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TSC2046IRGVT 데이터시트(PDF) 10 Page - Burr-Brown (TI) |
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TSC2046IRGVT 데이터시트(HTML) 10 Page - Burr-Brown (TI) |
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10 / 23 page  TSC2046 10 SBAS265C www.ti.com FIGURE 4. Simplified Diagram of Single-Ended Reference (SER/DFR high, Y switches enabled, X+ is ana- log input). FIGURE 5. Simplified Diagram of Differential Reference (SER/DFR low, Y switches enabled, X+ is analog input). Converter +IN +REF Y+ +V CC X+ Y– GND –REF –IN Converter +IN +REF Y+ +V CC V REF X+ Y– GND –REF –IN INTERNAL REFERENCE 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 INPUT 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. 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). 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 Buffer Band Gap Reference Power-Down To CDAC Optional V REF FIGURE 3. Simplified Diagram of the 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. 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 |
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