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ADN2850BRUZ25 Datasheet(PDF) 17 Page - Analog Devices |
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ADN2850BRUZ25 Datasheet(HTML) 17 Page - Analog Devices |
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17 / 30 page 
Data Sheet ADN2850 Rev. F | Page 17 of 30 DAISY-CHAIN OPERATION The serial data output pin (SDO) serves two purposes. It can be used to read the contents of the wiper setting and EEMEM values using Instruction 10 and Instruction 9, respectively. The remaining instructions (Instruction 0 to Instruction 8, Instruction 11 to Instruction 15) are valid for daisy-chaining multiple devices in simultaneous operations. Daisy-chaining minimizes the number of port pins required from the controlling IC (see Figure 28). The SDO pin contains an open-drain N-Ch FET that requires a pull-up resistor, if this function is used. As shown in Figure 28, users need to tie the SDO pin of one package to the SDI pin of the next package. Users may need to increase the clock period because the pull-up resistor and the capacitive loading at the SDO-to-SDI interface may require additional time delay between subsequent devices. When two ADN2850 devices are daisy-chained, 48 bits of data are required. The first 24 bits (formatted 4-bit command, 4-bit address, and 16-bit data) go to U2, and the second 24 bits with the same format go to U1. Keep CS low until all 48 bits are clocked into their respective serial registers. CS is then pulled high to complete the operation. CLK RP 2.2k Ω SDI SDO U2 CS CLK SDI SDO U1 ADN2850 CS VDD SCLK SS MOSI MICRO- CONTROLLER ADN2850 Figure 28. Daisy-Chain Configuration Using SDO TERMINAL VOLTAGE OPERATING RANGE The positive VDD and negative VSS power supplies of the ADN2850 define the boundary conditions for proper 2-terminal digital resistor operation. Supply signals present on Terminal B, and Terminal W that exceed VDD or VSS are clamped by the internal forward-biased diodes (see Figure 29). VSS VDD W B Figure 29. Maximum Terminal Voltages Set by VDD and VSS The GND pin of the ADN2850 is primarily used as a digital ground reference. To minimize the digital ground bounce, the ADN2850 ground terminal should be joined remotely to the common ground (see Figure 30). The digital input control signals to the ADN2850 must be referenced to the device ground pin (GND) and must satisfy the logic level defined in the Specifications section. An internal level-shift circuit ensures that the common-mode voltage range of the three terminals extends from VSS to VDD, regardless of the digital input level. Power-Up Sequence Because there are diodes to limit the voltage compliance at Terminal B, and Terminal W (see Figure 29), it is important to power VDD and VSS first before applying any voltage to Terminal B, and Terminal W. Otherwise, the diode is forward-biased such that VDD and VSS are powered unintentionally. For example, applying 5 V across Terminal W and Terminal B prior to VDD causes the VDD terminal to exhibit 4.3 V. It is not destructive to the device, but it might affect the rest of the user’s system. The ideal power-up sequence is GND, VDD and VSS, digital inputs, and VB, and VW. The order of powering VB, VW, and the digital inputs is not important as long as they are powered after VDD and VSS. Regardless of the power-up sequence and the ramp rates of the power supplies, when VDD and VSS are powered, the power-on preset activates, which restores the EEMEM values to the RDAC registers. Layout and Power Supply Bypassing It is a good practice to employ compact, minimum lead-length layout design. The leads to the input should be as direct as possible with a minimum conductor length. Ground paths should have low resistance and low inductance. Similarly, it is good practice to bypass the power supplies with quality capacitors for optimum stability. Bypass supply leads to the device with 0.01 μF to 0.1 μF disk or chip ceramic capacitors. Also, apply low ESR, 1 μF to 10 μF tantalum or electrolytic capacitors at the supplies to minimize any transient disturbance (see Figure 30). ADN2850 VDD GND VSS C3 10µF C4 10µF C2 0.1µF C1 0.1µF + + VDD VSS Figure 30. Power Supply Bypassing |
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