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AD7843ARU-REEL 데이터시트(PDF) 10 Page - Analog Devices |
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AD7843ARU-REEL 데이터시트(HTML) 10 Page - Analog Devices |
10 / 16 page REV. 0 AD7843 –10– Touch Screen Settling In some applications, external capacitors may be required across the touch screen to filter noise associated with it, e.g., noise generated by the LCD panel or backlight circuitry. The value of these capacitors will cause a settling time requirement when the panel is touched. The settling time will typically show up as a gain error. There are several methods for minimizing or elimi- nating this issue. The problem may be that the input signal, or reference, or both, have not settled to their final value before the sampling instant of the ADC. Additionally, the reference voltage may still be changing during the conversion cycle. One option is to stop, or slow down the DCLK for the required touch screen settling time. This will allow the input and reference to stabilize for the acquisition time. This will resolve the issue for both single-ended and differential modes. The other option is to operate the AD7843 in differential mode only for the touch screen, and program the AD7843 to keep the touch screen drivers ON and not go into power-down (PD0 = PD1 = 1). Several conversions may be required depending on the settling time required and the AD7843 data rate. Once the required number of conversions have been made, the AD7843 can then be placed in a power-down state on the last measure- ment. The last method is to use the 15 DCLK cycle mode, which maintains the touch screen drivers ON until it is commanded to stop by the processor. Reference Input The voltage difference between +REF and –REF (see Figure 4) sets the analog input range. The AD7843 will operate with a reference input in the range of 1 V to VCC. The voltage into the VREF input is not buffered and directly drives the capaci- tor DAC portion of the AD7843. Figure 5 shows the reference input circuitry. Typically, the input current is 8 µA with V REF = 2.5 V and fSAMPLE = 125 kHz. This value will vary by a few microamps, depending on the result of the conversion. The reference current diminishes directly with both conversion rate and reference voltage. As the current from the reference is drawn on each bit decision, clocking the converter more quickly during a given conversion period will not reduce the overall current drain from the reference. VREF ADC 3-TO-1 MUX X+ Y+ Figure 5. Reference Input Circuitry When making touch screen measurements, conversions can be made in the differential (ratiometric) mode or the single-ended mode. If the SER/ DFR bit is set to 1 in the control register, a single-ended conversion will be performed. Figure 6 shows the configuration for a single-ended Y coordinate measurement. The X+ input is connected to the analog to digital converter, the Y+ and Y– drivers are turned on and the voltage on X+ is digitized. The conversion is performed with the ADC refer- enced from GND to VREF. The advantage of this mode is that the switches that supply the external touch screen can be turned off once the acquisition is complete, resulting in a power saving. However, the on-resistance of the Y drivers will affect the input voltage that can be acquired. The full touch screen resistance may be in the order of 200 Ω to 900 Ω, depending on the manu- facturer. Thus if the on-resistance of the switches is approximately 6 Ω, true full-scale and zero-scale voltages cannot be acquired regardless of where the pen/stylus is on the touch screen. Note: The minimum touch screen resistance recommended for use with the AD7843 is approximately 70 Ω. X+ Y– Y+ +VCC GND IN– REF– IN+ ADC CORE REF+ VREF IN+ Figure 6. Single-Ended Reference Mode (SER/ DFR = 1) In this mode of operation, therefore, some voltage is likely to be lost across the internal switches and, in addition to this, it is unlikely that the internal switch resistance will track the resis- tance of the touch screen over temperature and supply, providing an additional source of error. The alternative to this situation is to set the SER/ DFR bit low. If one again considers making a Y coordinate measurement, but now the +REF and –REF nodes of the ADC are connected directly to the Y+ and Y– pins, this means the analog to digital conversion will be ratiometric. The result of the conversion will always be a percentage of the external resistance, independent of how it may change with respect to the on-resistance of the internal switches. Figure 7 shows the configuration for a ratiometric Y coordinate measurement. It should be noted that the differential Table I. Analog Input, Reference, and Touch Screen Control A2 1 A1 1 A0 1 SER/ DFR Analog In X Switches Y Switches +REF 2 –REF 2 0 011 X+ OFF ON VREF GND 0 1 0 1 IN3 OFF OFF VREF GND 1 011 Y+ ON OFF VREF GND 1 1 0 1 IN4 OFF OFF VREF GND 0 010 X+ OFF ON Y+ Y– 1 010 Y+ ON OFF X+ X– 1 100 Outputs Identity Code, 1000 0000 0000 NOTES 1All remaining configurations are invalid addresses. 2Internal node – not directly accessible by the user. |
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