AD7843(Rev0) Ver la hoja de datos (PDF) - Analog Devices
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AD7843 Datasheet PDF : 16 Pages
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AD7843
Table I. Analog Input, Reference, and Touch Screen Control
A21
A11
A01 SER/DFR
Analog In
X Switches
Y Switches
0
0
1
1
0
1
0
1
1
0
1
1
1
1
0
1
0
0
1
0
1
0
1
0
1
1
0
0
X+
OFF
ON
IN3
OFF
OFF
Y+
ON
OFF
IN4
OFF
OFF
X+
OFF
ON
Y+
ON
OFF
Outputs Identity Code, 1000 0000 0000
NOTES
1All remaining configurations are invalid addresses.
2Internal node – not directly accessible by the user.
+REF 2
VREF
VREF
VREF
VREF
Y+
X+
–REF 2
GND
GND
GND
GND
Y–
X–
Touch Screen Settling
In some applications, external capacitors may be required across
the touch screen to filter noise associated with it, e.g., noise
generated by the LCD panel or backlight circuitry. The value of
these capacitors will cause a settling time requirement when the
panel is touched. The settling time will typically show up as a
gain error. There are several methods for minimizing or elimi-
nating this issue. The problem may be that the input signal, or
reference, or both, have not settled to their final value before the
sampling instant of the ADC. Additionally, the reference voltage
may still be changing during the conversion cycle. One option is
to stop, or slow down the DCLK for the required touch screen
settling time. This will allow the input and reference to stabilize
for the acquisition time. This will resolve the issue for both
single-ended and differential modes.
The other option is to operate the AD7843 in differential mode
only for the touch screen, and program the AD7843 to keep the
touch screen drivers ON and not go into power-down (PD0 =
PD1 = 1). Several conversions may be required depending on
the settling time required and the AD7843 data rate. Once the
required number of conversions have been made, the AD7843
can then be placed in a power-down state on the last measure-
ment. The last method is to use the 15 DCLK cycle mode, which
maintains the touch screen drivers ON until it is commanded to
stop by the processor.
Reference Input
The voltage difference between +REF and –REF (see Figure 4)
sets the analog input range. The AD7843 will operate with a
reference input in the range of 1 V to VCC. The voltage into
the VREF input is not buffered and directly drives the capaci-
tor DAC portion of the AD7843. Figure 5 shows the reference
input circuitry. Typically, the input current is 8 µA with VREF =
2.5 V and fSAMPLE = 125 kHz. This value will vary by a few
microamps, depending on the result of the conversion. The
reference current diminishes directly with both conversion rate
and reference voltage. As the current from the reference is
drawn on each bit decision, clocking the converter more quickly
during a given conversion period will not reduce the overall
current drain from the reference.
X+
Y+
VREF
3-TO-1
MUX
ADC
Figure 5. Reference Input Circuitry
When making touch screen measurements, conversions can be
made in the differential (ratiometric) mode or the single-ended
mode. If the SER/DFR bit is set to 1 in the control register, a
single-ended conversion will be performed. Figure 6 shows the
configuration for a single-ended Y coordinate measurement.
The X+ input is connected to the analog to digital converter,
the Y+ and Y– drivers are turned on and the voltage on X+ is
digitized. The conversion is performed with the ADC refer-
enced from GND to VREF. The advantage of this mode is that
the switches that supply the external touch screen can be turned
off once the acquisition is complete, resulting in a power saving.
However, the on-resistance of the Y drivers will affect the input
voltage that can be acquired. The full touch screen resistance
may be in the order of 200 Ω to 900 Ω, depending on the manu-
facturer. Thus if the on-resistance of the switches is approximately
6 Ω, true full-scale and zero-scale voltages cannot be acquired
regardless of where the pen/stylus is on the touch screen.
Note: The minimum touch screen resistance recommended for
use with the AD7843 is approximately 70 Ω.
+VCC
Y+
VREF
X+
IN+
REF+
IN+ ADC CORE
IN– REF–
Y–
GND
Figure 6. Single-Ended Reference Mode (SER/DFR = 1)
In this mode of operation, therefore, some voltage is likely to be
lost across the internal switches and, in addition to this, it is
unlikely that the internal switch resistance will track the resis-
tance of the touch screen over temperature and supply, providing
an additional source of error.
The alternative to this situation is to set the SER/DFR bit low.
If one again considers making a Y coordinate measurement,
but now the +REF and –REF nodes of the ADC are connected
directly to the Y+ and Y– pins, this means the analog to digital
conversion will be ratiometric. The result of the conversion will
always be a percentage of the external resistance, independent of
how it may change with respect to the on-resistance of the internal
switches. Figure 7 shows the configuration for a ratiometric Y
coordinate measurement. It should be noted that the differential
–10–
REV. 0
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