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ADS7846N2K5 Datasheet(PDF) 10 Page - Burr-Brown (TI) |
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ADS7846N2K5 Datasheet(HTML) 10 Page - Burr-Brown (TI) |
10 / 30 page ADS7846 10 SBAS125H www.ti.com 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 ADS7846. Therefore, the input current is very low (typically < 13 µA). There is also a critical item regarding the reference when making measurements where the switch drivers are on. For this discussion, it is useful to consider the basic operation of the ADS7846 (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, 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 resis- tance of the touch screen, providing an additional source of error. 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, which makes 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 consid- eration regarding power dissipation when using the ratiometric mode of operation (see the Power Dissipation section for more details). 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 As a final note about the differential reference mode, it must be used with +VCC as the source of the +REF voltage and cannot be used with VREF. It is possible to use a high precision reference on VREF and single-ended reference mode for measurements which do not need to be ratiometric. In some cases, it is possible to power the converter directly from a precision reference. Most references can provide enough power for the ADS7846, but might not be able to supply enough current for the external load (such as a resistive touch screen). TOUCH SCREEN SETTLING In some applications, external capacitors may be required across the touch screen for filtering noise picked up by the touch screen (for example, noise generated by the LCD panel or backlight circuitry). These capacitors provide a low-pass filter to reduce the noise, but cause a settling time requirement when the panel is touched that typically shows up as a gain error. The problem is that the input and/or reference has not settled to the final steady-state value prior to the ADC sam- pling the input(s) and providing the digital output. Additionally, the reference voltage may still be changing during the mea- surement cycle. There are several methods for minimizing or eliminating this issue. Option 1 is to stop or slow down the ADS7846 DCLK for the required touch screen settling time. This allows the input and reference to have stable values for the Acquire period (3 clock cycles of the ADS7846; see Figure 9). This works for both the single-ended and the differential modes. Option 2 is to operate the ADS7846 in the differential mode only for the touch screen measurements and command the ADS7846 to remain on (touch screen drivers on) and not go into power-down (PD0 = 1). Several conversions are made depending on the settling time required and the ADS7846 data rate. Once the required number of conversions have been made, the processor commands the ADS7846 to go into the power-down state on the last measurement. This process is Converter +IN +REF Y+ +V CC V REF X+ Y– GND –REF –IN |
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Similar Description - ADS7846N2K5 |
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