25

LTC1197/LTC1197L

LTC1199/LTC1199L

S

APPLICATI

TYPICAL

C4

1000pF

C6

1000pF

C5

1000pF

R8

4.7k

R11

100k

R12

100k

R6

4.7k

R3

10

Ω

R7

100k

R1

100

Ω

LTC1199

R2

100

Ω

R7

100k

R9

100k

Q1

2N2907

Q3

2N2222A

Q2

2N2907

R10

4.7k

74HC14

R6

4.7k

5V

TOUCH SENSE

CHIP SELECT

SERIAL CLK

DATA IN

DATA OUT

Q4

2N2222A

C7

1000pF

C2

1

µF

C1

1

µF

1197/99 F13

C3

10

µF

1

2

3

4

8

7

6

5

CLK

CS

CH0

CH1

GND

+

Figure 13. The LTC1199 Digitizes Resistive Touchscreen X and Y Axis Voltages. The ADC’s Auto Shutdown Feature

Helps Maximize Battery Life in Portable Touchscreen Equipment

Resistive Touchscreen Interface

Figure 13 shows the LTC1199 in a 4-wire resistive touch-

screen application. Transistor pairs Q1-Q3, Q2-Q4 apply

5V and ground to the X axis and Y axis, respectively. The

LTC1199, with its 2-channel multiplexer, digitizes the

voltage generated by each axis and transmits the conver-

sion results to the system’s processor through a serial

interface. RC combinations R1C1, R2C2 and R3C3 form

lowpass filters that attenuate noise from possible sources

such as the processor clock, switching power supplies

and bus signals. The 74HC14 inverter is used to detect

screen contact both during a conversion sequence and to

trigger its start. Using the single channel LTC1197, 5-wire

resistive touchscreens are as easily accommodated.