Digital Signal Processing Reference
In-Depth Information
(a)
R
4
R
3
R
2
R
1
C
1
C
2
C
3
C
4
(b)
R
4
R
3
R
2
R
1
C
1
C
2
C
3
C
4
Fig. 5.12 a
The measured output of the 2
D
touch pad with the finger on two different positions
R
1
C
1
and
R
3
C
3
. The measurement is performed with a power supply voltage of 15V and at a sample
rate of 150S/S.
b
The measured sensor outputs when the finger is halfway between positions
R
3
C
2
and
R
3
C
3
and in the center of
R
1
R
2
C
1
C
2
at a 270S/s. Both measurements have been performed in
ambient with external selector signals and with a shielded finger
5.5 Discussion
The sensors presented in Sect.
5.4
are a proof of concept for electronic sensors im-
plemented in an organic electronics technology. They prove that not only at the tech-
nology level but also on the circuit level sensor design is possible in a given organic
technology without changing a single parameter in the production flow. However, a
few improvements of this sensor should be included in a redesign to obtain full and
correct operation. In the first place the selector circuit is not fully functional because
its output signals are not nicely digital. In some cases, their output voltage is around
V
DD
2
whereas it is expected to be high, i.e. equal to
V
DD
. This is caused by
NAND
gates that are not powerful enough to pull up the output signals when one of their
inputs is low and the other high. This problem can easily be solved in a redesign
through proper buffering of the digital signals with inverters.
In the second place a problem arises when the 2
D
sensor is touched with a human
finger. This is caused by the large capacitance (typically 100pF) of the human body
towards the environment. This capacitor creates a parasitic parallel path that hampers
the proper read-out of the different column lines. As a result, four peaks appear in
the serial output, instead of one expected peak. Those four peaks correspond to the
four pixels in the same row as the position of the finger. Practically, the circuit can
