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AD5280 Folha de dados(PDF) 6 Page - Analog Devices |
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AD5280 Folha de dados(HTML) 6 Page - Analog Devices |
6 / 10 page PRELIMINARY TECHNICAL DATA AD5280/AD5282 6 REV PrE 12 MAR 02 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 408 382-3107; FAX 408 382-2721; email; walt.heinzer@analog.com SCL SDA 1 9 1 0 1 1 0 AD0 AD1 R/ W ACK. BY AD5280 19 D7 D6 D5 D3 D4 D0 D1 D2 NO ACK. BY MASTER FRAME 1 Slave Address Byte START BY MASTER FRAME 2 Data From Select ed RDAC Regis ter STO P BY MASTER Figure 3. Reading Data from a Previously Selected RDAC Register OPERATION The AD5280/AD5282 provides a single/dual channel, 256- position digitally-controlled variable resistor (VR) device. The terms VR and RDAC are used interchangeably throughout this documentation. To program the VR settings, refer to the Digital Interface section. Both parts have an internal power ON preset that places the wiper in mid scale during power on, which simplifies the fault condition recovery at power up. In addition, the shutdown SHDN pin of AD5280/AD5282 places the RDAC in a zero power consumption state where terminal A is open circuited and the wiper W is connected to terminal B, resulting in only leakage currents being consumed in the VR structure. In shutdown mode the VR latch settings are maintained, so that, returning to operational mode from power shutdown, the VR settings return to their previous resistance values. RS D5 D4 D3 D2 D1 D0 RDAC LATCH & DECODER SHDN Ax Bx Wx RS RS RS D6 D7 Figure 4. AD5280/AD5282 Equivalent RDAC Circuit PROGRAMMING THE VARIABLE RESISTOR Rheostat Operation The nominal resistance of the RDAC between terminals A and B are available in 20K Ω, 50KΩ, and 200KΩ. The final three digits of the part number determine the nominal resistance value, e.g. 20K Ω = 20; 50KΩ = 50; 200KΩ = 200. The nominal resistance (RAB) of the VR has 256 contact points accessed by the wiper terminal, plus the B terminal contact. The eight bit data in the RDAC latch is decoded to select one of the 256 possible settings. Assume a 20K 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 connection yields a minimum of 60 Ω resistance between terminals W and B. The second connection is the first tap point corresponds to 138 Ω (RWB = RAB/256 + RW = 78Ω+60Ω) for data 01H. The third connection is the next tap point representing 216 Ω (78x2+60) for data 02H and so on. Each LSB data value increase moves the wiper up the resistor ladder until the last tap point is reached at 19982 Ω [R AB –1LSB+RW]. The wiper does not directly connect to the B terminal. See Figure 4 for a simplified diagram of the equivalent RDAC circuit. The general equation determining the digitally programmed output resistance between W and B is: 1 eqn. 256 ) ( W AB WB R R D D R + ⋅ = where D is the decimal equivalent of the binary code which is loaded in the 8-bit RDAC register, and RAB is the nominal end- to-end resistance. For example, RAB=20K Ω, when V B = 0V and A–terminal is open circuit, the following output resistance values RWB will be set for the following RDAC latch codes. Result will be the same if terminal A is tied to W: D RWB Output State (DEC) ( Ω) 256 19982 Ω Full-Scale (R AB - 1LSB + RW) 128 10060 Ω Mid-Scale 1 138 Ω 1 LSB 0 60 Ω Zero-Scale (Wiper contact resistance) Note that in the zero-scale 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 current of no more than 5mA. 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 resistance RWA. When these terminals are used the B–terminal should be let open or tied to the wiper terminal. 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: 2 eqn. 256 256 ) ( W AB WA R R D D R + ⋅ − = |
Nº de peça semelhante - AD5280 |
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Descrição semelhante - AD5280 |
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