
–
11
–
CXA3271AGE
Notes on Operation
Sr
S4
S3
Cf1
Cf2
S6
S7
S5
Sc
Cs
Cp
Cp'
Vcel
Vsl
S_Amp
O_Amp
VOS
Ch2
Ch1
Voi
Voo
Buf
Aout
Vdmy
Vsns
S11
S2
VL
VM
S1
VH
Aout output variance
Aout output variance can be broadly classified into two types. The first is variance intrinsic to the IC, and the
second is variance caused by the influence of external noise due to the extremely high sensitivity.
Variance intrinsic to the IC
1. The Aout output DC level fluctuates widely due to the IC.
This is caused by the Cp and Cp' capacitance values, the VM voltage level, the voltage differences VH
–
VM
and VM
–
VL, and the VOS voltage level in the figure above.
VOS, VH, VM and VL appear externally as pins. The Aout output level can be set to the desired potential
by applying the VOS voltage from an external source.
The Aout dynamic range is approximately 0.6 to 2.1V, so checking this output level and externally
applying the VOS voltage to set the optimum level is recommended.
2. 192 variances within one line
One line is comprised of 192 sensors. Each sensor is connected to a separate S_Amp, so the S_Amp
offset appears in the output. (approximately 100 to 200mV)
3. The DC level of a line changes with a certain regularity for some ICs. This is also caused by the S_Amp
DC offset.
Variance due to the influence of external noise
1. Output fluctuation due to cross talk from the power supply
Power supply fluctuation has a large influence on the Aout output of this IC.
In addition to the capacitances between the power supply and GND (approximately 1μF, both sides if
possible), attaching capacitances of approximately 0.1μF to VOS, VH, VM and VL is recommended.
2. Finger stabilization
The human body acts as an antenna, so the finger potential changes during the sensing period,
producing noise in the Aout output. To prevent this, the potential of the area around the finger being
sensed must be equalized with the sensor GND. Measures such as placing a metal plate connected to
GND around the sensor so that the finger touches this place during sensing are recommended.