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AD5235BRUZ250 Datasheet(PDF) 22 Page - Analog Devices |
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AD5235BRUZ250 Datasheet(HTML) 22 Page - Analog Devices |
22 / 32 page AD5235 Data Sheet Rev. F | Page 22 of 32 PROGRAMMING THE VARIABLE RESISTOR Rheostat Operation The nominal resistance of the RDAC between Terminal A and Terminal B, RAB, is available with 25 kΩ and 250 kΩ with 1024 positions (10-bit resolution). The final digits of the part number determine the nominal resistance value, for example, 25 kΩ = 24.4 Ω; 250 kΩ = 244 Ω. The 10-bit data-word in the RDAC latch is decoded to select one of the 1024 possible settings. The following description provides the calculation of resistance, RWB, at different codes of a 25 kΩ part. The first connection of the wiper starts at Terminal B for Data 0x000. RWB(0) is 30 Ω because of the wiper resistance, and it is independent of the nominal resistance. The second connection is the first tap point where RWB(1) becomes 24.4 Ω + 30 Ω = 54.4 Ω for Data 0x001. The third connection is the next tap point representing RWB(2) = 48.8 Ω + 30 Ω = 78.8 Ω for Data 0x002, and so on. Each LSB data value increase moves the wiper up the resistor ladder until the last tap point is reached at RWB(1023) = 25006 Ω. See Figure 45 for a simplified diagram of the equivalent RDAC circuit. When RWB is used, Terminal A can be left floating or tied to the wiper. CODE (Decimal) 100 75 0 0 1023 256 512 768 50 25 RWA RWB Figure 46. RWA(D) and RWB(D) vs. Decimal Code The general equation that determines the programmed output resistance between Terminal Bx and Terminal Wx is W AB WB R R D D R + × = 1024 ) ( (1) where: D is the decimal equivalent of the data contained in the RDAC register. RAB is the nominal resistance between Terminal A and Terminal B. RW is the wiper resistance. For example, the output resistance values in Table 12 are set for the given RDAC latch codes (applies to RAB = 25 kΩ digital potentiometers). Table 12. RWB (D) at Selected Codes for RAB = 25 kΩ D (Dec) RWB(D) (Ω) Output State 1023 25,006 Full scale 512 12,530 Midscale 1 54.4 1 LSB 0 30 Zero scale (wiper contact resistor) 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 20 mA to avoid degradation or possible destruction of the internal switches. Like the mechanical potentiometer that the RDAC replaces, the AD5235 part is symmetrical. The resistance between Wiper W and Terminal A also produces a digitally controlled complementary resistance, RWA. Figure 46 shows the symmetrical programmability of the various terminal connections. When RWA is used, Terminal B can be left 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 transfer equation for this operation is W AB WA R R D D R + × − = 1024 1024 ) ( (2) For example, the output resistance values in Table 13 are set for the given RDAC latch codes (applies to RAB = 25 kΩ digital potentiometers). Table 13. RWA(D) at Selected Codes for RAB= 25 kΩ D (Dec) RWA(D) (Ω) Output State 1023 54.4 Full scale 512 12,530 Midscale 1 25,006 1 LSB 0 25,030 Zero scale (wiper contact resistance) The typical distribution of RAB from channel to channel is ±0.2% within the same package. Device-to-device matching is process lot dependent upon the worst case of ±30% variation. However, the change in RAB with temperature has a 35 ppm/°C temperature coefficient. PROGRAMMING THE POTENTIOMETER DIVIDER Voltage Output Operation The digital potentiometer can be configured to generate an output voltage at the wiper terminal that is proportional to the input voltages applied to Terminal A and Terminal B. For example, connecting Terminal A to 5 V and Terminal B to ground produces an output voltage at the wiper that can be any value from 0 V to 5 V. Each LSB of voltage is equal to the voltage applied across Terminal A to Terminal B divided by the 2N position resolution of the potentiometer divider. |
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