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AD5161BRM5 Datasheet(PDF) 15 Page  Analog Devices 

AD5161BRM5 Datasheet(HTML) 15 Page  Analog Devices 
15 / 20 page Data Sheet AD5161 Rev. B  Page 15 of 20 THEORY OF OPERATION The AD5161 is a 256position digitally controlled variable resistor (VR)1 device. An internal poweron preset places the wiper at midscale during poweron, which simplifies the fault condition recovery at powerup. PROGRAMMING THE VARIABLE RESISTOR Rheostat Operation The nominal resistance of the RDAC between terminals A and B is available in 5 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ. The final two or three digits of the part number determine the nominal resistance value, e.g., 10 kΩ = 10; 50 kΩ = 50. The nominal resistance (RAB) of the VR has 256 contact points accessed by the wiper terminal, plus the B terminal contact. The 8bit data in the RDAC latch is decoded to select one of the 256 possible settings. Assume a 10 kΩ part is used, the wiper’s first connection starts at the B terminal for data 0x00. Since there is a 60 Ω wiper contact resistance, such connection yields a minimum of 60 Ω resistance between Terminals W and B. The second connection is the first tap point, which corresponds to 99 Ω (RWB = RAB/256 + RW = 39 Ω + 60 Ω) for data 0x01. The third connection is the next tap point, representing 177 Ω (2 × 39 Ω + 60 Ω) for data 0x02 and so on. Each LSB data value increase moves the wiper up the resistor ladder until the last tap point is reached at 9961 Ω (RAB – 1 LSB + RW). Figure 42 shows a simplified diagram of the equivalent RDAC circuit where the last resistor string will not be accessed; therefore, there is 1 LSB less of the nominal resistance at full scale in addition to the wiper resistance. B RDAC LATCH AND DECODER W A RS RS RS RS SD BIT D7 D6 D4 D5 D2 D3 D1 D0 Figure 42. AD5161 Equivalent RDAC Circuit 1 The terms digital potentiometer, VR, and RDAC are used interchangeably. The general equation determining the digitally programmed output resistance between W and B is W AB WB R R D D R 256 ) ( (1) where D is the decimal equivalent of the binary code loaded in the 8bit RDAC register, RAB is the endtoend resistance, and RW is the wiper resistance contributed by the on resistance of the internal switch. In summary, if RAB = 10 kΩ and the A terminal is open circuited, the following output resistance RWB will be set for the indicated RDAC latch codes. Table 9. Codes and Corresponding RWB Resistance D (Dec.) RWB (Ω) Output State 255 9,961 Full Scale (RAB – 1 LSB + RW) 128 5,060 Midscale 1 99 1 LSB 0 60 Zero Scale (Wiper Contact Resistance) Note that in the zeroscale condition a finite wiper resistance of 60 Ω is present. Care should be taken to limit the current flow between W and B in this state to a maximum pulse current of no more than 20 mA. Otherwise, degradation or possible destruction of the internal switch contact can occur. Similar to the mechanical potentiometer, the resistance of the RDAC between the wiper W and terminal A also produces a digitally controlled complementary resistance RWA. When these terminals are used, the B terminal can be opened. Setting the resistance value for RWA starts at a maximum value of resistance and decreases as the data loaded in the latch increases in value. The general equation for this operation is W AB WA R R D D R 256 256 ) ( (2) For RAB = 10 kΩ and the B terminal open circuited, the following output resistance RWA will be set for the indicated RDAC latch codes. Table 10. Codes and Corresponding RWA Resistance D (Dec.) RWA (Ω) Output State 255 99 Full Scale 128 5,060 Midscale 1 9,961 1 LSB 0 10,060 Zero Scale Typical device to device matching is process lot dependent and may vary by up to ±30%. Since the resistance element is processed in thin film technology, the change in RAB with temperature has a very low 45 ppm/°C temperature coefficient. 
