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SS7646

8/21/2004 Rev.2.01

A2A1A0

(S ho w n 101 )

Y-

X+

Y+

X-

VDD

B attery

M o nito r

/PEN IR Q

ADC

+ R E F

-R E F

IN

M O D E 0

(S ho w n 1)

Figure 5. Diagram of Input Channel and Reference Input

ANALOG INPUT MEASUREMENT

The SS7646 contains 6 channel inputs. X+, Y+ and

Y- inputs are for touch screen measurement, 2 auxiliary

is for temperature measurement. Figure 5 shows a

diagram of the analog input channel and reference

input. The input to the A/D converter is selected via the

s i x - c hannel m u ltiple x e r . ( s ee Blo c k

Diagram and Figure 5.)

Input Channel

Table III shown the input channel configuration of the

SS7646. The control bits are set via the DIN pin.

(see Control Byte section). The selected channel is for

A/D converter input. Please refer to Figure 5 for

detailed input channel multiplexer. For measuring X+

(Y-position) and Y+ (X-position), Y switches and X

switches are turned on respectively.

Internal Reference

The SS7646 has an internal 2.5V voltage

reference that can be turned ON with the power-down

address bit, PD1=”1”, at the configuration byte (See

Table V and Figure 1). The internal reference voltage

of SS7646 should be turned OFF (PD1=”0”) in

order to be compatible with the SS7643. If the

internal reference is turned OFF and the reference

is taken from the power supply directly, special care

must be taken to avoid noise from the power supply.

While the ADC can be powered up or down instantly,

the internal reference requires settling time to settle to

30

µs without a load. The internal reference voltage

can be turned ON or OFF independently of the ADC.

T

his allows extra time for the reference voltage to be

settled before a conversion. Although /CS=”1” will

put the SS7646 into a power-down mode

immediately, the internal reference does not turn

OFF with /CS going HIGH. An additional pattern

with PD1=”0” is required before /CS goes HIGH.