wm9713 Wolfson Microelectronics plc, wm9713 Datasheet - Page 66

• Manufacturer Part Number: wm9713
• Description: Ac?97 Audio + Touchpanel Codec
• Manufacturer: Wolfson Microelectronics plc

WM9713L
PRINCIPLE OF OPERATION - FOUR-WIRE TOUCHPANEL
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Four-wire touchpanels are connected to the WM9713L as follows:
The principle of operation is illustrated below (Note: the illustrations assume that the top plate is used
for X and the bottom plate for Y measurements, although the reverse is also possible).
Figure 25 X Co-ordinate Measurement on 4-wire Touchpanel
For an X co-ordinate measurement, the X+ pin is internally switched to TPVDD and X- to TPGND.
The X plate becomes a potential divider, and the voltage at the point of contact is proportional to its X
co-ordinate. This voltage is measured on the Y+ and Y- pins, which carry no current (hence there is
no voltage drop in R
Due to the ratiometric measurement method, the supply voltage does not affect measurement
accuracy. The voltage references VREF+ and VREF- are taken from after the matrix switches, so
that any voltage drop in these switches has no effect on the ADC measurement.
Figure 26 Y Co-ordinate Measurement on 4-wire Touchpanel
Y co-ordinate measurements are similar to X co-ordinate measurements, with the X and Y plates
interchanged.
•
•
•
•
Right side contact = X+ (pin 14)
Left side contact = X- (pin 16)
Top side contact = Y+ (pin 15)
Bottom side contact = Y- (pin 17)
Y+
or R
Y-
).
PP Rev 3.2 September 2008
Pre-Production
66