
Figure 4: Simplified model of mutual differential sensing.
Figure 5: Operation of CV amplifier when touch event occurs on