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AD7376 Fiches technique(PDF) 8 Page - Analog Devices |
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AD7376 Fiches technique(HTML) 8 Page - Analog Devices |
8 / 12 page AD7376 –8– REV. 0 0.1V CODE = OOH ISW VSS TO VDD W B DUT 0.1V ISW RSW = Figure 38. Incremental ON Resistance Test Circuit ICM VCM W B DUT VDD VSS NC A NC GND Figure 39. Common-Mode Leakage Current Test Circuit OPERATION The AD7376 provides a 128-position digitally-controlled vari- able resistor (VR) device. Changing the programmed VR set- tings is accomplished by clocking in a 7-bit serial data word into the SDI (Serial Data Input) pin, while CS is active low. When CS returns high the last seven bits are transferred into the RDAC latch setting the new wiper position. The exact timing require- ments are shown in Figure 1. The AD7376 resets to a midscale by asserting the RS pin, sim- plifying initial conditions at power-up. Both parts have a power shutdown SHDN pin which places the RDAC in a zero power consumption state where terminal A is open circuited and the wiper W is connected to 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 opera- tional mode from power shutdown, the VR settings return to their previous resistance values. D6 D5 D4 D3 D2 D1 D0 RDAC LATCH & DECODER RS RS RS RS SHDN A W B RS = RNOMINAL/128 Figure 40. AD7376 Equivalent RDAC Circuit PROGRAMMING THE VARIABLE RESISTOR Rheostat Operation The nominal resistance of the RDAC between terminals A and B are available with values of 10 k Ω, 50 kΩ, 100 kΩ and 1 MΩ. 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; 1 M Ω = 1M. The nominal resistance (R AB) of the VR has 128 contact points accessed by the wiper terminal, plus the B terminal contact. The 7-bit data word in the RDAC latch is decoded to select one of the 128 possible settings. The wiper’s first connection starts at the B terminal for data 00H. This B–termi- nal connection has a wiper contact resistance of 120 Ω. The second connection (10 k Ω part) is the first tap point located at 198 Ω (= R BA [nominal resistance]/128 + RW = 78 Ω + 120 Ω) for data 01H. The third connection is the next tap point repre- senting 156 + 120 = 276 Ω for data 02 H. Each LSB data value increase moves the wiper up the resistor ladder until the last tap point is reached at 10041 Ω. The wiper does not directly con- nect to the B terminal. See Figure 40 for a simplified diagram of the equivalent RDAC circuit. The general transfer equation that determines the digitally pro- grammed output resistance between W and B is: RWB(D) = (D)/128 × R BA + RW (1) where D is the data contained in the 7-bit VR latch, and RBA is the nominal end-to-end resistance. For example, when VB = 0 V and A–terminal is open circuit, the following output resistance values will be set for the following VR latch codes (applies to the 10 k Ω potentiometer). Table I. DRWB (DEC) ( ) Output State 127 10041 Full-Scale 64 5120 Midscale ( RS = 0 Condition) 1 276 1 LSB 0 198 Zero-Scale (Wiper Contact Resistance) Note that in the zero-scale condition a finite wiper resistance of 120 Ω is present. Care should be taken to limit the current flow between W and B in this state to a maximum value of 5 mA to avoid degradation or possible destruction of the internal switch contact. Like the mechanical potentiometer the RDAC replaces, it is totally symmetrical. 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. Setting the resistance value for RWA starts at a maxi- mum value of resistance and decreases as the data loaded in the latch is increased in value. The general transfer equation for this operation is: RWA(D) = (128-D)/128 × R BA + RW (2) where D is the data contained in the 7-bit RDAC latch, and RBA is the nominal end-to-end resistance. For example, when VA = 0 V and B–terminal is tied to the wiper W the following output resistance values will be set for the following RDAC latch codes. |
Numéro de pièce similaire - AD7376 |
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Description similaire - AD7376 |
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