Pixel sensing circuit with common mode cancellation
View Patent ↗An improved biometric data sensing circuit, for example adapted for fingerprint sensing, uses a charge subtraction technique at the input of the circuit integrator to cancel the so called “common mode” signal from the circuit output. The result is an output signal that is (a) linear, (b) free from any amplification effect due to the presence of the detected object (e.g. a finger), and (c) indicative of the detected object's fine surface geometry (i.e., indicative of the fingerprint's ridges and valleys).
1. A capacitive sensing pixel for a biometric data detection system, comprising:
a first conductor supported by a semiconductor substrate;
a second conductor supported by said semiconductor substrate, said first and second conductors being spaced apart from each other;
a first reference voltage source providing a first input voltage having a first phase;
a second reference voltage source providing a second input voltage having a second phase substantially opposite to said first phase;
an inverting amplifier having an input and output;
an input capacitor connected between said first reference voltage source and said input of said inverting amplifier;
said first and second conductors forming at least a portion of a sensing capacitor connected between said second reference voltage source and said input of said inverting amplifier; and
a feedback capacitor connected between said input and said output of said inverting amplifier.
2. The capacitive sensing pixel of claim 1 , further comprising a switch connected between said input and said output of said inverting amplifier for resetting said sensing pixel.
3. An array of capacitive sensing pixels of the type used for sensing the pattern of a user's fingerprint, comprising:
a substrate;
a plurality of pixel cells formed on said substrate, each said pixel cell comprising:
a first conductor supported by said substrate;
a second conductor supported by said substrate, said first and second conductors being spaced apart from each other;
a first reference voltage source providing a first input voltage having a first phase;
a second reference voltage source providing a second input voltage having a second phase substantially opposite to said first phase;
an inverting amplifier having an input and output;
an input capacitor connected between said first reference voltage source and said input of said inverting amplifier;
said first and second conductors forming at least a portion of a sensing capacitor connected between said second reference voltage source and said input of said inverting amplifier; and
a feedback capacitor connected between said input and said output of said inverting amplifier.
4. The array of capacitive sensing pixels of claim 3 , wherein each pixel cell further comprises a switch connected between said input and said output of said inverting amplifier for resetting said sensing pixel.
5. A method of obtaining improved image data in a fingerprint sensor, comprising:
providing an array of capacitive-type fingerprint sensor cells in which a change in output voltage, Δvout, represents data indicating the presence of a valley or a ridge of a fingerprint thereover, and wherein in each said cell Δvin is a changing input voltage to the cell, ΔvinN and ΔvinP are changing input voltages to the cell with substantially the same magnitude as Δvin but such that ΔvinN and ΔvinP are substantially out of phase with respect to each other, Cin is the capacitance at the input to the cell, Cf is the capacitance of a feedback capacitor within the cell, CS is a variable sensing capacitance within the cell which changes in the presence or absence of a fingerprint over the cell, and CS0 is CS immediately following a reset of the cell, and further wherein:
in the presence of a valley of a fingerprint Δvout is substantially given by
Δ
v
out
=
-
Δ
v
in
·
C
in
C
f
+
Δ
V
in
·
C
S
0
C
f
and in the presence of a ridge of a fingerprint Δvout is substantially given by
Δ
v
out
=
-
Δ
v
in
P
·
C
in
C
f
+
Δ
V
in
N
·
C
S
0
C
f
-
Δ
V
in
N
·
C
S
0
α
C
f
providing said sensor circuit such that each of said Cf, CS0, and Cin are substantially equal in value such that
Δ
v
out
=
-
Δ
V
in
N
·
C
S
0
α
C
f
obtaining, from each said cell, image data corresponding to a portion of a user's fingerprint;
whereby said data represent an image of a portion of said user's fingerprint having contributions due to a common mode of the array removed therefrom.