[AK4183]

MS0500-E-01

2008/12

6

The AK4183 incorporates a 12-bit successive approximation resistor (SAR) A/D converter for position measurement.

The architecture is based on a capacitive redistribution algorithm, and an internal capacitor array functions as the

sample/hold (S/H) circuit. The SAR A/D converter output is a straight binary format as shown below:

Input Voltage

Output Code

(

ΔVREF-1.5LSB)~ ΔVREF

FFFH

(

ΔVREF-2.5LSB) ~ (ΔVREF-

1.5LSB)

FFEH

---------

---------

0.5LSB ~ 1.5LSB

001H

0 ~ 0.5LSB

000H

ΔVREF: (VREF+) – (VREF-)

Table 1. Output Code

A position detecting (X, Y position) on the touch panel is selected by the control command via the A2, A1, A0 bits in

the control register. The mode of the position detecting is differential mode, the full scale (

ΔVREF) is the differential

voltage between the non-inverting terminal and the inverting terminal of the measured axis (e.g. X-axis measurement:

ΔVREF = V

The voltage difference on the A/D converter (

ΔAIN) is the voltage between non-inverting

terminals of the non-measured axis and the inverting terminal of the measured axis. (E.g.

ΔAIN= (AIN+) - (AIN-) =

current flows into the internal capacitor after the capacitor has been charged completely.

The required settling time to charge the internal capacitor array depends on the source impedance (Rin). If the source

impedance is 600 ohm, the settling time needs at least 2.5

μs (1 clock cycle period of SCL 400 KHz)

The position on the touch screen is detected by taking the voltage of one axis when the voltage is supplied between the

two terminals of another axis. At least two A/D conversions are needed to get the two-dimensional (X/Y axis) position.

The X-plate and Y-plate are connected on the dotted line when the panel is touched.

XP

XN

X-Plate (Top side)

Y-Plate (Bottom side)

c)

4-wire Touch Screen Construction

X-Plate

Y-Plate

X-Plate

YP

YN

YN

XN

YP

XN-Driver SW ON

VREF+

VREF-

ADC

AIN+

AIN-

XP

XP-Driver SW ON

a)

X-Position Measurement Differential Mode

b)

Y-Position Measurement Differential Mode

YN

XN

YP

YN-Driver SW ON

VREF+

VREF-

ADC

AIN+

AIN-

XP

YP-Driver SW ON

VCC

VCC

Touch Screen

Y-Plate

Figure 2. Axis Measurements