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ADS7846N Datasheet(PDF) 9 Page - Texas Instruments |
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ADS7846N Datasheet(HTML) 9 Page - Texas Instruments |
9 / 30 page ADS7846 9 SBAS125H www.ti.com INTERNAL REFERENCE The ADS7846 has an internal 2.5V voltage reference that can be turned on or off with the control bit, PD1 = 1 (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 ADS7846 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 Characteristics for power-up time of the reference from power-down). REFERENCE INPUT The voltage difference between +REF and –REF (shown in Figure 2) sets the analog input range. The ADS7846 oper- ates 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 FIGURE 2. Simplified Diagram of Analog Input. Converter –REF +REF +IN –IN V BAT AUX Battery On GND A2-A0 (Shown 001 B) 2.5V Reference Ref On/Off SER/DFR (Shown High) X+ X– +V CC TEMP1 PENIRQ Y+ Y– V REF TEMP0 7.5k Ω 2.5k Ω Buffer Band Gap Reference Power Down To CDAC Optional V REF FIGURE 3. Simplified Diagram of the Internal Reference. |
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