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AD5262BRU20 Folha de dados(PDF) 9 Page - Analog Devices |
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AD5262BRU20 Folha de dados(HTML) 9 Page - Analog Devices |
9 / 20 page REV. 0 –9– AD5260/AD5262 D7 D6 D5 D4 D3 D2 D1 D0 RDAC LATCH AND DECODE Ax Wx Bx RS = RAB/2N RS RS RS RS SHDN DIGITAL CIRCUITRY OMITTED FOR CLARITY Figure 11. Simplified RDAC Architecture PROGRAMMING THE VARIABLE RESISTOR Rheostat Operation The nominal resistances of the RDAC between terminals A and B are available with values of 20 k W, 50 kW, and 200 kW. The final three digits of the part number determine the nominal resistance value, e.g., 20 k W = 20; 50 kW = 50; 200 kW = 200. The nominal resistance (RAB) of the VR has 256 contact points accessed by the wiper terminal, plus the B terminal contact. The 8-bit data in the RDAC latch is decoded to select one of the 256 possible settings. Assuming a 20 k W part is used, the wiper’s first connection starts at the B terminal for data 00H. Since there is a 60 W wiper contact resistance, such connection yields a minimum of 60 W resistance between terminals W and B. The second connection is the first tap point corresponds to 138 W (RWB = RAB/256 RW = 78 W 60 W) for data 01H. The third connection is the next tap point represent- ing 216 W (78 2 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 W [RAB 1 LSB RW]. The wiper does not directly connect to the B terminal. See Figure 11 for a simplified diagram of the equivalent RDAC circuit. The general equation determining the digitally programmed output resistance between W and B is: RD D RR WB AB W () =¥ + 256 (1) 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 = 20 k W, when VB = 0 V and A–terminal is open circuit, the following output resistance values RWB will be set for the following RDAC latch codes. The result will be the same if terminal A is tied to W: DRWB (DEC) ( W)Output State 256 19982 Full-Scale (RAB – 1 LSB + RW) 128 10060 Mid-Scale 1 138 1 LSB 060 Zero-Scale (wiper contact resistance) Note that in the zero-scale condition a finite wiper resistance of 60 W is present. Care should be taken to limit the current flow between W and B in this state to no more than 20 mA to avoid degradation or possible destruction of the internal switches. Like the mechanical potentiometer the RDAC replaces, the AD5260/AD5262 parts are totally symmetrical. The resistance between the wiper W and terminal A also produces a digitally controlled complementary resistance RWA. Figure 12 shows the symmetrical programmability of the various terminal connections. When RWA is used, the B–terminal can be let floating or 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: RD D RR WA AB W () = - ¥+ 256 256 (2) For example, RAB = 20 k W, when VA = 0 V and B–terminal is open, the following output resistance RWA will be set for the following RDAC latch codes. The result will be the same if terminal B is tied to W: DRWA (DEC) ( W)Output State 256 60 Full-Scale 128 10060 Mid-Scale 1 19982 1 LSB 0 20060 Zero-Scale RWB RWA RAB = 20K D – CODE in decimal 20 064 128 192 256 16 12 8 4 0 Figure 12. AD5260/AD5262 Equivalent RDAC Circuit The typical distribution of the nominal resistance RAB from channel to channel matches within ±1%. Device-to-device match- ing is process lot dependent with the worst case of ±30% variation. On the other hand, since the resistance element is processed in thin film technology, the change in RAB with temperature has a low 35 ppm/ ∞C temperature coefficient. |
Nº de peça semelhante - AD5262BRU20 |
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Descrição semelhante - AD5262BRU20 |
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