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AD5241BRU10-REEL7 Scheda tecnica(PDF) 9 Page - Analog Devices |
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AD5241BRU10-REEL7 Scheda tecnica(HTML) 9 Page - Analog Devices |
9 / 16 page REV. B –9– AD5241/AD5242 OPERATION The AD5241/AD5242 provide a single-/dual-channel, 256- position digitally controlled variable resistor (VR) device. The terms VR, RDAC, and programmable resistor are commonly used interchangeably to refer to digital potentiometer. To program the VR settings, refer to the Digital Interface sec- tion. Both parts have an internal power ON preset that places the wiper in midscale during power-on, which simplifies the fault condition recovery at power-up. In addition, the shutdown SHDN Pin of AD5241/AD5242 places the RDAC in an almost zero power consumption state where Terminal A is open circuited and Wiper W is connected to Terminal B, resulting in only leakage current being consumed in the VR structure. During shutdown, the VR latch contents are maintained when the RDAC is inactive. When the part is returned from shutdown, the stored VR setting will be applied to the RDAC. SWSHDN SW N 2–1 R R SW N 2–2 RDAC LATCH AND DECODER R RAB/2 N B W DIGITAL CIRCUITRY OMITTED FOR CLARITY A SW1 SW0 R R D7 D6 D5 D4 D3 D2 D1 D0 SHDN Figure 4. Equivalent RDAC Circuit PROGRAMMING THE VARIABLE RESISTOR Rheostat Operation The nominal resistance of the RDAC between Terminals A and B is available in 10 k Ω, 100 kΩ, and 1 MΩ. The final two or three digits of the part number determine the nominal resistance value, e.g., 10 k Ω = 10; 100 kΩ = 100; 1 MΩ = 1 M. The nominal resistance (RAB) of the VR has 256 contact points accessed by the Wiper Terminal, plus the B Terminal con- tact. The 8-bit data in the RDAC latch is decoded to select one of the 256 possible settings. Assume a 10 k Ω part is used; the wiper’s first connection starts at the B Terminal for data 00H. Since there is a 60 Ω wiper contact resistance, such con- nection yields a minimum of 60 Ω resistance between Terminals W and B. The second connection is the first tap point that cor- responds to 99 Ω (RWB = RAB/256 + RW = 39 + 60) for data 01H. The third connection is the next tap point representing 138 Ω (39 × 2 + 60) for data 02 H, and so on. Each LSB data value increase moves the wiper up the resistor ladder until the last tap point is reached at 10021 Ω [R AB – 1 LSB + RW]. Figure 4 shows a simplified diagram of the equivalent RDAC circuit where the last resistor string will not be accessed; there- fore, there is 1 LSB less of the nominal resistance at full scale in addition to the wiper resistance. The general equation determining the digitally programmed resistance between W and B is: RD D RR WB AB W () =× + 256 (1) where: D is the decimal equivalent of the binary code between 0 and 255, which is loaded in the 8-bit RDAC register. RAB is the nominal end-to-end resistance. RW is the wiper resistance contributed by the on resistance of the internal switch. Again, if RAB = 10 k Ω and the A Terminal can be either open circuit or tied to W, the following output resistance at RWB will be set for the following RDAC latch codes. FREQUENCY – Hz 6 –36 –42 –48 –54 100k 10k 1k 100 –30 –24 –18 –12 –6 0 FFH 80H 40H 20H 10H 08H 04H 02H 01H TPC 13. AD5242 100 k Ω Gain vs. Frequency vs. Code FREQUENCY – Hz 6 –36 –42 –48 –54 100k 10k 1k 100 –30 –24 –18 –12 –6 0 FFH 80H 40H 20H 10H 08H 04H 02H 01H TPC 14. AD5242 1 M Ω Gain vs. Frequency vs. Code |
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