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AD7228ABQ Scheda tecnica(PDF) 7 Page - Analog Devices |
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AD7228ABQ Scheda tecnica(HTML) 7 Page - Analog Devices |
7 / 8 page AD7228A REV. A –7– DACs for correct timing relationships during the calibration cycle of the instrument. Figure 10b. AD7228A Timing Deskew Circuit COARSE/FINE ADJUST The DACs on the AD7228A can be paired together to form a coarse/fine adjust function as indicated in Figure 11. The func- tion is achieved using one external op amp and a few resistors per pair of DACs. DAC1 is the most significant or coarse DAC. Data is first loaded to this DAC to coarsely set the output voltage. DAC2 is then used to fine tune this output voltage. Varying the ratio of R1 to R2 varies the relative effect of the coarse and fine DACs on the output voltage. For the resistor values shown, DAC2 has a resolution of 150 µV in a 10 V output range. Since each DAC on the AD7228A is guaranteed monotonic, the coarse adjust- ment and fine adjustment are each monotonic. One application for this is as a set-point controller (see “Circuit Applications of the AD7226 Quad CMOS DAC” available from Analog Devices, Publication Number E873–15–11/84). Figure 11. Coarse/Fine Adjust Circuit SELF-PROGRAMMABLE REFERENCE The circuit of Figure 12 shows how one DAC of the AD7228, in this case DAC1, may be used in a feedback configuration to provide a programmable reference for itself and the other seven converters. The relationship of VREF to VIN is expressed by V REF = 1 + G () 1 + G • D 1 () •V IN where G = R2/R1 Figure 13 shows typical plots of VREF versus digital code, D1, for three different values of G. With VIN = 2.5 V and G = 3 the voltage at the output varies between 2.5 V and 10 V giving an effective 10-bit dynamic range to the other seven converters. For correct operation of the circuit, VSS should be –5 V and R1 greater than 6.8 k Ω. Figure 12. Self-Programmable Reference Figure 13. Variation of VREF with Feedback Configuration MICROPROCESSOR INTERFACING Figure 14. AD7228A to 8085A/Z80 Interface Figure 15. AD7228A to 6809/6502 Interface |
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