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AD5114 Datasheet(PDF) 20 Page  Analog Devices 

AD5114 Datasheet(HTML) 20 Page  Analog Devices 
20 / 24 page AD5111/AD5113/AD5115 Data Sheet Rev. 0  Page 20 of 24 Similar to the mechanical potentiometer, the resistance of the RDAC between the W terminal and the A terminal also produces a digitally controlled complementary resistance, RWA. RWA starts at the maximum resistance value and decreases as the data loaded into the latch increases. The general equations for this operation are AD5111: W AB AW R R R + = Bottom scale (7) W AB AW R R D D R + × − = 128 128 ) ( From 0 to 127 (8) TS AW R R = Top scale (9) AD5113: W AB AW R R R + = Bottom scale (10) W AB AW R R D D R + × − = 64 64 ) ( From 0 to 63 (11) TS AW R R = Top scale (12) AD5115: W AB AW R R R + = Bottom scale (13) W AB AW R R D D R + × − = 32 32 ) ( From 0 to 31 (14) TS AW R R = Top scale (15) where: D is the decimal equivalent of the binary code in the 5/6/7bit RDAC register; 128, 64, and 32 refer to top scale step. RAB is the endtoend resistance. RW is the wiper resistance. RTS is the wiper resistance at top scale. Regardless of which setting the part is operating in, take care to limit the current between A to B, W to A, and W to B, to the maximum continuous current of ±6 mA (5 kΩ and 10 kΩ) or ±1.5 mA (80 kΩ), or pulse current specified in Table 6. Otherwise, degradation or possible destruction of the internal switch contact can occur. PROGRAMMING THE POTENTIOMETER DIVIDER Voltage Output Operation The digital potentiometer easily generates a voltage divider at WtoB and WtoA that is proportional to the input voltage at AtoB, as shown in Figure 44. Unlike the polarity of VDD to GND, which must be positive, current across AtoB, WtoA, and WtoB can be in either direction. A VI W B VO Figure 44. Potentiometer Mode Configuration If ignoring the effect of the wiper resistance for simplicity, connecting Terminal A to 5 V and Terminal B to ground produces an output voltage at W to B ranging from 0 V to 5 V. The general equation defining the output voltage at VW with respect to ground for any valid input voltage applied to Terminal A and Terminal B, is B AB AW A AB WB W V R D R V R D R D V × + × = ) ( ) ( ) ( (16) where: RWB(D) can be obtained from Equation 1 to Equation 6. RAW(D) can be obtained from Equation 7 to Equation 14. Operation of the digital potentiometer in the divider mode results in a more accurate operation over temperature. Unlike the rheostat mode, the output voltage is dependent mainly on the ratio of the internal resistors, RWA and RWB, and not the absolute values. Therefore, the temperature drift reduces to 5 ppm/°C. TERMINAL VOLTAGE OPERATING RANGE The AD5111/AD5113/AD5115 are designed with internal ESD diodes for protection. These diodes also set the voltage boundary of the terminal operating voltages. Positive signals present on the A, B, or W terminals that exceed VDD are clamped by the forwardbiased diode. There is no polarity constraint between VA, VW, and VB, but they cannot be higher than VDD or lower than GND. 
