Proposed Approach

III. High Resolution Read-out IC for Mutual Capacitive Sensor

3.2 Proposed Approach

The proposed high resolution and low noise ROIC is adapted MCSM method. This MCSM method is a kind of noise averaging method. A receiver ROIC with MCSM improves SNR, area efficiency and overcomes the low sensitivity due to a thick cover glass of the display panel. MCSM method increases SNR through sequential operating of AFE and post-processing with MCU. And several sensing electrodes share one ROIC channel. So, it has area efficiency compared with non-MCSM case.

Capacitive fingerprint sensor has the equivalent circuit like the following figure. In conventional TISM mode TX and RX case, each charge amplifier output is proportional of CM/CF. CM is mutual capacitance of sensor and CF is feedback capacitance of charge amplifier.

C

M1

V

_TX

C

F

V

_OUT

= V

_TX

x (C

_M1

/C

_F

) C

F

V

OUT

= V

TX

x (C

M2

/C

F

) C

M2

Fig. 3. 4 Equivalent circuit of conventional ROIC

To increase signal, there is a method to adapt small size of CF. But small CF makes charge amplifier’s output saturation. In case of high voltage (=20 VPP) VTX, CM = 25-fF and CF = 400-fF, the output swing is 1.25 V peak to peak. This makes the charge amplifier output saturation at condition of 1.8 V MOSFET device. It is the trade-off of high gain.

So, to compensate this problem, DM-TISM method was suggested. To adapt DM-TISM method, each charge amplifier output is proportional to delta CM, not CM. Delta CM is a few atto-farads in 0.3T cover glass thickness. It has small output swing of charge amplifier compared with TISM method such as few femto-farads. So, it is possible to adapt small CF through this DM-TISM method. But, due to uniformity problem, it is hard to reduce CF is lower than few hundred femto-farads.

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With this DM-TISM method, this thesis suggests MCSM method to increase SNR through noise averaging method.

C

M1

V

_TX

C

F

V

OUT

= V

TX

x (C

M1

-C

M2

/C

F

) C

F

V

TX

C

_M2

C

M3

C

M4

V

OUT

= V

TX

x (C

M3

-C

M4

/C

F

)

Fig. 3. 5 Equivalent circuit of DM-TISM mode

Fig. 3. 6 MCSM method

Control <0:9>

V

_REF

Charge Amplifier

(+)

Charge Amplifier

(-)

-

-

+ Gain ^Diff.

Amplifier

Rx 1 Rx 2 Rx 3 Rx 4 Rx 5 Rx 6

Array T/G

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This figure shows the structure of MCSM with charge amplifier. There are 6 sensing electrodes as inputs of one receiver channel. And I tried to control these 6 inputs merged as maximum number of combinations. And the RX 1 as Standard channel. So, the number of maximum case is 10. Thus, T/G Array is consisted of 60 T/G. It operates 6 T/Gs in a time, others do not be operated. And 6 T/Gs are separated to 3 + and 3 – inputs. Control <0:9> signal change the phase of T/G Array. All phase has difference merge combination each other.

Fig. 3. 7 Transmission gate array structure

Let’s look the T/G Array deeper. Transmission Gate is a kind of switch. When the control input of individual T/G is high, the T/G is on. And CONT<0:9> goes high sequentially from 0 to 9. These are outputs of Ring Counter. And all phase is connected differently each other.

Rx 1 Rx 2 Rx 3

Rx 4 Rx 5 Rx 6

CONT

<0> CONT

<1> CONT

<9>

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Fig. 3. 8 MCSM operation @ CONT <0> is high

This figure shows the MCSM operation. I adopted Differential mode TX driving method.

Considering about a situation when CONT<0> is high, the equivalent circuit is the right of figure. So, Final Result is related with ΔC1 + ΔC2 + ΔC3 – (ΔC4 + ΔC5 + ΔC6)

RX<3>

RX<4>

RX <1>

RX <2>

RX<5>

RX<6>

TX <N-1> TX <N>

In-phase (+1)

Out-phase (-1)

ΔC1+ΔC2+ΔC3

ΔC4+ΔC5+ΔC6

In-phase (+1)

@ CONT <0> : High

Equivalent Circuit

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Fig. 3. 9 Sample fingerprint information, MCSM outputs & Simulation result

This figure shows the whole operation of MCSM. The sample of Fingerprint information is generated.

Black color represents ridge and white color is valley. It can be converted to this table. One on the left end is delta C1 and one on the right end is delta C6. The table of bottom is MCSM merge table. At Cont0 phase, RX1 to 3 are merged to +, RX4 to 6 are merged to –. Sequentially, At Cont9 phase, RX 1, 5 and 6 are merged to +, RX 2 to 4 are merged to –. The below Sum means sum of the capacitance difference of each phase. And the graph is simulated one about this sample fingerprint information.

Each Result represents the sum of each phase proportionally.

Table. 3. 1 Reconstruction of MCSM

Reconstruction is sum of 6 particular results. To reconstruct ΔC1 – ΔC2 data, results of CONT <4> to

<9> are summarized. Through this summarization, other data, delta C3 to C6, are cancelled out to zero.

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Then, Reconstruction result has only 6 times increased data of ΔC1 – ΔC2. In terms of noise, the summarization of uncorrelated noise is increased root N times. So, SNR has 7.8 dB improvement through MCSM method.

Cont 0

Cont 1

Cont 2

Cont 3

Cont 4

Cont 5

Cont 6

Cont 7

Cont 8

Cont 9

Rx 1

+ + + + + + + + + +

Rx 2

+ + + + - - - - - -

Rx 3

+ - - - + + + - - -

Rx 4

- + - - + - - + + -

Rx 5

- - + - - + - + - +

Rx 6

- - - + - - + - + +

Table. 3. 2. MCSM Control Table

Table. 3. 3. Comparison w/o MCSM and w/ MCSM

This table shows comparison w/o and w/ MCSM. I took w/o MCSM situation as standard. Second row is for multi sampling only. It takes 2 times longer TCH to increase SNR root 2 times higher. With MCSM cases, these can detect several capacitances at a time with higher SNR, and smaller Area. In terms of TCH, it seems MCSM case take much longer TCH. But to detect same number of capacitances, MCSM still has advantage of SNR and AREA. Last column represents Time, Area Efficiency. The value of Time Area efficiency is highest at MCSM_6 to merge more electrode, the efficiency goes down after MCSM_6.

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3.3 A Proposed Multi Channel Selective Merge (MCSM) System