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AD5235BRU250 Datasheet(PDF) 7 Page - Analog Devices |
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AD5235BRU250 Datasheet(HTML) 7 Page - Analog Devices |
7 / 10 page PRELIMINARY TECHNICAL DATA Nonvolatile Memory Digital Potentiometers AD5235 REV PrD 6 NOV, 2000 7 Information contained in this Product Concept data sheet describes a product in the early definition stage. There is no guarantee that the information contained here will become a final product in its present form. For latest information contact Walt Heinzer/Analog Devices, Santa Clara, CA. TEL(408)562-7254; FAX (408)727-1550; walt.heinzer@analog.com Detail Potentiometer Operation The actual structure of the RDAC is designed to emulate the performance of a mechanical potentiometer. The RDAC contains a string of connected resistor segments, with an array of analog switches that act as the wiper connection to several points along the resistor array. The number of points is the resolution of the device. The AD5235 has 1024 connection points allowing it to provide better than 0.5% set-ability resolution. Figure 3 provides an equivalent diagram of the connections between the three terminals that make up one channel of the RDAC. The SWA and SWB will always be ON while one of the switches SW(0) to SW(2 N-1) will be ON one at a time depends upon the resistance step decoded from the Data Bits. Note that there are two 50 Ω wiper resistances, R W. The resistance contributed by RW must be accounted for when calculating the output resistance. RW is the sum of the resistances of SWA + SWX and SWB + SWX for A-to-Wiper and B-to-Wiper respectively. A X R S R S R S W X B X RDAC WIPER REGISTER & DECODER R S = R AB /2 N SW A SW B SW(2 N -1) SW(2 N -2) SW(0 ) SW(1 ) DIGITAL CIRCUITRY OMITTED FOR CLARITY Figure 3. Equivalent RDAC structure PROGRAMMING THE VARIABLE RESISTOR Rheostat Operation The nominal resistance of the RDAC between terminals A and B are available with values of 25K Ω, and 250KΩ. The final digits of the part number determine the nominal resistance value, e.g., 25K Ω = 25; 250K Ω = 250. The nominal resistance (R AB) of the AD5235 VR has 1024 contact points accessed by the wiper terminal, plus the B terminal contact. The 10-bit data word in the RDAC latch is decoded to select one of the 1024 possible settings. The wiper's first connection starts at the B terminal for data 00H. This B– terminal connection has a wiper contact resistance, RW of 50 Ω, regardless of what the nominal resistance RAB is. The second connection (25K Ω part) is the first tap point where R WB =74 Ω [R WB =RAB/1024 + RW = 24Ω+50Ω)] for data 01H. The third connection is the next tap point representing RWB =49+50=99 Ω for data 02 H. Each LSB data value increase moves the wiper up the resistor ladder until the last tap point is reached at RWB=25025 Ω. See figure 3 for a simplified diagram of the equivalent RDAC circuit. The general equation, which determines the digitally programmed output resistance between Wx and Bx, is: RWB(Dx) = (Dx/2 N) * R AB + RW eqn. 1 Where N is the resolution of the VR, Dx is the data contained in the RDACx latch, and RAB is the nominal end-to-end resistance. Since N=10 and RW=50Ω for AD5235, eqn. 1 becomes: RWB(Dx) = (Dx/1024) * RAB + 50Ω eqn. 2 For example, when VB = 0V and A–terminal is open circuit the following output resistance values will be set for the following RDAC latch codes (applies to RAB=25KΩ potentiometers): Dx RWB Output State (DEC) ( Ω) 1023 25025 Ω Full-Scale 512 12500 Ω Mid-Scale 1 74 Ω 1 LSB 0 50 Ω Zero-Scale (Wiper contact resistance) Note that in the zero-scale condition a finite wiper resistance of 50 Ω is present. Care should be taken to limit the current flow between W and B in this state to no more than 20mA to avoid degradation or possible destruction of the internal switch contact. Figure 4. Symmetrical RDAC Operation Like the mechanical potentiometer the RDAC replaces, the AD5235 part is totally symmetrical. The resistance between the wiper W and terminal A also produces a digitally controlled resistance RWA. Figure 4 shows the symmetrical programmability of the various terminal connections. When these terminals are used, the B–terminal should be tied to the wiper. Setting the resistance value for RWA starts at a maximum value of resistance and decreases as the data loaded in the latch is increased in value. The general equation for this operation is: |
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