
–
9
–
CXA3271AGE
Sr
S4
S3
Cf1
Cf2
S6
S7
S11
S2
VL
VM
S1
VH
Sense amplifier block
(192)
Output block
(1)
Sensor block
(192
×
128)
S5
Sc
Cs
Cp
Cp'
Vcel
Vsl
S_Amp
O_Amp
VOS
Ch2
Ch1
Voi
Voo
Buf
Aout
Vdmy
Vsns
Fig. 2
Fingerprint sensor operation (Fig. 2)
Description of characters
Cs:
Capacitance formed between the finger and the metal electrode
Cp:
Parasitic capacitance formed between the metal electrode and the silicon substrate
Cp': Capacitance for canceling Cp (Cp
≈
Cp')
Ch
: Hold capacitance
Cf
: Feedback capacitance for determining the gain
S
:
Switch
V
:
Node voltage
VH
–
VM
≈
VM
–
VL
Detailed description of operation
(All switches are off in the default status.)
1.
S1, S4 and Sr are turned on, and Vcel is set to voltage VH.
Vcel accumulated charge (Cs + Cp) VH
2.
S1 and Sr are turned off.
3.
S11 and S3 are turned on, and Vdmy is set to voltage VL.
Vdmy accumulated charge Cp
’
VL
4.
S3 and S11 are set to off.
5.
S2 is turned on and Vsl is set to VM.
6.
S4 is turned off.
7.
Sr, S3 and S5 are turned on.
At this time, the charge level that moves from Vcel and Vdmy to Vsl (actually between capacitances) is:
(Cs + Cp) (VH
–
VM)
–
Cp' (VM
–
VL)
≈
Cs (VH
–
VM)
This means that the sense amplifier gain is determined independently of the parasitic capacitance,
making it possible to obtain the required large signal dynamic range.
Vsns = VM
–
Cs (VH
–
VM)/Cf1
The voltage Vsns determined as shown above is accumulated in Ch1.
8.
S5 is turned off.
9.
S6 is turned on and the Voi voltage is set to VOS.
10. S6 is turned off.
11. Sc and S7 are turned on.
At this time, the charge level that moves from Ch1 to Cf2 is:
(VOS
–
Vsns) Ch1
This determines the Voo voltage which is accumulated in Ch2 and output to Aout via the buffer.