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TSC2046EIPW Fiches technique(PDF) 11 Page - Burr-Brown (TI)

[Old version datasheet] Texas Instruments acquired Burr-Brown Corporation. Click here to check the latest version.
No de pièce TSC2046EIPW
Description  Low Voltage I/O TOUCH SCREEN CONTROLLER
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Fabricant  BURR-BROWN [Burr-Brown (TI)]
Site Internet  http://www.burr-brown.com
Logo BURR-BROWN - Burr-Brown (TI)

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TSC2046E
SBAS417 − JUNE 2007
www.ti.com
11
INTERNAL REFERENCE
The TSC2046E has an internal 2.5V voltage reference that
can be turned on or off with the control bit, PD1 (see Table 5
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
TSC2046E must be commanded to be off to maintain
compatibility with the ADS7843. Therefore, after power-up,
a write of PD1 = 0 is required to ensure the reference is off
(see the Typical Characteristics for power-up time of the
reference from power-down).
Buffer
Band
Gap
Reference
Power−Down
To
CDAC
Optional
V
REF
Figure 3. Simplified Diagram of the Internal
Reference
REFERENCE INPUT
The voltage difference between +REF and –REF (see
Figure 2) sets the analog input range. The TSC2046E
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 (referred to as LSB size) is also reduced. The LSB
(least significant bit) size 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.
The voltage into the VREF input directly drives the
capacitor digital-to-analog converter (CDAC) portion of the
TSC2046E. Therefore, the input current is very low
(typically < 13
µA).
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 TSC2046E (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 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, because 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.
Converter
+IN
+REF
Y+
+V
CC
V
REF
X+
Y
GND
−REF
−IN
Figure 4. Simplified Diagram of Single-Ended
Reference (SER/DFR high, Y switches enabled,
X+ is analog 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, making
the analog-to-digital conversion ratiometric. The result of
the conversion is always a percentage of the external
resistance, regardless of how it changes in relation to the
on-resistance of the internal switches. Note that there is an
important consideration regarding power dissipation when
using the ratiometric mode of operation (see the Power
Dissipation section for more details).


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