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AD7545A Datasheet(PDF) 7 Page - Analog Devices |
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AD7545A Datasheet(HTML) 7 Page - Analog Devices |
7 / 8 page AD7545A –7– REV. C The circuits of Figures 4, 5 and 6 can all be converted to single supply operation by biasing AGND to some voltage between VDD and DGND. Figure 10 shows the 2s Complement Bipolar circuit of Figure 5 modified to give a range from +2 V to +8 V about a “pseudo-analog ground” of 5 V. This voltage range would allow operation from a single VDD of +10 V to +15 V. The AD584 pin-programmable reference fixes AGND at +5 V. VIN is set at +2 V by means of the series resistors R1 and R2. There is no need to buffer the VREF input to the AD7545A with an amplifier because the input impedance of the D/A converter is constant. Note, however, that since the temperature coefficient of the D/A reference input resistance is typically –300 ppm/ °C, applications which experience wide temperature variations may require a buffer amplifier to generate the +2.0 V at the AD7545A VREF pin. Other output voltage ranges can be obtained by changing R4 to shift the zero point and (R1 + R2) to change the slope, or gain of the D/A transfer function. VDD must be kept at least 5 V above OUT1 to ensure that linearity is preserved. Figure 10. Single Supply "Bipolar" 2s Complement D/A Converter MICROPROCESSOR INTERFACING OF THE AD7545A The AD7545A can interface directly to both 8- and 16-bit microprocessors via its 12-bit wide data latch using standard CS and WR control signals. A typical interface circuit for an 8-bit processor is shown in Figure 11. This arrangement uses two memory addresses, one for the lower 8 bits of data to the DAC and one for the upper 4 bits of data into the DAC via the latch. Figure 11. 8-Bit Processor to AD7545 Interface Figure 12 shows an alternative approach for use with 8-bit pro- cessors which have a full 16-bit wide address bus such as 6800, 8080, Z80. This technique uses the 12 lower address lines of the processor address bus to supply data to the DAC, thus each AD7545A connected in this way uses 4k bytes of address loca- tions. Data is written to the DAC using a single memory write instruction. The address field of the instruction is organized so that the lower 12 bits contain the data for the DAC and the upper 4 bits contain the address of the 4k block at which the DAC resides. Figure 12. Connecting the AD7545A to 8-Bit Processors via the Address Bus SUPPLEMENTAL APPLICATION MATERIAL For further information on CMOS multiplying D/A converters the reader is referred to the following texts: Reference 1 CMOS DAC Application Guide available from Analog Devices, Publication Number G872a-15-4/86. Reference 2 Gain Error and Gain Temperature Coefficient of CMOS Multiplying DACs – Application Note, Publication Number E630c–5–3/86. Reference 3 Analog-Digital Conversion Handbook (Third Edition) available from Prentice-Hall. |
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