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## ADN2850BRUZ25 Datasheet(HTML) 19 Page - Analog Devices

 19 / 28 page Data SheetADN2850Rev. E | Page 19 of 28PROGRAMMING THE VARIABLE RESISTORThe nominal resistance of the RDAC between Terminal W andTerminal B, RWB, is available with 25 kΩ and 250 kΩ with1024 positions (10-bit resolution). The final digits of the partnumber 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 ofthe 1024 possible settings. The following description provides thecalculation of resistance, RWB, at different codes of a 25 kΩ part.The first connection of the wiper starts at Terminal B forData 0x000. RWB(0) is 30 Ω because of the wiper resistance, andit is independent of the nominal resistance. The second connectionis the first tap point where RWB(1) becomes 24.4 Ω + 30 Ω = 54.4 Ωfor Data 0x001. The third connection is the next tap pointrepresenting 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 theresistor ladder until the last tap point is reached at RWB(1023) =25006 Ω. See Figure 32 for a simplified diagram of the equivalentRDAC circuit.CODE (Decimal)100750010232565127685025RWBFigure 33. RWB(D) vs. Decimal CodeThe general equation that determines the programmed outputresistance between Terminal Bx and Terminal Wx isWNOMWBWBRRDDR+×=_1024)((1)where:D is the decimal equivalent of the data contained in the RDACregister.RWB_NOM is the nominal resistance valueRW is the wiper resistance.Table 13. RWB (D) at Selected Codes for RWB_NOM = 25 kΩD (Dec)RWB(D) (Ω)Output State102325,006Full scale51212,530Midscale154.41 LSB030Zero scale (wiper contact resistor)Note that, in the zero-scale condition, a finite wiper resistanceof 30 Ω is present. Care should be taken to limit the currentflow between W and B in this state to no more than 20 mA toavoid degradation or possible destruction of the internal switches.The typical distribution of RWB_NOM from channel to channel is±0.2% within the same package. Device-to-device matching isprocess lot dependent upon the worst case of ±30% variation.However, the change in RWBat full scale with temperature has a35 ppm/°C temperature coefficient.PROGRAMMING EXAMPLESThe following programming examples illustrate a typical sequenceof events for various features of the ADN2850. See Table 8 forthe instructions and data-word format. The instruction numbers,addresses, and data appearing at the SDI and SDO pins are inhexadecimal format.Table 14. Scratchpad ProgrammingSDISDOAction0xB001000xXXXXXXWrites Data 0x100 into RDAC1 register,Wiper W1 moves to 1/4 full-scaleposition.0xB102000xB00100Loads Data 0x200 into RDAC2 register,Wiper W2 moves to 1/2 full-scaleposition.Table 15. Incrementing RDAC Followed by Storing theWiper Setting to EEMEMSDISDOAction0xB001000xXXXXXXWrites Data 0x100 into RDAC1register, Wiper W1 moves to 1/4 full-scale position.0xE0XXXX0xB00100Increments RDAC1 register by one to0x101.0xE0XXXX0xE0XXXXIncrements RDAC1 register by one to0x102. Continue until desired wiperposition is reached.0x20XXXX0xXXXXXXStores RDAC2 register data intoEEMEM1. Optionally, tieAAWPEEAAto GND toprotect EEMEM values.The EEMEM values for the RDACs can be restored by power-on, by strobing theAAPREEAApin, or by the two commands shown inTable 16.Table 16. Restoring the EEMEM Values to RDAC RegistersSDISDOAction0x10XXXX0xXXXXXXRestores the EEMEM1 value to theRDAC1 register.