Electronic Components Datasheet Search 

AD5112 Datasheet(PDF) 25 Page  Analog Devices 

AD5112 Datasheet(HTML) 25 Page  Analog Devices 
25 / 28 page Data Sheet AD5110/AD5112/AD5114 Rev. 0  Page 25 of 28 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 also gives a maximum of 8% absolute resistance error. RWA starts at the maximum resistance value and decreases as the data loaded into the latch increases. The general equations for this operation are AD5110: W AB AW R R R + = Bottom scale (0xFF) (7) W AB AW R R D D R + × − = 128 128 ) ( From 0x00 to 0x7F (8) TS AW R R = Top scale (0x80) (9) AD5112: W AB AW R R R + = Bottom scale (0xFF) (10) W AB AW R R D D R + × − = 64 64 ) ( From 0x00 to 0x3F (11) TS AW R R = Top scale (0x40) (12) AD5114: W AB AW R R R + = Bottom scale (0xFF) (13) W AB AW R R D D R + × − = 32 32 ) ( From 0x00 to 0x1F (14) TS AW R R = Top scale (0x20) (15) where: D is the decimal equivalent of the binary code in the 5/6/7bit RDAC register. RAB is the endtoend resistance. RW is the wiper resistance. RTS is the wiper resistance at top scale. In the bottomscale condition or topscale condition, a finite total wiper resistance of 45 Ω is present. Regardless of which setting the part is operating in, take care to limit the current between Terminal A to Terminal B, Terminal W to Terminal A, and Terminal W to Terminal B, to the maximum continuous current of ±6 mA or to the pulse current specified in Table 6. Otherwise, degradation or possible destruction of the internal switch contact can occur. Calculating the Actual EndtoEnd Resistance The resistance tolerance is stored in the internal memory during factory testing. The actual endtoend resistance can, therefore, be calculated, which is valuable for calibration, tolerance matching, and precision applications. The resistance tolerance in percentage is stored in fixedpoint format, using an 8bit sign magnitude binary. The data can be read back by executing Command 6 and setting Bit DB0 (A0). The MSB is the sign bit (0 = − and 1 = +) and the next four bits are the integer part, the fractional part is represented by the three LSBs, as shown in Table 11. Table 11. Tolerance Format Data Byte DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0 Sign 24 23 22 21 . 21 22 23 For example, if RAB = 10 kΩ and the data readback shows 01010010, the endtoend resistance can be calculated as, if, DB[7] is 0 = negative DB[6:3] is 1010 = 10 DB[2:0] is 010 = 2 × 2−3 = 0.25 then, tolerance = −10.25% and, therefore, RAB = 8.975 kΩ PROGRAMMING THE POTENTIOMETER DIVIDER Voltage Output Operation The digital potentiometer easily generates a voltage divider at wipertoB and wipertoA that is proportional to the input voltage at A to B, as shown in Figure 48. Unlike the polarity of VDD to GND, which must be positive, voltage across AtoB, W toA, and WtoB can be at either polarity. A VI W B VO Figure 48. Potentiometer Mode Configuration Connecting Terminal A to 5 V and Terminal B to ground produces an output voltage at the Wiper W to Terminal 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 15. 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, RAW and RWB, and not the absolute values. Therefore, the temperature drift reduces to 5 ppm/°C. 
