INTERNAL REFERENCE
is made by connecting the X+ input to the ADC, turning on the
Y+ and Y– drivers, and digitizing the voltage on X+ (Figure 4
shows a block diagram). For this measurement, the resistance
in the X+ lead does not affect the conversion (it does affect the
settling time, but the resistance is usually small enough that
this is not a concern). However, since the resistance between
Y+ and Y– is fairly low, the on-resistance of the Y drivers does
make a small difference. Under the situation outlined so far, it
is not possible to achieve a 0V input or a full-scale input
regardless of where the pointing device is on the touch screen
because some voltage is lost across the internal switches. In
addition, the internal switch resistance is unlikely to track the
resistance of the touch screen, providing an additional source
of error.
The TSC2046 has an internal 2.5V voltage reference that can
be turned on or off with the control bit, PD1 (see Table V and
Figure 3). Typically, the internal reference voltage is only used
in the single-ended mode for battery monitoring, temperature
measurement, and for using the auxiliary input. Optimal touch
screen performance is achieved when using the differential
mode. The internal reference voltage of the TSC2046 must be
commanded to be off to maintain compatibility with the
ADS7843. Therefore, after power-up, a write of PD1 = 0 is
required to insure the reference is off (see the Typical Char-
acteristics for power-up time of the reference from power-
down).
Reference
Power-Down
+VCC
VREF
Y+
VREF
Band
Buffer
Gap
+REF
Converter
–REF
+IN
–IN
X+
Y–
Optional
To
CDAC
FIGURE 3. Simplified Diagram of the Internal Reference.
REFERENCE INPUT
GND
FIGURE 4. Simplified Diagram of Single-Ended Reference
SER/DFR high, Y switches enabled, X+ is ana-
The voltage difference between +REF and –REF (see Figure 2)
sets the analog input range. The TSC2046 operates with a
reference in the range of 1V to +VCC. There are several critical
items concerning the reference input and its wide voltage range.
As the reference voltage is reduced, the analog voltage weight
of each digital output code is also reduced. This is often referred
to as the LSB (least significant bit) size and is equal to the
reference voltage divided by 4096 in 12-bit mode. Any offset or
gain error inherent in the ADC appears to increase, in terms of
LSB size, as the reference voltage is reduced. For example, if
the offset of a given converter is 2LSBs with a 2.5V reference,
it is typically 5LSBs with a 1V reference. In each case, the
actual offset of the device is the same, 1.22mV. With a lower
reference voltage, more care must be taken to provide a clean
layout including adequate bypassing, a clean (low-noise, low-
ripple) power supply, a low-noise reference (if an external
reference is used), and a low-noise input signal.
(
log input).
This situation can be remedied as shown in Figure 5. By setting
the SER/DFR bit low, the +REF and –REF inputs are connected
directly to Y+ and Y–, respectively, which makes the analog-to-
digital conversion ratiometric. The result of the conversion is
+VCC
Y+
+REF
+IN
X+
The voltage into the VREF input directly drives the capacitor
digital-to-analog converter (CDAC) portion of the TSC2046.
Therefore, the input current is very low (typically < 13µA).
Converter
–IN
–REF
There is also a critical item regarding the reference when
making measurements while the switch drivers are ON. For this
discussion, it is useful to consider the basic operation of the
TSC2046, (see Figure 1). This particular application shows
the device being used to digitize a resistive touch screen. A
measurement of the current Y-Position of the pointing device
Y–
GND
FIGURE 5. Simplified Diagram of Differential Reference
(
SER/DFR low, Y switches enabled, X+ is
analog input).
TSC2046
10
SBAS265C
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