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AD5220WBRZ10-REEL7 Datasheet(PDF) 8 Page - Analog Devices |
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AD5220WBRZ10-REEL7 Datasheet(HTML) 8 Page - Analog Devices |
8 / 11 page AD5220 –8– REV. OPERATION The AD5220 provides a 128-position digitally controlled vari- able resistor (VR) device. Changing the VR settings is accom- plished by pulsing the CLK pin while CS is active low. The direction of the increment is controlled by the U/D (UP/DOWN) control input pin. When the wiper hits the end of the resistor (Terminals A or B) additional CLK pulses no longer change the wiper setting. The wiper position is immediately decoded by the wiper decode logic changing the wiper resistance. Ap- propriate debounce circuitry is required when push button switches are used to control the count sequence and direction of count. The exact timing requirements are shown in Figure 3. The AD5220 powers ON in a centered wiper position exhibit- ing nearly equal resistances of RWA and RWB. UP/ DOWN CNTR RS D E C O D E 7 40H POR EN AD5220 VDD A W B GND CLK CS U/ D Figure 35. Block Diagram DIGITAL INTERFACING OPERATION The AD5220 contains a three-wire serial input interface. The three inputs are clock (CLK), CS and UP/DOWN (U/D). The negative-edge sensitive CLK input requires clean transitions to avoid clocking multiple pulses into the internal UP/DOWN counter register, see Figure 35. Standard logic families work well. If mechanical switches are used for product evaluation they should be debounced by a flip-flop or other suitable means. When CS is taken active low the clock begins to incre- ment or decrement the internal UP/DOWN counter dependent upon the state of the U/D control pin. The UP/DOWN counter value (D) starts at 40H at system power ON. Each new CLK pulse will increment the value of the internal counter by one LSB until the full scale value of 3FH is reached as long as the U/D pin is logic high. If the U/D pin is taken to logic low the counter will count down stopping at code 00H (zero-scale). Additional clock pulses on the CLK pin are ignored when the wiper is at either the 00H position or the 3FH position. All digital inputs (CS, U/D, CLK) are protected with a series input resistor and parallel Zener ESD structure shown in Figure 36. LOGIC 1k Figure 36. Equivalent ESD Protection Digital Pins 20 A, B, W GND Figure 37. Equivalent ESD Protection Analog Pins D0 D1 D2 D3 D4 D5 D6 RDAC UP/DOWN CNTR & DECODE Wx Bx RS = RNOMINAL/128 RS RS RS Ax Figure 38. AD5220 Equivalent RDAC Circuit PROGRAMMING THE VARIABLE RESISTOR Rheostat Operation The nominal resistance of the RDAC between terminals A and B is available with values of 10 k Ω, 50 kΩ, and 100 kΩ. The final three characters of the part number determine the nominal resistance value, e.g., 10 k Ω =10; 50 kΩ = 50; 100 kΩ = 100. The nominal resistance (RAB) of the VR has 128 contact points accessed by the wiper terminal, plus the B terminal contact. At power ON the resistance from the wiper to either end Terminal A or B is approximately equal. Clocking the CLK pin will in- crease the resistance from the Wiper W to Terminal B by one unit of RS resistance (see Figure 38). The resistance RWB is determined by the number of pulses applied to the clock pin. Each segment of the internal resistor string has a nominal resis- tance value of RS = RAB/128, which becomes 78 Ω in the case of the 10 k Ω AD5220BN10 product. Care should be taken to limit the current flow between W and B in the direct contact state to a maximum value of 5 mA to avoid degradation or possible de- struction of the internal switch contact. Like the mechanical potentiometer the RDAC replaces, it is totally symmetrical (see Figure 38). The resistance between the Wiper W and Terminal A also produces a digitally controlled resistance RWA. When these terminals are used the B–terminal should be tied to the wiper. The typical part-to-part distribution of RBA is process lot depen- dent having a ±30% variation. The change in R BA with tempera- ture has a 800 ppm/ °C temperature coefficient. The RBA temperature coefficient increases as the wiper is pro- grammed near the B-terminal due to the larger percentage con- tribution of the wiper contact switch resistance, which has a 0.5%/ °C temperature coefficient. Figure 14 shows the effect of the wiper contact resistance as a function of code setting. An- other performance factor influenced by the switch contact resis- tance is the relative linearity error performance between the 10 k Ω, and the 50 kΩ or 100 kΩ versions. The same switch contact resistance is used in all three versions. Thus the perfor- mance of the 50 k Ω and 100 kΩ devices which have the least impact on wiper switch resistance exhibits the best linearity error, see Figures 7 and 8. A |
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