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TSC2003IPWR датащи(PDF) 11 Page - Texas Instruments |
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TSC2003IPWR датащи(HTML) 11 Page - Texas Instruments |
11 / 22 page TSC2003 11 SBAS162D www.ti.com REFERENCE MODE 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 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 A/D converter, turning on the Y+ and Y– drivers, and digitizing the voltage on X+, as shown in Figure 4. 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, 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 would not be 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. This situation is remedied, as shown in Figure 5, by using the differential mode: the +REF and –REF inputs are connected directly to Y+ and Y–, respectively. This makes the A/D converter ratiometric. The result of the conversion is always a percentage of the external reference, regardless of how it changes in relation to the on-resistance of the internal switches. REFERENCE INPUT The voltage difference between +REF and –REF (see Figure 2) sets the analog input range. The TSC2003 will operate with a reference in the range of 2V 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 A/D converter will appear 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 will typically be 2.5LSBs with a 2V 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 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. FIGURE 4. Simplified Diagram of Single-Ended Reference. FIGURE 5. Simplified Diagram of Differential Reference (Y Switches Enabled, X+ is Analog Input). Converter +IN +REF Y+ +V DD X+ Y– GND –REF –IN Converter +IN +REF Y+ +V DD V REF X+ Y– GND –REF –IN |
Аналогичный номер детали - TSC2003IPWR |
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Аналогичное описание - TSC2003IPWR |
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