Internal Reference

Buffer

Band

Gap

Reference

Power Down

To

CDAC

Optional

V

REF

Reference Input

Reference Mode

TSC2003-Q1

www.ti.com ........................................................................................................................................................................................... SBAS454 – DECEMBER 2008

The TSC2003 has an internal 2.5-V voltage reference that can be turned on or off with the power-down control

bits, PD0 and PD1 (see Table 2 and Figure 5). The internal reference is powered down when power is first

applied to the device.

The internal reference voltage is only used in the single-ended reference mode for battery monitoring,

temperature measurement, and for measuring the auxiliary input. Optimal touch screen performance is achieved

when using a ratiometric conversion; thus, all touch screen measurements are done automatically in the

differential mode.

Figure 5. Simplified Diagram of the Internal Reference

The voltage difference between +REF and –REF (see Figure 4) sets the analog input range. The TSC2003

operates with a reference in the range of 2 V to VDD. 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 (256 if in 8-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 2 LSBs with a 2.5-V reference, it is typically 2.5 LSBs with a 2-V reference. In each case, the actual

offset of the device is the same, 1.22 mV. 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 is not buffered, and directly drives the capacitor digital-to-analog converter

(CDAC) portion of the TSC2003. Therefore, the input current is very low, typically < 6

µA.

There is 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 TSC2003 (see Figure 3). 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+, as shown in Figure 6. For this measurement, the resistance in the X+ lead does

not affect the conversion; it does, however, 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 would not be possible to

achieve a 0-V 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.

Copyright © 2008, Texas Instruments Incorporated

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